Maria Rita Cicconi

Daniel R Neuville

Natural glasses

On Earth, natural glasses are typically produced by rapid cooling of melts, and as in the case of minerals and rocks, natural glasses can provide key information on the evolution of the Earth. However, we are aware that natural glasses are products not solely terrestrial and that the formation mechanisms give rise to a variety of natural amorphous materials. On the Earth´s surface, glasses are scarce compared to other terrestrial bodies (i.e. Moon), since the conditions on the surface give rise to devitrification or weathering.

In order to provide an exhaustive overview, we are going to classify natural glasses based on the mechanisms by which they were formed. When talking about natural glasses, the first image is that of an obsidian, a volcanic glass that has been used since prehistoric times and has accompanied and influenced human evolution. Indeed, the largest volumes (km 3 ) of natural glasses (e.g. obsidian, perlite and pitchstone) are linked to volcanic activity and associated with cooling close to the surface. Tektites and impact glasses are formed in a completely different way since their formation is related to the impact of an extraterrestrial body on the surface of the Earth. Therefore, since glasses are formed by different processes,

we will provide a subdivision based on the formation mechanisms: temperature related, temperature-pressure related, temperature-pressure-volatiles related, and others:

-Quenched glasses -Impact glasses -Obsidian -Others.

At the end we will provide some insights into the structure and properties of natural glasses and melts.

Several reviews, for the different natural glasses, have been published and the readers are referred to these exhaustive papers: i.e. [START_REF] Weeks | Libyan Desert glass: A review[END_REF][START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF][START_REF] Koeberl | Geochemistry and tektites and impact glasses[END_REF][START_REF] Glass | Tektites and microtektites: key facts and inferences[END_REF][START_REF] Aboud | Libyan Desert Glass: has the enigma of its origin been resolved?[END_REF][START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF][START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF]. An interesting nomenclature for natural glasses was provided by Heide and Heide [START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF], which divided natural non-crystalline solids into four main groups, depending on their origin: magmatic, metamorphic, sedimentary and biogenetic.

The chemical composition of natural glasses varies from mafic to felsic. Figure 1 reports the Total Alkali vs. Silica (TAS) diagram, where the compositions of many natural glasses (Table 1) have been recalculated to 100% excluding water and carbon dioxide. The enormous variability of natural glasses composition can be appreciated in the TAS diagram, since they are differentiated as many common types of volcanic rocks ( [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF]). Indeed, the diagram shows how widely natural glasses can vary in their composition, with SiO 2 contents ranging from 30 to 99wt%, and total alkali contents ranging from 0% up to 15wt%. The list presented is, of course, not exhaustive, but represents many different glasses and their average compositions.

Table 1 -Average composition of the major oxides (wt%) in several natural glasses. 1).

Important abbreviations

Ab = albite -NaAlSi 3 O 8 An = anorthite -CaAl 2 Si 2 O 8 Basalt = extrusive igneous rock (forms from lava that cools rapidly at Earth's surface) Deep Sea Drilling Project (DSDP) = Multi-institution US programme operating a drilling ship for scientific sampling of the oceans from 1968 to 1985 [9] Effusion rate = the volume rate of lava production (m 3 /s) [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF] FAMOUS = French-American Mid-Ocean Undersea Study Feldspar: group of silicate minerals (e.g. Anorthite, Albite, Orthoclase) Felsic = (e.g. rhyolite) term used for magmas/rocks enriched in lighter elements: SiO 2 , alkali elements and Al 2 O 3 .

Hypervelocity impacts = Impacts, involving impacting bodies that are traveling at speed (generally greater than a few km/s) higher enough to generate shock waves upon impact.

-QUENCHED GLASSES

Basalt is a mafic igneous rock with a SiO 2 content that lies between 45-52 wt%.

Basaltic melts are produced in a wide variety of tectonic environments on Earth (and also occur on other terrestrial bodies including the Moon) [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF], and can be classified depending on the geological setting in which they have formed.

Mid-ocean Ridges, where new oceanic lithosphere is continually built from mantle melting, has a total length of more than 60000 km and produce ~ 21 km 3 of lava per year. Mid -ocean ridge basalts (MORBs) are tholeiitic basalts (mafic magmas enriched in Mg and Fe) that commonly contain phenocrysts of silicate minerals and oxides (e.g. olivine (Mg,Fe) 2 SiO 4 , chromite (Mg,Fe)Cr 2 O 4 , etc…) [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF]. The most distinctive aspect of Mid -ocean ridge basalts is their chemical composition: i.e. most MORBs (normal MORB) have similar major element compositions, and are characterized by low contents of K 2 O and other incompatible elements.

Another class of MORB, referred to as E-MORB (enriched MORB), has very different trace element compositions and is significantly enriched in the more incompatible elements [9]. The chemical variations observed between basalts from different terrestrial tectonic settings can provide key information about the processes by which basaltic melts are generated in the Earth' s interior [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF].

Basalt flows, erupted under water, differ in form from those erupted on land, and a variety of lava flow morphologies (typically classified as pillowed, lobate, or sheet flows on the basis of their morphology) is found at Mid-ocean Ridges. Pillow lavas (see Fig. 2) are approximately spherical or cylindrical bodies result from lower effusion rates, whereas sheet flows form from higher effusion rates [9]. The lobate morphologies is a type of bulbous lava flow wider and flatter than pillow lava, typical of intermediate effusion rates or flow on moderate sea floor slopes [9]. Indeed, Gregg and Fink [10] by carrying out 182 controlled laboratory simulations, in order to quantify the effects of effusion rate, cooling rate, and sea floor slope on lava morphologies, suggested that the variation from sheet flows to pillows reflects a decreasing effusion rate for a given viscosity and sea floor slope [START_REF] Gregg | Quantification of submarine lava-flow morphology through analog experiments[END_REF].

Rapid quenching in cold seawater commonly results in a glassy matrix. Because of the fast quench of basaltic melt causes fragmentation, submarine lava flows are often associated with deposits of fine, glassy pieces called hyaloclastite (from the Greek: "glassy fragment").

The glassy fragments alter to a yellow or green-brown hydrated mass called palagonite (see later). Several studies have been devoted to understanding the oxidation state of Mid -ocean ridge basalt glasses since the redox state of the mantle is reflected in the oxidation state of MORBs (e.g. [START_REF] Byers | Volatiles in basalts and andesites from the Galapagos Spreading Center, 85° to 86°W[END_REF][START_REF] Christie | Oxidation states of mid-ocean ridge basalt glasses[END_REF][START_REF] Bézos | The Fe3+/??Fe ratios of MORB glasses and their implications for mantle melting[END_REF][START_REF] Frost | The Redox State of Earth's Mantle[END_REF] and references therein). The understanding of the mantle redox equilibria is deeply linked to the magmatic differentiation, element partitioning, and speciation: e.g. C-O-H-S equilibria in magma and during degassing.

-Volcanic glasses

Volcanism is not exclusively basaltic in all tectonic environments on Earth, even if basalts are found on the surface of the Earth in greater volume than any other volcanic type.

Basaltic glasses have an average composition of about (wt%) 49-54% SiO 2 , 15-20% Al 2 O 3 , 8-10% FeO tot , 2-5% alkali (K 2 O + Na 2 O), 10-20% alkali-earth (CaO + MgO) [START_REF] Bouška | Terrestrial and Lunar , Volcanic and Impact Glasses , Tektites , and Fulgurites[END_REF] and their low viscosity favors crystallization (devitrification). Volcanic glass commonly produced upon rapid cooling of melts of basaltic composition is called sideromelane. Basaltic glasses also occur as volcanic ash, fibers and teardrops (i.e. Pele's Hair and Pele's Tears) and more rarely form solidified foamreticulite (Fig. 3) [START_REF] Keefe | Natural glass[END_REF]. Pele's tears are spherical pyroclasts with sizes varying from few μm to hundreds of μm of diameter, and the droplet shape is mainly controlled by the surface tension, the acceleration of the droplet after eruption and air friction [START_REF] Heiken | Morphology and petrography of volcanic ashes[END_REF]. In contrast, Pele's hairs have cylindrical forms (1 to 500 μm in diameter) and are extremely delicate (Fig. 4). Pele´s hair and tears form during fountaining of the fluid lava (hawaiian-style eruptions) with an extremely high cooling rate. Hawaiian tholeiitic basalt magmas have temperatures of 1050°-1200°C, and gas contents ranging from 0.5 to 2 wt% [START_REF] Macdonald | Physical properties of erupting Hawaiian magmas[END_REF] hence Hawaiian eruptions are characterized by the emission of very fluid lava. Shimozuru [START_REF] Shimozuru | Physical parameters governing the formation of Pele's hair and tears[END_REF] suggested that Pele´s hairs are produced when the velocity of erupting magmas is high and Pele's tears when it is "relatively" lower. Moune et al. [START_REF] Moune | Pele's hairs and tears: Natural probe of volcanic plume[END_REF] observed that Pele's tears can be associated with Pele's hairs after their formation. Pele's hairs have vesicles, typically parallel to the axis of elongation that break and often form long open cavities. Pele´s tears can be also trapped on the walls and/or in these cavities [START_REF] Moune | Pele's hairs and tears: Natural probe of volcanic plume[END_REF]. These pyroclastic products are not solely associated to Hawaii volcanisms, since many other volcanoes produce them (e.g. Etna, Réunion, Masaya volcano, etc… ). Fiber is usually one of basic reinforcing elements of composite materials, and in the framework of developing environmentally sustainable composites, the use of natural fibers (e.g. basaltic fibers) has been an active field of study in the last decade [e.g. [START_REF] Fiore | A review on basalt fibre and its composites[END_REF]]. Indeed, basalt fibers have properties, comparable to those of the synthetic ones, are cheaper of carbon fibers, and have even better mechanical properties than most types of E-Glass [START_REF] Dhand | A short review on basalt fiber reinforced polymer composites[END_REF]. Fiore et al. [START_REF] Fiore | A review on basalt fibre and its composites[END_REF] in their review report the main properties and several applications of basalt fibers.

Here we report a few of the advantages of the use of these fibers:

-use of natural basalt rocks as raw material, thus no additive needed;

-the manufacturing process of basalt fibers is similar to that of other glass fibers, but with less energy consumed (lower temperatures required), which makes it cheaper than glass or carbon fibers [START_REF] Fiore | A review on basalt fibre and its composites[END_REF][START_REF] Dhand | A short review on basalt fiber reinforced polymer composites[END_REF][START_REF] Singha | A Short Review on Basalt Fiber[END_REF];

-mechanical properties similar to those of synthetic glasses (e.g. E-Glass) [START_REF] Fiore | A review on basalt fibre and its composites[END_REF][START_REF] Singha | A Short Review on Basalt Fiber[END_REF];

-high thermal and chemical stability [START_REF] Fiore | A review on basalt fibre and its composites[END_REF][START_REF] Ramachandran | Chemical durability studies on basalt fibres[END_REF];

Among the volcanic glasses, an interesting melt/glass composition is the phonolitic one. There are few long-lived lava lakes, and among them the phonolitic lava lake of the Erebus volcano (Ross Island, Antarctic) has attracted many scientists. Both chemical and isotopic compositions of the magma in the Erebus lake have remained almost constant since the 1970s, when it was discovered [START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross island, Antarctica[END_REF][START_REF] Kelly | Geochemistry and mineralogy of the phonolite lava lake, Erebus volcano, Antarctica: 1972-2004 and comparison with older lavas[END_REF]. Erebus phonolitic magma/glass (Fig. 5) is Na enriched (average composition: SiO 2 ~55wt%, Al 2 O 3 ~20wt%, FeO tot >5wt% and alkali (Na 2 O + K 2 O) >14wt%; see Table 1) and has low viscosities (for an anhydrous and crystalfree melt, in the temperature range ~ 670-770°C, viscosity ranges between 10 10 and 10 12 Pa*s [START_REF] Losq | Rheology of phonolitic magmas -the case of the Erebus lava lake[END_REF]).

Interestingly, foamed phonolite has been observed in some lava bombs from the Erebus. Kyle [START_REF] Kyle | Mineralogy and glass chemistry of recent volcanic ejecta from Mt Erebus, Ross Island, Antarctica[END_REF] describes some phonolitic bombs as extremely breakable samples, since they consist of highly pumiceous -dark green to black -glass with anorthoclase phenocrysts.

The bombs are "fusiform" and highly vesiculated with the size of vesicles from 1-2 mm up to 40 mm on the surface and with some of the holes crossed by fine glassy hairs. The small thickness of basaltic glasses that are formed by natural processes, along with a strong tendency to devitrify, result in the alteration of basaltic glass in relatively short time (a few thousand to tens of thousands of years) [START_REF] Friedman | Volcanic glasses, their origins and alteration processes[END_REF]. The alteration processes of volcanic glasses, and in particular the formation of secondary minerals, is an extremely important topic of study, and in a review paper, Stroncik and Schmincke [START_REF] Stroncik | Palagonite -A review[END_REF] listed three main reasons: i) the alteration of volcanic glass influences the chemistry of natural waters; ii) the economic relevance of bentonite deposits (clay mineral) formed by the alteration of pyroclastic materials; iii) the alteration processes of natural glasses can be used as analog for modeling the long-term stability of glasses used for nuclear waste immobilization (see below).

In altered basaltic glasses of hyaloclastite deposits from Sicily (Monti Iblei, Sicily) it was first described a resin-like transparent substance that thereafter was called "palagonite" [START_REF] Stroncik | Palagonite -A review[END_REF]. Palagonite is considered the first stable product of the alteration caused by the interaction of glass and aqueous solutions (Fig. 6). Indeed, basaltic glass in contact with water forms this alteration zone whose thickness increases linearly with time [START_REF] Friedman | Volcanic glasses, their origins and alteration processes[END_REF]. The occurrence of this layer has been confirmed under various alteration conditions, and it can be more or less crystalline and commonly associated with the presence of clays (e.g. [START_REF] Stroncik | Palagonite -A review[END_REF][START_REF] Parruzot | Long-term alteration of basaltic glass: Mechanisms and rates[END_REF]).

Applications: alteration of natural glasses as analogue of nuclear waste glasses

The study of basaltic glasses is not only related to Geoscience (s.l.) for understanding the eruptive processes, or for understanding the formation of economic mineral deposits.

Natural basaltic glasses have been widely used as analogues in studies for evaluation of the long-term stability of nuclear waste form glasses [START_REF] Ewing | Natural Glasses: Analogues for Radioactive Waste Forms[END_REF][START_REF] Chapman | The potential of natural analogues in assessing systems for deep disposal of high-level radioactive waste[END_REF] because a challenging aspect of nuclear waste repositories is the extrapolation of short-term laboratory data (hours to years) to the long time periods (hundred to thousand years) [START_REF] Ewing | Natural analogues: their application to the prediction of the long-term behavior of nuclear waste forms[END_REF]. Therefore, in the last decades many studies have been devoted to get information from natural analogues, in order to corroborate predictive models of the long-term processes which affect the stability of radioactive waste repositories, since they provide the opportunity to examine processes occurring over geological timescales (natural basalt provide data on the alteration behavior of amorphous materials over tens or hundreds of thousands years) (e.g. [START_REF] Zielinski | Stability of glass in the geologic environment: some evidence from studies of natural silicate glasses[END_REF][START_REF] Malow | Alteration effects and leach rates of basaltic glasses: Implications for the long-term stability of nuclear waste form borosilicate glasses[END_REF][START_REF] Byers | Basalt glass: an analogue for the evaluation of the long-term stability of nuclear waste form borosilicate glasses[END_REF][START_REF] Techer | Basaltic glass: alteration mechanisms and analogy with nuclear waste glasses[END_REF]).

From all the studies it has been highlighted that basaltic glasses and borosilicate nuclear waste form glasses appear to be similar in their corrosion rates and mechanisms despite the significant compositional differences. Thus, the alteration processes observed for natural basalt glasses are relevant to understanding the alteration of nuclear waste glass, as both appear to react via similar processes. An early study from Zielinski [START_REF] Zielinski | Stability of glass in the geologic environment: some evidence from studies of natural silicate glasses[END_REF] on the stability of glass in geological environments report three main observations for natural silicate glasses:

1 -Breakdown of glass in a water-free environment is too slow to be of concern for the periods of time required for safe waste disposal;

2-Contact between glass and a hot aqueous phase accelerates all processes of glass alteration.

Glass hydration and dissolution rates become fast enough that significant alteration of glass can occur in the period required for hazardous radionuclides to decay;

3-Other parameters that can influence hydration/dissolution rates of glass/water interaction, include pH, composition, salinity, etc... [START_REF] Zielinski | Stability of glass in the geologic environment: some evidence from studies of natural silicate glasses[END_REF].

Byers et al. [START_REF] Byers | Basalt glass: an analogue for the evaluation of the long-term stability of nuclear waste form borosilicate glasses[END_REF] reported that the alteration of basalt glasses in nature is driven by a hydrolytic dissolution/re-precipitation mechanism which results in the formation of welldefined iron-rich aluminosilicate surface layers (palagonite). Natural glass alteration is also accompanied by the precipitation of authigenic cements (e.g., zeolites, clay minerals), and the clay mineralization observed on natural glasses is very similar to that produced on laboratory altered glasses [START_REF] Berger | Behavior of Li, Rb and Cs during basalt glass and olivine dissolution and chlorite, smectite and zeolite precipitation from seawater: Experimental investigations and modelization between 50° and 300°C[END_REF][START_REF] Crovisier | Nature and role of natural alteration gels formed on the surface of ancient volcanic glasses (Natural analogs of waste containment glasses)[END_REF]. More recently, Techer and coauthors [START_REF] Techer | Basaltic glass: alteration mechanisms and analogy with nuclear waste glasses[END_REF], Poinssot and Gin [41] and Parruzot et al. [31] corroborate the analogy of alteration kinetics/mechanisms for natural and synthetic basaltic glasses, archaeological glasses, and the SON68 (or R7T7) nuclear waste glass (a simulated French inactive nuclear reference glass).

Lunar Glasses

Lunar glasses with colours from red through green are widely distributed in lunar soil. and FeO ~16-24 wt%; see Fig. 1 and Table 1).

Since the Apollo missions, several homogeneous glass samples have been returned from six different lunar locations. Delano [START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF] distinguished 25 different types of lunar volcanic ejecta, which differ conspicuously from terrestrial volcanic ones both in composition and in shape, with the lunar ones, typically having extremely low silica content (see Table 1)

and spherical shape [START_REF] Keefe | Natural glass[END_REF]. The high variability of TiO 2 contents led to several studies to understand the origin and differences of these glasses. Indeed, as pointed out by Delano [START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF] the TiO 2 abundance variability could reflect the complex processes associated with lunar magma petrogenesis. Figure 7 shows the variation of SiO 2 and FeO against TiO 2 content in lunar glasses (data and references in [START_REF] Brown | Origin of the Apollo 14, 15, and 17 yellow ultramafic glasses by mixing of deep cumulate remelts[END_REF] and some black glasses. . Modified after Delano [START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF] and Brown and Grove [START_REF] Brown | Origin of the Apollo 14, 15, and 17 yellow ultramafic glasses by mixing of deep cumulate remelts[END_REF] and references therein).

Lunar glasses were at first believed to be of impact origin, but later studies pointed to volcanism. The Lunar Magmatism (so called Mare basalts) is represented by the dark smooth areas on the Moon. In addition to the eruption of large volumes of lava, the colored volcanic glass beads were produced by lava fountaining. These glass beads, as homogeneous quenched materials, provide some of the most important constraints to models of mare-basalt petrogenesis and for studying the characteristics of lunar interiors (composition, phase stability, thermodynamic parameters) [START_REF] Shearer | Basaltic magmatism on the Moon: A perspective from volcanic picritic glass beads[END_REF]. Indeed, several studies on lunar glasses have been performed in order to provide constrains on models used to represent the lunar interior and insights into the thermal histories of lunar regions (experimental constraints on the origin of lunar ultramafic glasses e.g. [START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF][START_REF] Ma | A chemical study of individual green glasses and brown glasses from 15426-Implications for their petrogenesis[END_REF][START_REF] Delano | Mare glasses from Apollo 17: constraints on the moon's bulk composition[END_REF][START_REF] Longhi | On the Connection between Mare Basalts and Picritic Volcanic Glasses[END_REF][START_REF] Hughes | Apollo 15 yellow-brown volcanic glass: Chemistry and petrogenetic relations to green volcanic glass and olivine-normative mare basalts[END_REF][START_REF] Shearer | Ion microprobe studies of trace elements in Apollo 14 volcanic glass beads: Comparisons to Apollo 14 mare basalts and petrogenesis of picritic magmas[END_REF][START_REF] Wagner | Experimental constraints on the origin of lunar high-Ti ultramafic glasses[END_REF][START_REF] Elkins Tanton | Re-examination of the lunar magma ocean cumulate overturn hypothesis: Melting or mixing is required[END_REF][START_REF] Krawczynski | Experimental investigation of the influence of oxygen fugacity on the source depths for high titanium lunar ultramafic magmas[END_REF].

An examination of experimental data and thermal models is reported in a review by Shearer et al. [START_REF] Shearer | Thermal and Magmatic Evolution of the Moon[END_REF]. From low-pressure phase equilibria experiments it was highlighted that glasses are characterized by (1) high liquidus temperatures, (2) olivine ((Mg, Fe) 2 SiO 4 ) (± Chrome spinel, (Mg,Fe)(Al,Cr) 2 O 4 ) as the only liquidus phase, and (3) the remaining silicate and oxide phases are relegated to much lower temperatures [START_REF] Shearer | Thermal and Magmatic Evolution of the Moon[END_REF]. These features suggest that the glass beads are primitive (least fractionated) magmas generated at considerable depths in the lunar mantle. The phase equilibria experiments on glasses provided range of pressure from 1.5 to 2.4 GPa and high temperatures (1430-1560°C) [START_REF] Shearer | Thermal and Magmatic Evolution of the Moon[END_REF].

Particular attention has been given to quantifying the redox conditions on the Moon (the readers are referred to the exhaustive review by Wadhwa [START_REF] Wadhwa | Redox Conditions on Small Bodies, the Moon and Mars[END_REF]). First of all, it is important to point that in Geosciences, the oxidation-reduction conditions of a system usually are described by using "redox buffer", that is the fugacity-temperature stability field for minerals and oxides (the phases coexist only at the buffer line). Examples of such redox buffers are:

-magnetite-hematite (MH)

2Fe 3 O 4 + ½O 2 = 3Fe 2 O 3 -nichel-nichel oxide (NNO) 2Ni + O 2 = 2NiO -iron-wüstite (IW) 2Fe + O 2 = 2FeO
Iron is one of the most abundant elements that exists in more than one oxidation state (Fe 0 , Fe 2+ and Fe 3+ ), thus, generally the redox is expressed in terms of iron oxidation state, i.e., the relative proportions of the different iron valences [START_REF] Herd | Basalts as Probes of Planetary Interior Redox State[END_REF]. Fe 2+ and Fe 3+ are most applicable to the range of oxygen fugacity (fO 2 ) in terrestrial and martian basalts [START_REF] Papike | Comparative planetary mineralogy: Valence state partitioning of Cr, Fe, Ti, and V among crystallographic sites in olivine, pyroxene, and spinel from planetary basalts[END_REF]. Lunar basalts formed under significantly more reducing conditions than terrestrial basalts and the estimations of the fO 2 conditions for lunar basalts and lunar glasses have been made using a variety of methods (e.g. [START_REF] Sato | Oxygen fugacity values of Apollo 12, 14, and 15 lunar samples and reduced state of lunar magmas[END_REF][START_REF] Fogel | Magmatic volatiles in primitive lunar glasses: I. FTIR and EPMA analyses of Apollo 15 green and yellow glasses and revision of the volatileassisted fire-fountain theory[END_REF][START_REF] Sutton | Vanadium K edge XANES of synthetic and natural basaltic glasses and application to microscale oxygen barometry[END_REF][START_REF] Karner | Application of a new vanadium valence oxybarometer to basaltic glasses from the Earth, Moon, and Mars[END_REF]). Based on the several experimental works done, Moon redox conditions are estimated to range from ~2 log units below the Iron-Wüstite buffer (~IW-2) to close to the IW buffer [START_REF] Wadhwa | Redox Conditions on Small Bodies, the Moon and Mars[END_REF]. For comparison, the range of oxygen fugacity on Earth can be represented by oxygen fugacity in the range IW+2 to IW+6. Papike et al. [START_REF] Papike | Comparative planetary mineralogy: Valence state partitioning of Cr, Fe, Ti, and V among crystallographic sites in olivine, pyroxene, and spinel from planetary basalts[END_REF] provided a schematic representation of the range of valence states of several multivalent elements with the range of fO 2 of planetary basalts (relative to the IW buffer) (Fig. 1 in [START_REF] Papike | Comparative planetary mineralogy: Valence state partitioning of Cr, Fe, Ti, and V among crystallographic sites in olivine, pyroxene, and spinel from planetary basalts[END_REF]).

Few Si-enriched glasses (with SiO 2 contents > 60wt%) have been found in soil (and breccia) samples from the Apollo 11,[START_REF] Christie | Oxidation states of mid-ocean ridge basalt glasses[END_REF][START_REF] Frost | The Redox State of Earth's Mantle[END_REF][START_REF] Bouška | Terrestrial and Lunar , Volcanic and Impact Glasses , Tektites , and Fulgurites[END_REF] and 17 landing sites and from the Luna 16 and 20 sites [START_REF] Glass | High-silica (>60%) lunar glasses in an Apollo 14 soil sample: Evidence for silicic lunar volcanism?[END_REF]. Among them Glass [START_REF] Glass | High-silica (>60%) lunar glasses in an Apollo 14 soil sample: Evidence for silicic lunar volcanism?[END_REF] found only one colorless (without vesicles or crystalline inclusions) high-silica spherule. The analysis of these glasses results in an average composition of the major elements similar to that of terrestrial volcanic glasses, even if the lunar glasses have higher K 2 O and lower Na 2 O contents than the terrestrial ones. The author studied both petrology and chemistry of several Si-enriched glasses and described the origin/source of these material not as residual glass or impact-produced glasses from a granitic source, but as products of lunar acidic volcanism [START_REF] Glass | High-silica (>60%) lunar glasses in an Apollo 14 soil sample: Evidence for silicic lunar volcanism?[END_REF].

-Impact glasses

The generic term impactite refers to a large variety of rocks formed by the melting of crustal rocks during the impact of a large extraterrestrial body. A series of distinct materials (impactites) is generated by the forces of a hypervelocity impact, starting with shock compression and followed by decompression from peak shock pressures, with associated heat generation [START_REF] Reimold | <em>IMPACT!</em> -BOLIDES, CRATERS[END_REF]. Depending on the pressure generated, several distinct markers will be formed, and by increasing pressure there are: planar deformation features (PDFs), diaplectic mineral glasses (produced without fusion), fused mineral glasses (produced with fusion), and melts [START_REF] Reimold | <em>IMPACT!</em> -BOLIDES, CRATERS[END_REF].

We will mainly discuss the most homogeneous glasses created by such events, i.e. tektites. To distinguish "impact glasses" and tektites, Koeberl [START_REF] Koeberl | The Geochemistry and Cosmochemistry of Impacts[END_REF] provides the following characteristics for tektites: i) are amorphous and fairly homogeneous, ii) contain lechatelierite (amorphous SiO 2 ), iii) occur within definite areas, called strewn-fields (SF), iv) tektites do not occur directly in or around a source crater (distal ejecta), and have v) low water content (<0.02wt%, at least an order of magnitude lower than the H 2 O content of volcanic glasses).

Moreover, studies on iron oxidation state show that Fe is generally highly reduced in tektites, with almost all occurring as Fe 2+ [START_REF] Fudali | The oxidation state of iron in tektite glass[END_REF][START_REF] Giuli | Iron local structure in tektites and impact glasses by extended X-ray absorption fine structure and high-resolution X-ray absorption near-edge structure spectroscopy[END_REF][START_REF] Giuli | Iron oxidation state and local structure in North American tektites[END_REF][START_REF] Giuli | Australasian microtektites from Antarctica: XAS determination of the Fe oxidation state[END_REF].

Tektites and microtektites

Tektites are small, black, glassy objects that are found only in certain areas of the Earth's surface. Most of the tektites are Si-rich glasses of various sizes (usually > 1 cm), with typical aerodynamic shapes and very characteristic surface features . Microtektites are microscopic tektites (< 0.1mm) found in deep-sea sediments [START_REF] Glass | Tektites and microtektites: key facts and inferences[END_REF]. Based on their shapes it is possible to distinguish three types of tektites: Muong Nong-Type (or layered), splash forms, and ablated/flanged tektites. Splash-form tektites (the most common form), include spheres, flattened ellipsoids, tear-shaped bodies (Fig. 8), rod-shaped bodies (generally thickened at both ends), canoes, and saucer-shaped objects [START_REF]The origin of tektites[END_REF]. To date, four main groups of tektites, associated with separate impacts and strewn fields (SF), are known (e.g., [START_REF] Glass | Chemical composition of Ivory Coast microtektites[END_REF][START_REF] Folco | Transantarctic Mountain microtektites: Geochemical affinity with Australasian microtektites[END_REF][START_REF] Glass | Distal impact ejecta layers: Spherules and more[END_REF][START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF]): the North American, the Ivory Coast, the central European and the Australasian (Fig. 10). The source craters have been located for three of the four tektite strewn fields, based on geographic location, geochemical evidences, and composition [START_REF] Engelhardt | Origin of moldavites[END_REF][START_REF] Koeberl | The geochemistry of tektites: an overview[END_REF][START_REF] Deutsch | Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence[END_REF].

The oldest strewn field is the North American (NA) field of ~ 35.5 Ma age associated with the ~ 40 km wide Chesapeake Bay impact structure [START_REF] Poag | The Chesapeake Bay Crater[END_REF] and includes Bediasites, Georgianites, Barbados and Cuba tektites, respectively found in Texas, Georgia, Barbados and Cuba (Fig. 10). The central European (CE) or moldavite strewn field of 14.4 Ma age is associated with the Ries crater of about 24km in diameter (Nördlinger Ries, Bavaria, D).

There is another impact crater, the Steinheim crater, ~ 3.8 km in diameter, located about 42 kilometers west-southwest from the centre of Ries. These two craters are believed to have formed nearly simultaneously by the impact of a binary asteroid [START_REF] Stöffler | Modeling the Ries-Steinheim impact event and the formation of the moldavite strewn field[END_REF]. The Ivory Coast (IC) tektite strewn field is associated with the 1.07 Ma old Bosumtwi crater (10.5 km diameter, Ghana, Africa). The youngest SF, of about 0.8 Ma, is the Australasian one, for which no source crater has been identified so far. Tektites of the Australasian strewn field (AA) include australites, thailandites, indochinites, philippinites and javanites and spread from the southeastern region of Asia down to Australia (Fig. 10). Recently, glass spherules (microtektites) were discovered on the Victoria Land Transantarctic Mountains (Antarctica)

and there is clear evidence [START_REF] Folco | Transantarctic Mountain microtektites: Geochemical affinity with Australasian microtektites[END_REF][START_REF] Folco | Microtektites from Victoria Land Transantarctic Mountains[END_REF]78] that these microtektites represent a major southeastward extension of the Australasian strewn field.

Even if no source crater has been discovered yet, several authors suggest a location for the AA crater in the Indochina region (see [START_REF] Glass | Geographic variation in Australasian microtektite concentrations: Implications concerning the location and size of the source crater[END_REF][START_REF] Lee M-Y | Australasian microtektites in the South China Sea and the West Philippine Sea: Implications for age, size, and location of the impact crater[END_REF][START_REF] Glass | Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba[END_REF][START_REF] Ma | Beryllium-10 in Australasian tektites: Constraints on the location of the source crater[END_REF]). Three of the four tektite strewn fields so far known also present microtektites (diameter < 0.1mm): the North American, the Ivory Coast, and the Australasian SF. These spherules have been found in deep-sea deposits (see, e.g. [START_REF] Glass | Tektites and microtektites: key facts and inferences[END_REF][START_REF] Glass | Microtektites in deep-sea sediments[END_REF][START_REF] Glass | Australasian microtektites and the stratigraphic age of the australites[END_REF][START_REF] Glass | Bottle Green Microtektites[END_REF]) and are very important for defining the extension of the strewn fields (e.g., [START_REF] Glass | North American microtektites in Deep Sea Drilling Project cores from the Caribbean Sea and Gulf of Mexico[END_REF]), for constraining the stratigraphic age of tektites, and to provide an indication regarding the location of possible source craters (e.g., [START_REF] Glass | Geographic variation in Australasian microtektite concentrations: Implications concerning the location and size of the source crater[END_REF]). Trace element abundances confirm that microtektites are genetically related to tektites in the associated strewn field [START_REF] Frey | Microtektites: a chemical comparison of bottle-green microtektites, normal microtektites and tektites[END_REF], but microtektites usually show a wider compositional range than tektites. Glass [START_REF] Glass | Tektites and microtektites: key facts and inferences[END_REF] assigns the different methods of analysis used, to explain the compositional differences between microtektites and tektites. A difference between microtektite and tektite samples was recently found by Giuli and coauthors [START_REF] Giuli | Amorphous Materials: Properties, structure, and durability: North American microtektites are more oxidized than tektites[END_REF]. The authors have shown that some North American microtektites present higher Fe 3+ /Fe 2+ ratio (up to 0.61), compared to the respective tektites, implying that, probably, different formation mechanisms are involved for the formations of such small objects [START_REF] Giuli | Amorphous Materials: Properties, structure, and durability: North American microtektites are more oxidized than tektites[END_REF].

Muong Nong-Type tektites (MN -named after a region in Laos) are a subgroup of tektites that are unusually large (up to several tens of centimeters in size) and with layered structures. MN tektites are enriched in volatile trace elements (e.g., Cl, Br, Zn, Cu, Pb), present chemical heterogeneity (darker and lighter layers), and may contain relict mineral grains (e.g. corundum, quartz, chromite and cristobalite) and bubbles [START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF]. Muong Nong-Type tektites strongly differ from volcanic glasses because of the presence of shocked mineral inclusions, for differences both in major and trace element contents (e.g. REE patterns), very low water content, highly reduced iron and the presence of 10 Be [START_REF] Koeberl | Geochemistry and origin of Muong Nong-type tektites[END_REF]. Koeberl [START_REF] Koeberl | Geochemistry and origin of Muong Nong-type tektites[END_REF]observed that Muong Nong-Type tektites contain higher abundances of the most volatile elements (the halogens, Cu, Zn, Ga, As, Se, Pb) compared to the splash-form tektites (the most common tektites). The presence of relict mineral grain, and the relatively higher water amount in Muong Nong-Type tektites suggest that MN have experienced the lowest temperatures of all tektites [START_REF] Koeberl | Geochemistry and origin of Muong Nong-type tektites[END_REF]. Is assumed that Muong Nong-type tektites have been deposited closer to the source crater and that they derive from a greater depth in the target deposits than were most of the other tektites [START_REF] Ma | Beryllium-10 in Australasian tektites: Constraints on the location of the source crater[END_REF].

The details of tektite and microtektite formation and on their distribution from the source crater are unknown and greatly debated. Numerical modeling indicates that highvelocity impacts (35-40 km/s) into a dry target with impact angles of 30° to 50° may provide the best conditions for tektite production [START_REF] Artemieva | Numerical modeling of tektite origin in oblique impacts : 91. Artemieva N, Pierazzo E[END_REF]91]. Indeed, the position of the strewn fields, with respect the inferred parent crater, indicates that tektites formed after oblique impacts.

However, the tektite-producing impact processes still have several open questions [START_REF] Pierazzo | Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets[END_REF] because physical and mathematical models are hampered by the limited information available (e.g. the importance of superheating), and because the impact process is a highly nonequilibrium and heterogeneous process. For instance, the exact target rocks from which tektites have been produced are not known yet, because it would require an accurate understanding of the physical-chemical processes, which may alter the chemical composition of the target rocks during impact. Moreover, the extent of volatilization, the state of tektite material after impact melting, or, what the size distribution of the melt droplets is [START_REF] Montanari | Impact Stratigraphy[END_REF], are still not understood. Engelhardt and coauthors [START_REF] Engelhardt | Origin of moldavites[END_REF] suggested that the tektite material is completely vaporized to a plasma state, and then the condensation from the plasma could form coalescing droplets. The hypothesis of tektite formation from a vapour plume might explain the ejecta homogeneity, and also the rapidity of homogenization of large volumes and the long distance transport. More recently, Johnson and Melosh [START_REF] Johnson | Formation of spherules in impact produced vapor plumes[END_REF] investigated droplets formation in impact produced vapor plumes and defined a linear correlation between the size of the ejected droplets and the size of the impacting object.

-K-Pg (KT) spherules

These small spherules (100-500m), that resemble microtektites, were first detected in the Cretaceous-Paleogene K-Pg (formally known as KT-Cretaceous-Tertiary) layer in Gubbio (I) [START_REF] Alvarez | Extraterrestrial Cause for the Cretaceous-Tertiary Extinction[END_REF] and are associated with the most recent major impact event on Earth. Around 65-66 Ma ago there was the collision of an asteroid that caused a massive extinction with an impact crater of about 180 km identified on the Yucatan Peninsula and known as Chicxulub crater [START_REF] Schulte | The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary[END_REF]. This large (cataclysmic) event is responsible for the formation of worldwide ejecta horizons, and caused the end-Cretaceous mass extinction, around 66 Ma ago.

K-Pg distal impact ejecta layers are associated with Ir enrichments, siderophile element anomalies, shock metamorphosed mineral (quartz grains, coesite and stishovite) and rock debris. Because of the poor preservation of the claystone K-Pg boundary, there was an early discussion about the origin of these spherules. Some authors supported the impact hypothesis (e.g. [START_REF] Smit | Sanidine spherules at the Cretaceous-Tertiary boundary indicate a large impact event[END_REF]), whereas others attributed an authigenic origin for those microspherules (e.g. [START_REF] Izett | Authigenic †TM spherules' in K-T boundary sediments at Caravaca, Spain, and Raton Basin, Colorado and New Mexico, may not be impact derived[END_REF]). However, Sigurdsson et al. [99], by studying the glasses preserved at the K-Pg layer at Beloc in Haiti, provide clear evidences of an impact event. Moreover, Koeberl [100] and Koeberl and Sigurdsson [101] also provided exhaustive geochemical data for the impact origin of these Haitian Si-rich glasses and reported the occurrence of rare inhomogeneous glasses with lechatelierite and other mineral inclusions, which are typical for an origin by impact [100]. In particular, the identification of Planar deformation features (PDF) in quartz in the K-Pg boundary-event bed was the key for the acceptance of this layer as an impact horizon [102]. Usually PDFs occur in silicate minerals, such as quartz and feldspars, which develop PDFs at pressures between 10-15 and 35 GPa. [START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF] The study of the K-Pg boundary ejecta provided the most influence for the discussion about the importance of impact events with respect to the evolution of the planet and of life, and the detailed study of a K-Pg distal impact ejecta layers had led to the discovery of one of the largest impact structures on Earththe 180 km Chicxulub crater [START_REF] Montanari | Impact Stratigraphy[END_REF].

-Younger Dryas Spherules

The Younger Dryas (YD) event is the name of a hypothesized impact event that may have occurred at the beginning of the Younger Dryas (~12.8 ka), and as emphasized by Bunch et al. [103], "impact" denotes a collision by a cosmic object either with Earth's surface, producing a crater, or with its atmosphere, producing an airburst.

This hypothetical impact event seems to be supported by several markers, listed by Firestone and coauthors [104]. The authors describe the occurrence of a <5-cm-thick sediment layer dated ca. 12.9 ka in several sites in North America ([105] expand the list and report also a few layers in Europe and in Syria). The markers include: magnetic microspherules, grains enriched in Ir, vesicular carbon spherules, glass-like carbon, nanodiamonds, fullerenes containing extraterrestrial concentrations of 3 He, the presence of singular forms of soot, and charcoal.

Younger Dryas boundary (YDB) spherules resemble the K-Pg spherules. Wittke et al.

[105] report detailed geochemical and morphological analyses of nearly 700 glass spherules from quite a few sites. These samples are Fe-and/or Si-rich magnetic spherules with dimensions ranging from 10 μm to 5.5 mm in diameter, with shapes from simple spheroids, to teardrops and dumbbells. The authors describe three groups of YDB objects based on chemistry: i) an Al/Si group, ii) a Fe-rich group, and iii) a high-Si/low-Al group. Some scorialike objects and spherules are mainly composed of shock-fused vesicular lechatelierite, whereas other present enrichment in Fe. Wittke et al. [105] conclude that these spherules are geochemically and morphologically comparable to cosmic ejecta associated to other highenergy events. Despite all this evidence, the hypothesis of the YD impact or airburst is still largely debated, since most of the reported markers could not be confirmed in independent studies. In the last few years several arguments for and against this hypothesis have been

-Enigmatic Impact glasses

Glass and Simonson [START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF] in their review on distal impact ejecta describe the occurrence of glasses, believed (almost certainly) to be of impact origin, but not found in stratigraphic contexts. These natural glasses have been found in several locations on Earth, and we are going to describe two of the most famous: i) Libyan Desert Glass, and ii) Darwin glass.

-Libyan Desert Glass -LDG

In an area of about 6500 km 2 in southwest Egypt, close to the border with Libya, fragments of a natural silica-rich glass, known as Libyan Desert Glass (or LDG, Fig. 11), of age 28-29 million years, are found [111, 112]. Since its discovery, early in the 20th century [113] the origin of the Libyan Desert Glass still represents an unanswered enigma to all scientists and researchers. However, an origin by impact seems the most plausible mechanism. In fact, LDG fragments are thought to be the remains of a glassy surface layer, resulting from high temperature melting of sandstones/desert sand, caused either by a meteorite impact, or -to explain the absence of an impact crater-by airburst (shock melting caused by a cosmic object exploding in the atmosphere) [START_REF] Aboud | Libyan Desert Glass: has the enigma of its origin been resolved?[END_REF]114].

LDG is very silica-rich (about 96.5-99 wt% SiO 2 ; see Table 1) and shows a limited variation in major and trace element abundances. The remaining few wt% are oxides of iron, titanium, calcium, and magnesium plus a few other oxides. LDG occurs as centimetre-to decimeter-sized, irregularly shaped, and strongly wind-eroded glass pieces (Fig. 11). The age of the LDG was mainly made by fission-track methods. Indeed, due to the low K content of the glass, the age errors from the K-Ar determinations are too high to be meaningful ([115] and references therein). The fission track dating from different investigation provides similar ages: 28.5 ± 2.3 Ma to 29.4 ± 0.5 Ma [116], and 28.5 ± 0.8 Ma [111].

Evidences for an impact origin include the presence of detectable amount of Ir [117] lechatelierite and baddeleyite [118] and, more in general, high pressure-temperature phases Based on the presence of phases, stable both at high and at low temperatures and pressures, the authors delimited a range of T and P conditions during the impact event: from 300 to >1470 °C, and from 10 to >30 GPa [115].

In 2007 a crater-like feature (Kebira crater, Gilf Kebir region, Egypt) was discovered by using satellite images and initially it was inferred as the source of LDG, because of its size, geographic location and topography. However, almost immediately it was disregarded since it lacked the geologic features associated with impact craters, such as impactites, breccias, and shatter cones.

-Darwin glass

The Darwin Glass is found in a strewn-field of about 400Km 2 in western Tasmania (Australia). The age of this glass, estimated by Ar-Ar methods, is around 800 ka [122]. The glass generally occurs as irregular fragments, or masses, but small glasses (spheres and teardrops <5mm) can be found across the Darwin SF. The color ranges from white, gray, light or dark green, dark brown, to black, and the glasses are generally vesicular and often exhibit flow structure marked by bands of elliptical vesicles [START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF]. Interesting, the proportion of white glasses is greatest in the proximity of the crater, and the proportion of darker glasses increase with distance from the crater [123,124].

The Darwin crater, a small (~1.2 km) simple impact crater formed in sedimentary target rocks, has been proposed as the source of Darwin glasses [125]. Howard and Haines [126] carried out a detailed petrographic study of the crater-filling samples, but no conclusive evidence of shock metamorphism has been found (e.g., shocked quartz grains, PDFs).

The chemistry suggests the presence of two main glass groups: the first one richer in SiO 2 (average ~ 85wt.%), and depleted in CaO and Na 2 O, and a second one with a lower average abundance of SiO 2 and a significant higher content of MgO and FeO [127].

Moreover, the second group is enriched in Ni, Co and Cr, with contents higher than in the surrounding sedimentary rocks. The enrichment in these elements (likely a meteoritic contamination), and the presence of coesite (high-pressure quartz polymorph) and lechatelierite seem to confirm the impact origin of these glasses [START_REF] Glass | Distal Impact Ejecta Layers -A Record of Large Impacts in Sedimentary Deposits[END_REF].

-Fulgurites

Fulgurite is a glass formed as a result of fusion of rock by lightning in desert sands and in many other soils. Usually fulgurites consist of irregularly shaped tubes (Fig. 12) A subdivision in five groups of fulgurites has been proposed by Pasek and coauthors [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF] based on the different morphology and petrology occurring as a result of target material composition. According to Pasek and coauthors [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF] it is possible to divide fulgurites in four main types of morphologies (plus a minor type):

ranging
 Type I fulgurites are formed in quartz sand and usually have thin glassy walls; Type I can contain one or two melts consisting prevalently of lechatelierite, and sometimes, also a SiO 2 -rich melt with higher concentrations of Al and/or Fe. In the groundmass has been reported also enrichments in Zr oxide-and Fe-Ti oxide-rich glass [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF].

 Fulgurites type II has a lower amount of lechatelierite (<50%), but higher glass thicknesses compared to fulgurites type I. The melt is more compositionally varied, because formed in different environments than quartz sand soils (e.g. soils composed also of clays minerals, quartz, and/or small rocks) [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF].

 Type III consists of lechatelierite and feldspar glasses, and a calcite-rich matrix.

Fulgurite type III, are mostly found in calcite-rich soils and are the densest (average density 1 2.1 ± 0.5 g/cm 3 ) [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF]. No zircons were observed in the type III fulgurites.

 Type IV fulgurites are heterogeneous melts: with the outer portion consisting of unmelted (or partially melted) rocks and minerals. Type IV fulgurites form usually in bulk rocks and have densities similar to those of the target rocks.

 Type V droplet fulgurites are thoroughly mixed and have a homogeneous melt. The two main oxides contained in type V droplets are enriched in SiO 2 and K 2 O relative to the originating fulgurite, whereas other oxides are depleted [START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF].

Usually the outer portion of a fulgurite tube is very rough and this is attributed to fragments of unaltered, or partially melted, minerals and rocks (Fig. 13). On the other hand the inner portion is mainly smooth and glassy (typically lechatelierite) and may also contain Several studies have reported the occurrence of metallic phases [128,134,135] and several explanations have been proposed to clarify the metallic oxide reduction occurring during fulgurite formation. Essene and Fisher [128], explained the occurrence of metallic globules rich in silicon, or spheroids of silicon-bearing metals (99.5 at% of metallic silicon phase with minor amounts of titanium, iron, and phosphorus), via thermodynamic calculations that indicate that extremely high temperatures (> 2000 K) and reducing conditions close to the SiO 2 -Si buffer were needed. Thus, the formation of coexisting metallic and silicate liquids was attributed to thermodynamic instability -at high temperature and strong reducing conditions -of the oxide species with respect to the metals. However the authors were not able to rule our other mechanisms, such as the presence of carbon, degassing of oxygen or formation of nitrogen oxide gases. Rowan and Ahrens [136] produced, by shock experiments, Fe-, Si-and Mo-rich metallic microspheres embedded in a shocked glass. Thus, there is the possibility that shockwaves induce reduction. Jones et al. [131], artificially (triggeredlightning) produced fulgurite specimens composed of 99.9% pure binary oxides of manganese and nickel in order to study the reduction mechanisms, and while they observed the formation of nickel oxide particles, the manganese oxide fulgurite showed no metallic phase formation.

Hence, the mechanism proposed by Essene and Fisher [128] the thermodynamic stability of an oxideis the most likely.

-OBSIDIAN

The most common natural glass deserves a special section. The Elder Pliny in his famous 'Natural History', first described "obsiana", so named from its similarity to a stone found in Ethiopia. The shape, the color and the properties of this volcanic glass (Fig. 14) have

been key factors for its use and contribution to Human history. Indeed, in prehistoric times it was widely used for cutting tools or arrowheads (Fig. 15), but also for the production of decoration and mirrors. The origin of the most common natural glass is not a simple matter of fast cooling.

The glass-forming processes of obsidian melt are strongly influenced by the contents of volatile components such as water, fluorine and chlorine, sulphur and carbon oxides [START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF]138].

Indeed, small variations in volatile content can cause notable changes in the physical properties, such as rheology, which strongly influences flow dynamics of obsidian melts.

Moreover, depending on various parameters such as temperature, pressure, chemical composition, water content, strain rate and also microlite (acicular crystals typically less than 10 μm in length) preferred orientation [139], obsidian may exhibit elastic, viscous, or brittle behavior [140].

There are many studies focused on the chemistry and hydration of obsidians, but curiously, there are not many studies on magnetical, mechanical and optical properties of these specimens that can provide strong insights on formation mechanisms and the nature of

obsidians. An early study by Ericson et al. [141] Obsidian is a preferred material for provenience studies (to determine from which location an "archaeological" obsidian came), and in particular, there are several studies on obsidians geochemical composition, fission-track dating, and chemical analyses [143].

Moreover, Mössbauer, Electron Spin Resonance, Raman spectroscopies, and magnetization have been largely used in provenance studies [144][145][146][147]. On the other hand, it is interesting to note that there are not many studies dedicated to the investigation of the redox state of obsidians. Indeed, as pointed out by Heide and Heide [START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF], many analyses have been concentrated only on the determination of the total Fe content.

Another interesting point of obsidians is their colour. The obsidian colour depends upon the presence of various transition elements along with the formation mechanisms, but obsidian is typically black or grey and sometimes occurs banded or streaky. However, it is possible to find several obsidian specimens with interesting optical properties, such as the socalled "rainbow", "sheen", or "fire" obsidian.

Rainbow obsidian shows iridescence bands of various colors, ranging from red through purple. Ma et al. [148] investigated the optical properties of rainbow obsidians with several techniques and they observed a correlation of the color bands both with the thickness and with the position of microcrystallites (pyroxenes or feldspars) within the samples. Moreover, they identify as possible cause of the rainbow effect a thin-film interference.

Arrays of crystallites produce interference effects, and the crystallites size and spacing control the rainbow effect.

Sheen obsidian usually is black to dark-brown and displays a single-color sheen, generally characterized by either a silvery or a golden hue. For a long time the sheen was assumed to be produced by reflections from oriented bubbles. However, Ma et al. [148] found that two kinds of vesicles coexist in these obsidians: vesicles filled with a slightly different glass or unfilled. In conclusion, Ma and coauthors [148] attribute the sheen to differences in indices of refraction between the glassy obsidian matrix and the lower indices of refraction of either gas-filled or glass-filled vesicles.

Fire obsidians have thin layers showing various colors. Indeed, under bright light, these glasses reflect colored bands resembling an iridescent oil slick. Ma et al. [149] reported that the iridescent layers were made of nano-particles of magnetite (Fe 3 O 4 ) that increased the refractive index, giving rise to thin-film interference and are the cause of the "fire" coloration

[149].

-Hydrated and Altered obsidians

Natural glasses are thermodynamically unstable at ambient temperatures and pressures and they slowly hydrate through the diffusion of water into the outer surface and along cracks.

Natural rhyolitic glasses hydrated at temperatures below the glass transition (Tg) are volcanic glasses that lately had a significant economic use in industry (e.g. supports of heterogeneous metal catalysts). The water-rich glasses are pumice, perlite and pitchstone.

Pumices are microvesicular, volcanic "foam" materials formed by decompression of volatile -rich melts [START_REF] Gill | Igneous Rocks and Processes -A Practical Guide[END_REF]. Thus, pumices have compositions close to that of obsidian, while after ejection they release the enclosed gas, forming a porous material. Perlite is a hydrated obsidian and the perlite deposits mostly occur as lava flows, dykes and domes. Perlitic glass which is relatively more altered, fractured and contains water > 4 wt% is often called pitchstone [150].

Rhyolitic obsidian glasses from different locations are plotted in the TAS diagram (Fig. 16), along with data for perlite and pitchstone, and it is possible to observe that the composition of the latters resembles that of obsidians (see Fig. 16). . See also Table 1.

Obsidian is frequently accompanied by the formation of pumice (Fig. 17). Indeed, pumice is usually considered to be foamed obsidian, and often has water contents similar to that of perlite glasses [START_REF] Bouška | Terrestrial and Lunar , Volcanic and Impact Glasses , Tektites , and Fulgurites[END_REF]. A beautiful example for a combined obsidian-pumice deposit can be found at the Rocche Rosse on Lipari Island (I).

Usually pumice samples have pale colors that range from white, grey or blue, to darker colors green-brown or black, depending on the presence/amount of iron. The samples are highly vesiculated with porosity up to ~ 90%, with the remaining solid part being mainly amorphous. As comparison, the volcanic scoria is less vesiculated (denser), thus it sinks rapidly, whereas pumice samples float on water. Bubbles form a percolating network at porosities between 30-80 vol.%, depending on melt viscosity, crystallinity, magnitude of shear, and bubble expansion rate [154][155][156][157]. Several studies have been done in order to explain the absence of vesicles in obsidian and thus, to provide insights on the change in eruptive behaviour (from explosive to effusive) in silicic volcanic systems (e.g. [157]).

Experimental works have shown that cooling and heating kinetics strongly control the release of gases of obsidian melts. Indeed, depending on the heating rate, an obsidian glass can transform into pumice or form a melt flow ( [START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF] and references therein). By thermal analysis, Bagdassarov and coauthors [150] observed that the main factor governing the uniform vesiculation and expansion of perlites is due to the microfracture pattern, which is developed during the dehydration of perlites below the glass transition temperature. Thus, the higher amount of water in perlite glasses is due to diffusion of meteoric water into the glass, and not from the original magma.

-Others

All those glasses that sometimes cannot be classified as volcanic or impact-related, and neither purely "natural" nor of "inorganic-origin" will be grouped under "Others". The so-called trinitite glasses (first described by Ross [166]) are a record of the first atomic bomb blast on July 16, 1945 (Fig. 19). The resulting fireball that burnt the arkosic sand desert formed a crater glazed with green fused silica sand. Ross [166] studied the optical properties of the amorphous layers and reported the occurrence of two different glasses. The first one with higher index of refraction (RI), and the other with a lower index of refraction (close to 1.46), indicating that this material was almost pure silica glass. Ross recognized also the occurrence of gray and red glasses (with dispersed copper). Several years later the early studies of Ross were confirmed and, according to Eby et al. [167], the detonation of the 21 kilotons plutonium bomb, produced four different type of glass: 1) the top part of the layer mainly consist of glassy and vesicular green fragments (Fig. 19), 2) a Cu-rich glass (red trinitite) containing metallic chondrules, (3) scoriaceous trinitite fragments; and (4) ejecta which includes aerodynamically shaped droplets, beads, and dumbbell glasses. The latter were compared to tektite and microtektites and Glass and coauthors [168] described many similarities between tektites and trinitite beads. Moreover, Giuli et al. [169] reported the Fe redox ratio (Fe 3+ /Fe 2+ ) in a trinitite glass to be close to 0.1, thus similar to tektites values.

Many studies have been done on the radioactive nuclides present in the materials (e.g.

[167, 170-172]). Indeed, the migration of actinides at historical test sites has been well studied, since it is closely related to the waste management and to the storage of high-level nuclear waste [173][174][175][176]. Eby et al. [177] recently published a detailed mineralogy and petrology study of the different trinitite glasses. The authors pointed out that to explain the physical processes occurring during the glasses formation, two main factors must be considered: the temperature and the duration of high temperatures. Through macroscopic measurements and theoretical calculations, Hermes and Strickfaden [START_REF] Folco | A common volatilization trend in Transantarctic Mountain and Australasian microtektites: Implications for their formation model and parent crater location[END_REF], estimated backward to the yield, fireball temperature, fireball duration, heat in the rising fireball, and the spread of ejecta from the Trinity test. The authors estimate that the yield of the Trinity event was 9-18 kt, and an average fireball temperature of 8430 K and a crater depth of approximately 4 ft [START_REF] Folco | A common volatilization trend in Transantarctic Mountain and Australasian microtektites: Implications for their formation model and parent crater location[END_REF].

Moreover, Hermes and Strickfaden [START_REF] Folco | A common volatilization trend in Transantarctic Mountain and Australasian microtektites: Implications for their formation model and parent crater location[END_REF] report that the duration of heating was very short (~3s) and this could explain why some minerals (zircon and quartz grains) were only partially melted.

Interestingly, Eby et al. [177] reports the occurrence of quartz with planar deformation features, typical evidence of shock metamorphism in impactites. Based on the mineralogy, petrology of the glass samples, and on previous time and temperature estimations, Eby and coauthors concluded that at the instant of detonation, "pressures of at least 8 GPa and temperatures of >8000 K occurred in the fireball" (cf. [177]).

The study of the first nuclear glass, beside applications for waste management and storage of high-level nuclear waste, has nuclear forensic applications since it provides information on the type of device that was detonated and its origin. Recently, Molgaard and coauthors [179] produced synthetic nuclear glasses, comparable with trinitite glasses, as surrogates that could be used to simulate a variety of scenarios (simulated nuclear event parameters such as, fuel type, weapon yield, and emplacement scenario), and could be used as a tool for developing and validating (nuclear) forensic analysis methods.

-Friction melts: Frictionites/Pseudotachylite

The formation of "friction melts" is associated either with large impact structures, earthquake-generated layers, and very large rock avalanches (e.g. see reviews [180,181]).

A generic definition for pseudotachylite (generic name for friction melt) is "dense rock produced in the compression and shear associated with intense fault movements, involving extreme mylonitization (i.e. the process of formation of a fine-grained rock produced by bending/internal slip of grains, and recrystallization) and/or partial melting"

(from [180]).

Friction melts (and especially those with a pumice texture) were first associated exclusively to volcanic origin or to impact events. Differences between impact-related and fault-related pseudotachylites occur in their thickness and their formation history (single-slip event, multiple stick-slip motion) [182]. The exclusive volcanic origin or impact-associated origin hypotheses have been both ruled out from the study of pumiceous rocks from Ötz Valley (Köfels landslide, A. [183,184]). Indeed in the last decades several studies focused on fault related friction melts (fault pseudotachylytes) that form by coseismic high velocity friction (Fig. 20).

Well studied fault pseudotachylites are from giant rockslides in Himalaya (Nepal) and Köfels (Austria). Masch and Preuß [185] report a detailed study of both events and they observed that glass matrix is chemically heterogeneous with schlieren (elongate segregations of mafic minerals), bubbles, and rock relicts from the parent material (partial to almost complete melting of host rocks of granitic to granodioritic composition). Moreover they report the occurrence of glasses with pure quartz, plagioclase and alkali-feldspar compositions. It's worth mentioning that the giant rockslides in the Himalayas (dislocation of ~170m) caused the formation of a homogeneous glassy crust of thickness between 1-3cm

[185]. Weidinger et al. [186] report a review of 19 basal deposits of giant rockslides and many had both micro-breccias and frictionite melts.

For the formation of fault pseudotachylite, the estimated melting temperatures derived from the mineral geothermometer of microlites (mineral systems used to estimate the temperatures) or from the chemical compositions of matrices (e.g. [187]). Artificial pseudotachylites have been produced by direct high-speed friction experiments (e.g. [182,187,188]) and the estimated melt temperatures of natural and experimental pseudotachylytes are in the range of 750-1400°C. [187]. Heide and Heide [START_REF] Heide | Vitreous state in nature-Origin and properties[END_REF] in their review report melting temperatures of 1700°C because of the presence of lechatelierite inclusions in frictionite melts.

-Bioglasses: amorphous hydrated silica

Biomineralization processes form biogenic materials that are considered eco-friendly and thus they captured the attention either of organic-/inorganic-chemists and materials scientists. Carbonate and phosphate are the most abundant biominerals in nature. On the other hand, amorphous SiO 2 , even if less abundant in biomineralizing organisms, has been widely studied (biomimetic studies -the use of models that partially reproduce the natural conditions of biomineral formation. [190])

The low-temperature hydrated variety of silica, opal (SiO 2 *nH 2 O), is a biomineral.

Opal is composed of differing amounts and arrangements of structural units of amorphous SiO 2 , water, and the quartz polymorphs: crystobalite, and tridymite [191] and it is possible to distinguish three types:

-opal-C, with cristobalite;

-opal-CT, with cristobalite and tridymite; -opal-A, X-ray-"amorphous" opal.

Moreover, the amorphous opal-A can be further divided in opal-AN (e.g., hyalite) and opal-AG with an amorphous silica gel structure. In a "maturation process" (Ostwald ripening) opals are transformed as follows ([191] and references therein):

Opal-AG → OpalCT → Opal-C → microcrystalline quartz Several siliceous marine organisms exhibit discontinuous, three-dimensional frameworks of short chains of SiO 4 tetrahedra, bonded with apical hydroxyls [192]. The most studied biomineralized species are fossil diatom frustules and sponge spicule: e.g. [193] report the study of the evolution of biogenic silica produced in marine environments both for fossils and living organisms, and they identify two different networks. The fossils have a highly condensed and well-organized silica network whereas the living diatoms is much less condensed, with the silica network suggested to be linked to the bio-organic components of the cell, in agreement with the biosilicification mechanisms [193].

Usually, the production of silica requires high temperatures a/o pressures, a/o extremes pH ranges. On the other hand, living organisms are able to form silica under ambient conditions, with low temperatures and pressures and almost neutral pH. Hence, understanding the biosilica formation mechanisms is of high importance for applications [194]. Indeed, the structure and the evolution of diatoms have several implications for Materials science and palaeoenvironmental research, and in the last years many studies have been devoted to biogenic silica. In the following a few examples are provided.

Bio-optical filters/fibers

Siliceous spicules from siliceous sponges are composed of siliceous layers superposed in a stratified pattern around a central axial filament [cf. 190], and are excellent light transmitters, and very good optical Bandpass filters. Indeed, exclusively wavelengths between 615 nm and 1310 nm can pass through these natural spicule fibers [195]. Moreover, [196,197] demonstrated that some spicules have compositional variations in the glass/organic composite and a variation in the refractive index profile: with a high refractive index in the spicule core and a low refractive index in the outer portion ("cladding"), thus presenting optical properties similar to those of commercial telecommunication fibers [196,197].

Metal Oxide production

The enzymes involved in silica formation have attracted increasing attention because of their potential applications in nano-biotechnology and biomedicine [194]. Indeed, these enzymes, at low temperature and near neutral pH, are also able to catalyze nanoparticles of metal oxides, such as TiO 2 , ZrO 2 , nanocrystalline Ga 2 O 3 , GaOOH, and also nanocrystalline Perovskite-like Barium Oxofluorotitanate (BaTiOF 4 ) [194,198,199].

Insights into the structure and properties of glasses

This section is an attempt to highlight the structure and the physical properties of natural glasses. Natural glasses, being a multicomponent system, require a deep understanding of many factors, thus the understanding of the evolution of natural glasses must be considered an interdisciplinary problem.

Physical properties of silicate glasses/melts

When talking about the physical properties of magma, one of the most important parameters is the viscosity (), i.e. the measure of a fluid's resistance to flow. It controls magma flow rates, rates of volatiles exsolution, rates of diffusion and crystal growth and volcano morphology. Depending on the bulk chemistry we have a wide range of viscosities, as reported in Figure 21, which shows the variation of the viscosity against the reciprocal temperature for a pure silica glass (SiO 2 ) and some glasses with mineral compositions of three The rheology of an obsidian flow is a key parameter governing the textural and structural evolution in obsidian flows. Rhyolitic magmas have a greater melt viscosity due to the high silica content, with respect to basaltic melts (Fig. 21a). Viscosity of rhyolitic magma ranges from 10 8 to 10 11 Pa*s at temperatures of 700-750°C, though viscosity strongly decreases with increasing volatile content (e.g. H 2 O, CO 2 ).

In tektites, the study of physical properties (e.g. density, viscosity), along with the redox conditions, can provide insights on their thermal history. Figure 21b shows the variation of the viscosity () against the reciprocal temperature for a moldavite (mol) and an Australasian tektite (chi). These tektites are highly polymerized glasses and the measurements show the very high viscosities of these glasses, and their nearly Newtonian behaviour (Fig. 21b). It is interesting to notice that the Australasian tektite has a viscosity an order of magnitude lower than the moldavite. In Table 2 are reported the compositions and some properties for the unpublished data shown here. To clarify the nature of natural glasses, beside physical properties and redox conditions, it would be helpful to understand the evolution of glass structure upon cooling.

The glass properties depend on the process by which it is formed, and close to Tg chemical/ physical properties are extremely sensitive to temperature [202]. For example, in silica this dependence can be well-represented as Arrhenian behaviour (exhibiting approximate linearity) and the liquid is called "strong". On the other hand, a liquid is called "fragile", where the fragility reflects what degree the temperature dependence of the viscosity deviates from Arrhenian behavior. The glass transitions of the two tektites considered here (taken for log=12Pa*s) are 1014 and 1070K, respectively for chi and mol and by using the Angell plot

[203] it is possible to do a Tg-scaled Arrhenius (Tg/T) representation of liquid viscosities (Fig. 22).

Strong liquids, such as SiO 2 , exhibit Arrhenius behaviour, indicative of a temperatureindependent activation energy [202]. Both moldavite (mol) and Australasian (chi) tektites show a nearly linear behaviour, and thus they can be considered as strong liquids.

Structure of silicate glasses

The knowledge of the structure of silicate melt/glass, along with its composition-and temperature-dependence is crucial to explain many properties of silicate systems. In the literature there are several investigations on silicate glasses structure, and many studies (e.g.

[204-206]) refer to the "stuffed tridymite" model proposed by Taylor and Brown [207].

Based on X-ray radial distribution analysis of glasses in the system SiO 2 -Ab-Or, Taylor and Brown [207] found several similarities and suggested a stuffed tridymite-like structure for anhydrous Si-rich melts. This model is similar to that proposed by Konnert et al. [208], where silica glasses have a tridymite-like bonding topology based on six-membered rings of SiO 4 tetrahedra (cf. Taylor and Brown [207]). Wright et al. [209] made distinctions between silica (fulgurite) and silicate glasses (obsidian and tektite) based on neutron diffraction measurements. Okuno et al. [210] studied molten and untreated obsidian and also found that the basic structures of samples may be explained by the "stuffed tridymite" model. studied obsidian with wide angle X-ray scattering and concluded that the structure of obsidian is a superposition of quartz-and cristobalite-like structures, in agreement with studies by Wright and Leadbetter [212].

A powerful tool to investigate the structure of silicate glasses is Raman spectroscopy, which provides information on the short-to medium-range order. Raman spectra have been measured on a collection of obsidians, tektites, and related glasses. These spectra will be presented and discussed in terms of the interpretative framework that has developed from studies on synthetic glasses [213].

Raman spectra of silicate glasses consist of three major portions: a high wavenumber region extending from 800 to 1200 cm -1 , which provides information on the T-O-T stretching mode (e.g. tetrahedron T = Si, Al), an intermediate-wavenumber region from 400 to 650 cm -1 , which gives information on the T-O-T rocking, bending mode and ring distributions, and a low wavenumber region below 250 cm -1 which provides information on tetrahedral arrangement (see [213] for more details). The high wavenumber region, also referred as Qrange, contains polarized bands that have been associated with the symmetric stretching motions of silica tetrahedra. Indeed, the short-range order of silicate glasses can be described through the abundance of the Q n species, where n is the number of bridging oxygens atoms (BO) and Q represents the four-fold coordinated cation -e.g. Si, Al (i.e. fully polymerized = Q 4 ).

A fully-polymerized silica glass (SiO 2 -Fig. 23) network has only weak high frequency bands (frequency range ~ 980-1350cm -1 ), but may be characterized by its strong asymmetric band in the low frequency region (400-500 cm -1 ) [214, 215] and from the well pronounced D 1 (~490 cm -1 ) and D 2 (~ 600 cm -1 ) "defect" lines, associated with the breathing modes of (SiO) 4 -and (SiO) 3 -rings, respectively (see SiO 2 in Fig. 23). By decreasing glass polymerization (e.g. by adding network modifiers), the main peak in the low-frequency region, increases in intensity and shifts to higher wavenumbers, whereas the Q-range increases in intensity because other bands, related to different Q species (Q 3 , Q 2 ) appear in the 800-1200 cm -1 region [213].

Figure 23 shows the unpolarized Raman spectra of some Si-rich natural glasses in comparison with a synthetic pure SiO 2 glass. For some spectra the background was subtracted since there was a strong luminescence both with the excitation lasers at 488nm and 532nm.

The shape of the Raman spectra for the natural glasses presented here fit well with those reported in literature (Figs. 23a,b) [124,132,147,216].

The spectra for the fulgurite were collected from the sample reported in Figure 13. A cross section of the specimen was cut and the Raman spectra were collected with an excitation laser at 532 nm. The spectra for the inner glassy portion (fulgurite) and for the outer part (fulgurite2) are reported in Figure 23a. Both spectra resemble that of the pure SiO 2 glass, with the typical doublet 440-490 cm -1 .

Raman spectra for obsidian samples are reported in Figure 23a. Bellot-Gurlet et al.

[147] measured, by Raman spectroscopy, obsidians from the western Mediterranean area. The authors suggest that the detailed analysis of the Raman spectra in the high wavenumber region could be used to distinguish between obsidians originating from Pantelleria and Sardinia and/or Lipari and Palmarola, and thus that Raman spectroscopy could be a complementary technique in archaeological obsidians provenance studies. The obsidian samples, from 3 different locations, shown here have Raman spectra similar to those reported by Bellot-Gurlet et al [147], with the Pantelleria one showing a strong contribution around 975 cm -1 . This feature could be ascribed to the high amount of iron in these glasses [217]. The Raman spectra for two different opals (Fig. 23b) show strong sharp peaks corresponding to the main presence of cristobalite, but also some contributions from a minor presence of tridimyte can be observed, in agreement with the observation done by Ilieva et al.

[218]. Indeed, the two sharp peaks respectively at 410 and 226 cm -1 are related to a cristobalite-type atomic arrangement, whereas the small broader contributions at ~350 and 300 cm -1 are related to a tridymite-type arrangement [218]. Hyalite (Fig. 23b) is the opal-AN form (see section 4.3) and the Raman spectrum highlights the amorphous character, since it presents much broader bands, and no sharp peaks typical of crystalline phases.

Trinitite Raman spectrum (Fig. 23b) was collected on the outer portion of the specimen reported in Figure 19. The Trinitite sample is particularly interesting because it seems to consist of the overlap of a highly polymerized glass and of a more depolymerized glass, since it presents both the characteristic features: D lines and Q 2 -Q 3 peaks.

LDG (Fig. 23b) has the same features of the silica glass Raman spectrum. Galeener et al. [219] shown how the fractional areas under the lines D 1 and D 2 vary with fictive temperature in pure SiO 2 and they estimated a fictive temperature for LDG ~ 1000 (+50) °C and a cooling rate ranging from a few minutes to a few days. However the authors in this early work stated that further work on the subject should be done, especially on the effect of radiation, relaxation time and pressure on the D lines. Champagnon et al. [216] measured

Raman spectra for some natural glasses and they show a correlation between the position of the Boson peak (at very low frequency: near 40-60 cm -1 ) and the intensities of the D defect lines. Knowledge of the relationship of the "defect" lines to the Boson peak can help understand the fictive temperature, and in turn, the thermal history.

The high spatial resolution provided by Raman (micro-) spectroscopy is well suited for the study of heterogeneous samples. Many silicic obsidian specimens contain only very small amounts of crystallized phases, even if it is unusual to find glass samples completely devoid of crystals, and often the crystalline assemblage includes Ti and Fe oxides. We show an example of Raman mapping done on a spherulitic obsidian sample on the boundary between the crystalline portion (spherulite) and the glass portion (map dimension ~ 50x30 micrometers in Fig. 24). In Figure 24 are reported the photo of an obsidian specimen, along with the microscopy image of the mapped portion and some of the Raman spectra collected ( 7discrete points) in the range 200-1000 cm -1 . All the spectra, and in particular the glass portion has a strong luminescence (likely caused by the presence of transitions elements and REE) at higher wavenumbers (above 1000cm -1 ). Moving from the spherulite to the glass part, we observed the presence of feldspar minerals (blue and violet points), but also the presence of magnetite/ilmenite, and hematite (orange, green and red points). The spectra for the glassy portion (black and cyan points) have the same contributions observed for the other obsidians.

The detection and the study of Fe-Ti oxides is very important, because these oxides are particularly susceptible to variations in the redox conditions.

Redox of natural glass and reduction during high-energy events

There are many studies (experimental and theoretical) devoted to the understanding of melt/glass redox equilibria since the proportion of each species occurring in the melt is controlled by the bulk composition, temperature, oxygen fugacity, concentration, and presence and amount of volatiles and other redox species. Zotov [206] report a review of the techniques employed to study the structure of natural glasses and of the element-specific spectroscopic methods used to investigate redox and coordination of different major, minor and trace elements in natural volcanic glasses. Most of the studies have been done on the fourth most abundant element in the Earth (Fe) since it is the only major element with more than one electronic configuration -oxidation state and spin state (e.g. see [220][221][222][223]).

The data obtained from collision of cosmic objects, or from an airburst, or from lightning (i.e. tektites, impact melts, trinitite, fulgurites) suggest that these events led to the loss of oxygen and consequently to the production of extremely reduced melt phases. Indeed, the high temperatures, speeds, and heating/cooling rates produce exceptional alterations in the involved rocks. Wilding et al. [224] determined the quench rate for distal ejecta (tektites) by calorimetric measurements and reported a cooling rate of a few degree/s at temperature around the glass transition temperature. Wasserman and Melosh [135] report similar values from simulations of simplified systems, and a "blocking temperature" when the evaporation rate of the liquid becomes so slow that there is no time for the phases to equilibrate. If this "blocking temperature" happens at a temperature higher than 2800K, O 2 gas will be lost from the liquid, leaving the remaining liquid reduced [135].

The knowledge of the fraction of reduced phases (metallic Si, or reduced iron and phosphorous) in glasses, created from such high energy events, could provide constraints for the formation temperatures, and thus to help to shed some light on the major physical and chemical processes occurring. Many early studies on tektites report Fe 3+ /Fe 2+ values between 0.02 and 0.23 (e.g. [START_REF] Fudali | The oxidation state of iron in tektite glass[END_REF][225][226][227]). In a study done by Schreiber et al. [227] the authors, by using three different methods, estimated the Fe redox ratio values in Australasian tektites and reported that iron in those glasses to be almost all as Fe 2+ . Moreover, by remelting the samples at different temperatures and under controlled reducing conditions they tried to constrain the T-fO 2 (oxygen fugacity) regimes for tektites formation. More recent studies on tektites have been carried out by using different techniques: e.g. Mössbauer, ESR (Electron Spin Resonance), XAS (X-ray Absorption Spectroscopy) (e.g. [64-66, 88, 228-230]). As already stressed, except for few early studies, all the investigations done point to Fe in tektites being highly reduced. Only recently, Giuli et al. [88,231] by studying several microtektites from the three different strewn fields, reported the occurrence of higher Fe 3+ /Fe tot ratios for some North American microtektites. Since alteration of the specimens was ruled out by further analyses (i.e. water content) a different mechanisms was proposed (interaction of melt droplets with a H 2 O-rich vapor plume). A slightly different Fe redox state was also reported for MN-type tektites. These glasses have lighter and darker layers, with the lighter ones enriched in Al and Fe with respect the darker ones. Giuli et al. [232] report that MN dark layers are slightly more oxidized respect to the light layers.

An attempt to understand the redox ratio variations for tektites has been done by collecting at high temperatures XAS data at the Fe K-edge (see [233] for details on the experimental procedure). Fe redox kinetics were studied in these highly polymerized glasses/melts from room temperature (RT) to 1680°C and the data obtained were compared to the theoretical model of Kress and Carmichael [220]. The XAS data analysis and the Fe redox estimation have been carried out according to the procedure reported in Cicconi et al. [234,235] for synthetic glasses. At room temperature the indochinite tektite shows, as expected, a

Fe 3+ /Fe ratio close to 0 (~ 0.05 ± 0.05, Fig. 25), in agreement with XAS studies done on several samples by . When heated up at temperature just above Tg (1014K), the tektite Fe redox ratio increases up to 0.55 (± 0.05) (non equilibrium conditions), and by further increasing the temperature to 1500K and maintaining it as long as needed to reach the equilibrium, the redox ratio goes up to Fe 3+ /Fetot ~ 0.7 (± 0.05) (see Fig. 25). By additionally increasing the temperature, the Fe oxidation state starts to decrease, in good agreement with the trend suggested by the theoretical model of Kress and Carmichael [220] (K&C) for such compositions at ambient pressure (see Fig. 25). Of course, being in an oxygen depleted atmosphere (reducing conditions) will move the Fe buffer to lower temperatures.

However, to reach this very high amount of reduced iron, and to preserve it even in the glass, it would be reasonable to consider that the temperatures approached values higher than 2000K and the cooling rate must have been in the order of few °/s. These observations suggest that further studies must be done in order to full understand the mechanisms occurring during these high-energetic events. Experimental data on simplified systems along with thermodynamic models could help to understand the main factors involved.

Conclusions and Future Directions

The study of natural glasses has led to many important discoveries for technological applications, like in the case of nuclear waste management, and biomimetic studies. Indeed, since dawn of humanity, humankind tried to understand first, and then reproduce natural processes, and sometimes scientists achieved to exceed what nature does. Hence we hope that this chapter has provided fresh impetus for the understanding/study of natural glasses, not only in the framework of geosciences (s.l.), but also for many practical uses, or in general to provide useful insights into processes involved in the manufacture of glasses.

The key information on natural glasses provided here offers a basis for better synergy between disciplines such as Earth Sciences and Materials Science. Indeed, the cooperation between researches from different "sectors" could provide the ideas needed for moving forward.

Figure captions

Fig. 1 -Total alkali vs. silica (TAS) diagram for several natural glasses (some of the data and references are reported in Table 1). Table 1). Modified after [START_REF] Delano | Pristine lunar glasses: Criteria, data, and implications[END_REF] and Brown and Grove (2015 and references therein). [147]. See also Table 1. When a single value is given in the two colons, it refers to the total.

Fig. 1 -

 1 Fig. 1 -Total alkali vs. silica (TAS) diagram for several natural glasses (most of the data and references are reported in Table1).

K

  -Pg = Cretaceous-Paleogene (K-Pg) boundary (~66 million years ago) KT = Cretaceous-Tertiary boundary (former name for K-Pg) Lechatelierite = shock-fused SiO 2 glass LDG = Libyan Desert Glass Mafic rocks = (e.g. basalt) magmas/rocks with ~50wt% SiO 2 , enriched in Fe, Mg and Ca, and usually dark in color. MN = Muong Nong-Type tektites MORB = Mid-Ocean Ridge Basalt Or = orthoclase -KAlSi 3 O 8 Planar deformation features (PDFs) = microscopic parallel, isotropic features in minerals that originate from elevated shock metamorphism. Pyroxenes = group of silicate minerals (e.g. diopside CaMgSi 2 O 6 ) SF = tektite/impact glass strewn fields TAS = Total-Alkalis versus Silica diagram Tektites = millimeter-to centimeter-scale, glassy particles of ballistically transported impact melt, formed by the impact of an extraterrestrial projectile. Microtektites are less than 1 mm in size. Tg = Glass Transition temperature YD = Younger Dryas is a geological period from ~ 12,900 to ~ 11,700 BP YDB = Younger Dryas boundary Ultramafic rocks = (e.g. komatiite) magmas/rocks with very low silica content (

Figure 2 -

 2 Figure 2 -Basaltic glass from French-American Mid-Ocean Undersea Study (FAMOUS) area on the Mid-Atlantic Ridge. Image copyright: D.R. Neuville.

Figure 3 -

 3 Figure 3 -Reticulite from Erebus volcano. This highly foamed sample is mainly composed of glass. Image copyright: D.R. Neuville (sample dimension 7x6x5cm).

Figure 4 -

 4 Figure 4 -Photograph of Pele's hairs along with few melt droplets (Pele´s tears). Samples from Hawaiian Islands. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 5 -

 5 Figure 5 -Bomb sample from Erebus volcano with the surface covered by olive green and black glass with a metallic lustre, and anorthosite crystals. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 6 -

 6 Figure 6 -Palagonite specimen. Image copyright: D.R. Neuville.

Figure 7 -

 7 Figure 7 -Variation of SiO 2 and FeO contents vs. TiO 2 content in lunar glasses (data in

Figure 8 -

 8 Figure 8 -Tektites specimens with the typical aerodynamic shapes and characteristic surface features. Image copyright: D.R. Neuville (square dimension=5mm)

Figure 9 -

 9 Figure 9 -Photos of some tektites, including a moldavite (length ~ 18 mm) and two indochinites (length ~ 25 mm). Image copyright: M.R. Cicconi

Figure 10 -

 10 Figure 10 -Approximate location and extension of the four strewn fields: NA (North American), IC (Ivory Coast), CE (central Europe) and Australasian (AA). The location of the known source craters are Chesapeake Bay (NA), Ries (CE) and Bosumtwi crater (IC). Image copyright: M.R. Cicconi

  published, and the readers are referred to: (e.g. Surovell et al. [106], Pinter et al. [107], Boslough et al. [108], Van Hoesel et al. [109]).The YD impact is not the only debated event, and recently, Reimold and coauthors published a detailed paper on impact controversies[110]; e.g. the Maniitsoq structure (Greenland), and the unconfirmed Khebira (Egypt), Arkenu (Libya), Bedout (Australia) craters(cf. [110]).

[

  119]. Moreover, the high concentration of Platinum-group elements [117], osmium isotope data of included dust, the presence of graphite-rich bands [120], and reduced Fe [121], are additional evidence for the impact origin. Recently, Gomez-Nubla and coauthors [115] performed several spectroscopy measurements on LDG specimens (SEM-EDS, energy-dispersive micro X-ray fluorescence, electron microprobe, and optical cathodoluminescence) in order to identify and characterize the different phases. The authors identified several mineral phases, such as coesite, cristobalite, corundum, rutile (TiO 2 polymorph), and also not-high temperature phases, such as anhydrite (CaSO 4 ), gypsum (CaSO 4 •2H 2 O), calcite, and aragonite (calcite polymorph).

Figure 11 -

 11 Figure 11 -Photo of a LDG fragment. The sample has a clear pale yellow color and the typical wind erosion surface. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 12 -

 12 Figure 12 -Some fulgurite specimens with the characteristic irregular tube shapes. Image copyright: D.R. Neuville (square dimension=5mm).

  spherical inclusions: i.e.Pasek and Block [133] report iron phosphide spherules and Ca-P-Si oxide rich grains in type II fulgurites.

Figure 13 -

 13 Figure 13 -Photographs of a fulgurite sample. The inner part is more smoothed and has a heterogeneous glassy material, whereas the outer portion is very irregular. Image copyright: M.R. Cicconi. Length of the specimen: ~5.5 cm.

Figure 14 -

 14 Figure 14 -Obsidian from Lipari (I). Image copyright: D.R. Neuville (square dimension=5mm).

Figure 15 -

 15 Figure 15 -Arrowhead made of obsidian. Image copyright: D.R. Neuville (square dimension=5mm).

  FeTiO 3 …), and in particular, obsidians have several coexisting paramagnetic, ferrimagnetic and/or superparamagnetic phases. To explain the complex magnetic behavior of obsidian, recently,Mameli et al [142] studied the magnetic and microstructural properties of 12 obsidian samples from Sardinia (I). By coupling several techniques the authors were able to discern that the presence of magnetite nanolites, dispersed into the obsidian glassy matrix, is responsible for the ferrimagnetism and superparamagnetism behaviours. Moreover, the coexistence of other two Fe-minerals could be responsible for the antiferromagnetic and paramagnetic behaviour[142].

Figure 16 -

 16 Figure 16 -TAS diagram for obsidians, perlites and pitchstones. Obsidian Data for 1) subalkalic silicic obsidians [137]; 2) from [151]; 3) from [152]; 4) from [153]and 5) from[147]. See also Table1.

Figure 17 -

 17 Figure 17 -Photograph of a pumice specimen from Glass Mountain (USA). The sample has the typical highly vesicular irregular texture. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 18 -

 18 Figure 18 -Photograph of spherulitic obsidians (length ~ 4 cm; unknown locality). Spherulites are distributed homogeneously all over the samples with size ranging from 3.0 to 8.0 mm. Image copyright: M.R. Cicconi

4. 1 -

 1 Glasses from nuclear explosion: Trinitite Detonation of nuclear weapons has created glasses, i.e. from the first atomic bomb test in Alamogorgo (Trinity test, 1945) or from the first underground nuclear explosion (Rainier test, 1957).

Figure 19 -

 19 Figure 19 -Glass sample from Trinity site, Tularosa Basin, Alamogorgo (USA). This green trinitite fragment is glassy and vesiculated and is a product of the first atomic bomb blast on July 16, 1945. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 20 -

 20 Figure 20 -Microphotography of a pseudotachylite layer (dark layer) from the Nojima fault, Japan [189]. (Rectangular dimension: 0.5mm). Image copyright: D.R. Neuville.

  tectosilicate and one pyroxene compositions (respectively, Ab = NaAlSi 3 O 8 , Or = KAlSi 3 O 8 , An = CaAl 2 Si 2 O 8 , Wo = CaSiO 3 ). Nevertheless for a constant composition, there are rapid variations in viscosity when crystals form and/or gas bubbles exsolve: e.g. the viscosity of basaltic melts varies from <10 2 Pa*s to >10 4 Pa*s for Hawaiian to Plinian eruptions, respectively ([6] and references therein).

Figure 21 -

 21 Figure 21 -Low-and high-temperature measured values of viscosity for SiO 2 , tectosilicates (Ab, An and Or) and wollastonite (Wo) melts as a function of reciprocal temperature. (a) The viscosity measurements for a rhyolitic (Rhy), an andesitic (And) and a basaltic (Bas) melt are reported as symbols. (b) The viscosity measurements for a moldavite (mol) and an Australasian

Figure 22 -

 22 Figure 22 -Tg-scaled Arrhenius representation of liquid viscosities showing Angell's strongfragile pattern. Both tektites, mol and chi, exhibit approximate linearity, and thus a nearly Arrhenian behaviour.

Figure 23a -

 23a Figure 23a -Raman spectra for some natural silicate glasses and for a pure SiO 2 glass in the range 300 -1250 cm -1 . The shape of the spectra change depending on the polymerization of the glasses, and on the presence/amount of other elements (such as Fe).

Figure 24 -

 24 Figure24-Photo and light microscopy images of an obsidian specimen and the crystalline portion (spherulite). The Raman spectra collected both in the glassy and crystalline parts are reported. Beside the glass portion it was possible to identify feldspar minerals (blue and violet points), and magnetite/ilmenite, and hematite (orange, green and red points).

Figure 25 -

 25 Figure 25 -Fe redox ratio vs T. Empty circles represent the theoretical Fe 3+ /Fe values calculate for the indochinite tektite composition by using the model of Kress and Carmichael [220], K & C). The black squares are the Fe 3+ /Fe estimated from the XAS data analysis at high temperatures, whereas the empty square represents the value at room temperature (~ 0.05 ± 0.05).

Figure 2 -

 2 Figure 2 -Basaltic glass from French-American Mid-Ocean Undersea Study (FAMOUS) area on the Mid-Atlantic Ridge. Image copyright: D.R. Neuville.

Figure 3 -Fig. 4 -

 34 Figure 3 -Reticulite from Erebus volcano. This highly foamed sample is mainly composed of glass. Image copyright: D.R. Neuville (sample dimension 7x6x5cm)..

Figure 5 -

 5 Figure 5 -Bomb sample from Erebus volcano with the surface covered by olive green and black glass with a metallic lustre, and anorthosite crystals. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 6 -

 6 Figure 6 -Palagonite specimen. Image copyright: D.R. Neuville.

Figure 7 -

 7 Figure 7 -Variation of SiO2 and FeO contents vs. TiO2 content in lunar glasses (data in

Figure 8 -Figure 9 -Figure 10 -

 8910 Figure 8 -Tektites specimens with the typical aerodynamic shapes and characteristic surface features. Image copyright: D.R. Neuville (square dimension=5mm)

Figure 11 -

 11 Figure 11 -Photo of a LDG fragment. The sample has a clear pale yellow color and the typical wind erosion surface. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 12 -

 12 Figure 12 -Some fulgurite specimens with the characteristic irregular tube shapes. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 13 -Figure 14 -Figure 15 -

 131415 Figure 13 -Photographs of a fulgurite sample. The inner part is more smoothed and has a heterogeneous glassy material, whereas the outer portion is very irregular. Length of the specimen: ~55 mm. Image copyright: M.R. Cicconi

Figure 16 -

 16 Figure 16 -TAS diagram for obsidians, perlites and pitchstones. Obsidian Data for 1) subalkalic silicic obsidians [137]; 2) from [151]; 3) from [152]; 4) from [153] and 5) from

Figure 17 -

 17 Figure 17 -Photograph of a pumice specimen from Glass Mountain (USA). The sample has the typical highly vesicular irregular texture. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 18 -Figure 19 -

 1819 Figure 18 -Photograph of spherulitic obsidians (length ~ 4 cm; unknown locality). Spherulites are distributed homogeneously all over the samples with size ranging from 3.0 to 8.0 mm. Image copyright: M.R. Cicconi Figure 19 -Glass sample from Trinity site, Tularosa Basin, Alamogorgo (USA). This green trinitite fragment is glassy and vesiculated and is a product of the first atomic bomb blast on July 16, 1945. Image copyright: D.R. Neuville (square dimension=5mm).

Figure 20 -

 20 Figure 20 -Microphotography of a pseudotachylite layer (dark layer) from the Nojima fault, Japan [189]. (Rectangular dimension: 0.5mm). Image copyright: D.R. Neuville.

Figure 21 -

 21 Figure 21 -Low-and high-temperature measured values of viscosity for SiO2, tectosilicates (Ab, An and Or) and wollastonite (Wo) melts as a function of reciprocal temperature. (a) The viscosity measurements for a rhyolitic (Rhy), an andesitic (And) and a basaltic (Bas) melt are reported as symbols. (b) The viscosity measurements for a moldavite (mol) and an Australasian tektite (chi) are reported as symbols. . Lines are only guides for the eyes. Data in Tab. 2 and from [200]. Viscosity measurements were obtained following [201].

Figure 22 -

 22 Figure 22 -Tg-scaled Arrhenius representation of liquid viscosities showing Angell's strongfragile pattern. Both tektites, mol and chi, exhibit approximate linearity, and thus an Arrhenian behaviour.

Figure 23a -

 23a Figure23a-Raman spectra for some natural silicate glasses and for a pure SiO 2 glass in the range 300 -1250 cm -1 . The shape of the spectra change depending on the polymerization of the glasses, and on the presence/amount of other elements (such as Fe).

Figure 23b -

 23b Figure 23bcontinue of Fig. 23a

Figure 24 -

 24 Figure[START_REF] Ramachandran | Chemical durability studies on basalt fibres[END_REF] -Photo and light microscopy images of an obsidian specimen and the crystalline portion (spherulite). The Raman spectra collected both in the glassy and crystalline parts are reported. Beside the glass portion it was possible to identify feldspar minerals (blue and violet points), and magnetite/ilmenite, and hematite (orange, green and red points).

Figure 25 -

 25 Figure 25 -Fe redox ratio vs T. Empty circles represent the theoretical Fe3+/Fetot values calculate for the indochinite tektite composition by using the model of Kress and Carmichael (1991, K & C). The black squares are the Fe3+/Fetot estimated from the XAS data analysis at high temperatures, whereas the empty square represents the value at room temperature (~ 0.05 ± 0.05).

  Lunar glasses differ completely from terrestrial ones by the very low content of water or other volatiles, and really high contents of MgO and FeO. The volcanic lunar glasses are mainly homogeneous spherules (50 m to 1 mm in diameter) with compositions from basic to ultrabasic (SiO 2 ~33-48 wt%, Al 2 O 3 ~4.6-9 wt%, TiO 2 ~0.26-16 wt%, MgO ~11-20 wt%,

Table

Table

  beside the silica glassy matrix, small inclusions of α-cristobalite, and iron or iron/nickel oxides. In one specimen the authors also report the presence of secondary phases (formed or incorporated to the glass matrix after the impact, most likely due to weathering)[124].

Recently,

did a detailed investigation of Darwin glasses by using Raman spectroscopy, Energy Dispersive X-Ray Fluorescence, SEM-EDS and Electron Probe Microanalysis. The authors report that the same major elements were found in all the samples they analysed, with compositions ranging from: SiO 2 = 80-90 wt.% (excluding the SiO 2 pure inclusions), Al 2 O 3 = 5-9 wt.%, FeO = 2-4 wt.%, MgO = 0.3-0.8 wt.%, K 2 O = 1.8-2.3 wt.%, CaO = 0.01-0.03 wt.% and TiO 2 = 0.35-0.6 wt.% [124]. Data from Raman spectroscopy identified,

  provided an exhaustive study of 28 obsidian specimens and results from many techniques: thermal expansion, density measurements, Vicker's hardness, chemical durability, electrical properties, Mössbauer and infrared spectroscopy studies. The mechanical properties of obsidians are characteristic and differ from other glasses, such as windows glasses. However, the authors report also that obsidian is has high chemical durability, comparable to Pyrex, and high hardness, comparable with SiO 2

glass [141]. Despite the dominant glassy matrix, obsidian specimens have several crystalline inclusions, such as Feldspars, pyroxenes, silica (and polymorphs), Fe a/o Ti, a/o Fe-Ti oxides (e.g. hematite Fe 2 O 3 , ilmenite

  Usually there are several polymineralic aggregates, such as intergrowths of quartz, feldspar and magnetite. The formation conditions of spherulitic textures and the kinetics of spherulite growth in natural silicate are still much

	debated. E.g. Castro et al. [161] determined the kinetics of spherulite growth in Island
	obsidians, and the water diffusion modeling yields spherulite growth rates of a few tenths to
	hundredths of a millimeter per day, depending on temperature [161]. Watkins et al. [162]
	found that spherulites can grow on the order of days to months at temperatures above the
	glass transition temperature. Arzilli et al. [160] by using several techniques demonstrated that
	Denton et al. [158] report a
	study on the crystalline and volatile contents in obsidians and perlites. The authors observed
	that the volatile enrichment in perlites is often accompanied by secondary crystalline phases
	growth (zeolites).

During devitrification the glass slowly crystallizes

[159]

. Figure

18

shows spherulites in black obsidians. Spherulites are confocal radial polycrystalline aggregates that commonly occur in glassy felsic materials [160]. the development of spherulites is dominated by heterogeneous nucleation and the growth can occur in a short time in water saturated trachytic melts, reaching ~400 μm diameter in a few hours [160].

Table 1 -Average composition of the major oxides (wt%) in several natural glasses.

 1

	2054
	Sample	SiO 2	TiO 2	Al 2 O 3	Cr 2 O 3	Fe 2 O 3 or § Fe 2 O 3 tot	§ FeO or FeOtot	MnO	MgO	CaO	Na 2 O	K 2 O	(*) H 2 O or H 2 O +	P 2 O 5	Reference
	Obsidian
		76.27	0.06	13.09		0.52		0.07	0.05	0.51	4.07	4.39		0
	Armenian and Caucasian	76.7 72.84 75.15	0.1 0.21 0.11	13.5 14.52 13.95		0.66 1.8 0.94		0.1 0.06 0.07	0.15 0.48 0.22	0.54 1.67 0.99	4.56 4.56 4.29	4.65 3.76 4.26		0.02 0.09 0.03	[151]
		77.37	0.08	12.64		0.59		0.06	0.04	0.48	4.18	4.56		0
		74.72	0.09	13.4		1.25		0.08	0.08	0.59	3.44	5.26		0.06
	Mediterranian	73.77	0.18	13.68		1.31		0.11	0.13	0.75	3.34	5.63		0.04	[153]
		75.05	0.13	12.97		1.17		0.08	0.11	0.57	3.34	5.51		0.04
	Monte Arci (I)	75.57	0.09	13.88		1.28		0.05		0.57	3.55	4.71
	Monte Arci (I)	74.84	0.14	14.02		1.48		0.04		0.7	3.49	4.88
	Monte Arci (I)	75.96	0.13	13.44		1.23		0.03		0.54	3.36	4.99
	Monte Arci (I)	73.68	0.3	14.33		1.75		0.03		0.85	3.3	5.31			[147]
	Lipari (I)	74.87	0.07	13.25		1.63		0.06		0.69	4.16	4.66
	Palmarola (I)	74.51	0.08	13.45		1.61		0.07		0.44	4.79	4.43
	Pantelleria (I)	71.2	0.19	7.66		8.11		0.27		0.25	6.84	3.72
		75.55	0.06	12.83			1.6	0.06	0.04	0.69	4.13	5.03	0.21	0.01
		75.36	0.06	12.83			1.59	0.06	0.04	0.72	4.16	5.02	0.16	0.01
		75.48	0.07	12.82			1.58	0.07	0.04	0.7	4.16	5.04	0.16	0
		75.57	0.06	12.86			1.6	0.06	0.04	0.69	4.16	5.04	0.14	0.02
	Lipari (I)	75.62 75.47	0.08 0.07	12.63 12.82			1.48 1.47	0.07 0.06	0.04 0.04	0.72 0.72	4.2 4.19	5.15 5.16	0.15 0.14	0.01 0.01	[152]
		75.37	0.07	12.84			1.57	0.07	0.04	0.72	4.19	5.13	0.14	0.01
		75.45	0.08	12.82			1.48	0.07	0.04	0.72	4.21	5.12	0.15	0.01
		74.42	0.08	12.77			1.5	0.07	0.04	0.72	4.2	5.18	0.14	0.01
		75.54	0.08	12.82			1.47	0.06	0.05	0.72	4.17	5.1	0.15	0.01
	Subalkalic	73.1 66.8	0.31 0.64	14.02 16.13		0.46 0.54	1.30 3.00	0.05 0.08	0.39 1.33	1.58 3.57	4.54 4.84	3.20 2.32	0.08 0.51	0.04 0.11	[137]

Notes: §

Fe oxide content is reported in literature in different ways: FeO total, or Fe 2 O 3 total. Few authors reported both the Fe oxides.

Table 2A -Composition (wt%) and some properties for tektites (chi and mol) shown in Fig. 19, 20 and 21A.

 2A

		Moldavite (mol)	Indochinite (chi)
	SiO 2	78.12	71.85
	Al 2 O 3	9.56	12.94
	TiO 2	0.32	0.83
	CaO	2.73	2.48
	MgO	2.09	1.96
	Cr 2 O 3	0.01	0.03
	MnO	0.06	0.14
	FeO	1.61	4.44
	Na 2 O	0.36	1.21
	K 2 O	3.49	2.43
	density (g/cm 3 ) (±0.0005)	2.3717	2.4277
	Tg (K)	1070	1014
	S conf (Tg)(J/molK)	13.4	11.9

The density value reported is related to the density of the material as approximated by a whole fulgurite cylinder[START_REF] Pasek | Fulgurite morphology: A classification scheme and clues to formation[END_REF]..

Acknowledgements

Some of the data here shown have been acquired at FAME beamline and we thanks the European Synchrotron Radiation Facility (Grenoble, France) for provision of synchrotron radiation facilities. The authors thank D. de Ligny and G. Henderson for the useful discussions, B. Cochain for help during HT XANES experiments, and J. Stebbins for Libyan Desert glass sample. MRC thanks E. Guillaud for pictures of natural glasses, and S. Wolf for useful discussions on biomimetic materials. DRN thanks J.C. Bouillard, Curator of the Collection de Minéraux University Pierre and Marie Curie, Paris, to provide fulgurite samples.