Michael J Heap
email: heap@unistra.fr

H Albert Gilg

Kai-Uwe Hess

Lea Mertens

Gisela Pösges

Thierry Reuschlé

K.-U Hess

Conservation and restoration of St. George's church (Nördlingen, Germany), a 15th century Gothic church built using suevite from the Ries impact crater

Keywords: suevite, conservation, porosity, permeability, uniaxial compressive strength

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Introduction

An asteroid impacted a 600-700 m thick layer of sedimentary rocks overlying a crystalline basement [START_REF] Pohl | The Ries impact crater[END_REF][START_REF] Engelhardt | Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany-A review[END_REF][START_REF] Stöffler | Ries crater and suevite revisited-Observations and modeling Part I: Observations[END_REF] in southwest Germany ca. 15 Ma [START_REF] Rocholl | Comment on "A high-precision 40 Ar/39 Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events[END_REF] (Fig. 1a). In the aftermath of the impact, "clouds" of hot debris and gas dispersed radially [START_REF] Siegert | Density current origin of a melt-bearing impact ejecta blanket (Ries suevite, Germany)[END_REF] forming a subcircular blanket of poorly sorted clastic material that subsequently welded to form a rock called suevite. Two layers of suevite exist: (1) "crater suevite", which is up to 300 m thick and covered by Miocene lake sediments and (2) "outer suevite", which is only up to 25 m thick and outcrops up to 22 km from the centre of the structure [START_REF] Stöffler | Ries crater and suevite revisited-Observations and modeling Part I: Observations[END_REF]. The Ries impact crater, a double-layer rampart crater [START_REF] Sturm | The Ries impact, a double-layer rampart crater on Earth[END_REF], has an inner-and outer-ring diameter of ca. 12 and 24 km, respectively [START_REF] Wünnemann | Is Ries crater typical for its size? An analysis based upon old and new geophysical data and numerical modeling[END_REF]Fig. 1b). The suevite contains centimetric-sized clasts of impact melt glass (aerodynamically shaped bombs and angular fragments; [START_REF] Stähle | Impact glasses from the suevite of the Nördlinger Ries[END_REF][START_REF] Osinski | Impact glasses in fallout suevites from the Ries impact structure, Germany: An analytical SEM study[END_REF] and crystalline and sedimentary rocks [START_REF] Graup | Terrestrial chondrules, glass spherules and accretionary lapilli from the suevite, Ries Crater, Germany[END_REF][START_REF] Engelhardt | Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany-A review[END_REF][START_REF] Engelhardt | Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts[END_REF].

These clasts are hosted within a matrix of clays (and sometimes zeolites) that formed as a result of post-deposition hydrothermal alteration, impact melt glass, rock and mineral fragments, and crystallites [START_REF] Newsom | Fluidization and hydrothermal alteration of the suevite deposit at the Ries crater, West Germany, and implications for Mars[END_REF][START_REF] Engelhardt | Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany-A review[END_REF][START_REF] Engelhardt | Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts[END_REF][START_REF] Osinski | Hydrothermal activity associated with the Ries impact event, Germany[END_REF]Osinski et al., 2010;[START_REF] Muttik | Stable isotope composition of smectite in suevites at the Ries crater, Germany: Implications for hydrous alteration of impactites[END_REF][START_REF] Muttik | Boron isotope composition of secondary smectite in suevites at the Ries crater, Germany: boron fractionation in weathering and hydrothermal processes[END_REF][START_REF] Sapers | Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany[END_REF].

Coesite is also present [START_REF] Stöffler | Zones of impact metamorphism in the crystalline rocks of the Nördlinger Ries crater[END_REF][START_REF] Stähle | Shock-induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the Ries impact crater (Germany)[END_REF], the mineral that confirmed the impact origin of these deposits [START_REF] Shoemaker | New evidence for the impact origin of the Ries Basin, Bavaria, Germany[END_REF]. Diamond and other high-pressure minerals have also been identified [START_REF] Stöffler | Ries crater and suevite revisited-Observations and modeling Part I: Observations[END_REF].

The town of Nördlingen (Fig. 1) was founded within the impact crater and suevite was used in its construction. Parts of the city hall, the city fortifications (walls and gates), and the impressive 15 th century Gothic church (St. George's church; Fig. 1c) were constructed using suevite. A visual inspection of St. George's church, and other buildings constructed using suevite, highlights its strong weathering susceptibility. There is evidence of alveolisation (detachment of clasts/fragments), scaling, exfoliation, and disaggregation (Fig. 2a) and, as a result, many of the suevite dimension stones of St. George's church have been recently (starting from the 1970s) replaced with blocks of synthetic and natural suevite and a white sandstone (Fig. 2b and2c).

Laboratory experiments on clay-bearing sandstones [START_REF] Dyke | Evaluating the strength and deformability of sandstones[END_REF][START_REF] Hawkins | Sensitivity of sandstone strength and deformability to changes in moisture content[END_REF][START_REF] Shakoor | Relationship between unconfined compressive strength and degree of saturation for selected sandstones[END_REF][START_REF] Wasantha | Water-weakening behavior of Hawkesbury sandstone in brittle regime[END_REF][START_REF] Verstrynge | Multi-scale analysis on the influence of moisture on the mechanical behavior of ferruginous sandstone[END_REF]Heap et al., 2018a[START_REF] Heap | Rock mass strength and elastic modulus of the Buntsandstein: an important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben[END_REF] and tuffs (e.g., [START_REF] Zhu | Micromechanics of brittle faulting and cataclastic flow in Alban Hills tuff[END_REF]Heap et al., 2018b) suggest that clays may promote rock weakening in the presence of water. The metric "water-weakening", the ratio of the wet to dry strength of a material, is often used to describe this affect, where low values (close to zero) indicate a strong water-weakening effect and values at or close to unity indicate that there is little or no water-weakening. The high clay content of the suevite [START_REF] Newsom | Fluidization and hydrothermal alteration of the suevite deposit at the Ries crater, West Germany, and implications for Mars[END_REF][START_REF] Osinski | Hydrothermal activity associated with the Ries impact event, Germany[END_REF][START_REF] Muttik | Stable isotope composition of smectite in suevites at the Ries crater, Germany: Implications for hydrous alteration of impactites[END_REF][START_REF] Muttik | Boron isotope composition of secondary smectite in suevites at the Ries crater, Germany: boron fractionation in weathering and hydrothermal processes[END_REF][START_REF] Sapers | Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany[END_REF] may therefore help explain its high susceptibility to weathering (Fig. 2).

Research aim

Our objective is twofold. First, to inform on the suitability of the replacement stones [START_REF] Přikryl | Understanding the earth scientist's role in the pre-restoration research of monuments: an overview[END_REF], we assess the physical properties of the suevite originally used in the construction of the church and the replacement suevite and sandstone. Second, to better understand the observed weathering susceptibility (Fig. 2), we assess the water-weakening of the original suevite and the replacement stones.

The two suevite blocks used in this study were sourced from the quarries at Altenbürg (thought to be the quarry from where the rock for the church was sourced; Fig. 1b) and Seelbronn (the quarry from where the suevite used to restore the church is sourced; Fig. 1b; see also [START_REF] Poschlod | Erfassung historischer Naturwerksteinvorkommen als Grundlage für deren umweltverträgliche Reaktivierung zwecks Restaurierung national bedeutender Kulturgüter in Bayern[END_REF]. Although the Altenbürg quarry is thought to be the source site for the suevites used in the original construction, it cannot be definitively ruled out that some blocks were sourced from other quarries.

Nevertheless, we consider that the block we collected from Altenbürg well represents the colour and therefore alteration of the blocks used in the original construction. The sandstone used in the restoration of the church is Schönbrunn sandstone (from Schönbrunn-Hermannsberg quarry in Bavaria, Germany).

Both suevites are poorly sorted clastic rocks that contain millimetric and centimetric-sized clasts of glass (aerodynamically shaped bombs and angular fragments), crystalline rocks (Variscan granites and pre-Variscan gneisses and amphibolites), and sedimentary rocks within a fine-grained matrix (von [START_REF] Engelhardt | Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany-A review[END_REF][START_REF] Engelhardt | Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts[END_REF][START_REF] Stöffler | Ries crater and suevite revisited-Observations and modeling Part I: Observations[END_REF]. Macroscopically, the only observable difference between the blocks is that the suevite blocks from Seelbronn and Altenbürg are bluegrey and yellow-green in colour, respectively (Figs. 2 and3). The replacement sandstone is very well sorted and contains faint bedding-parallel laminations.

A total of 62 cylindrical samples were cored from the three blocks: 20 samples of Schönbrunn sandstone (SBS), 20 samples of suevite from Seelbronn (SEEL), and 22 samples of suevite from Altenbürg (ALT). The sandstone samples were cored to a diameter of 20 mm and cut and precision-ground to a nominal length of 40 mm. Due to the heterogeneity of the suevite blocks, the suevite samples were prepared to a diameter of 25 mm and cut and precision-ground to a nominal length of 60 mm. The suevite samples were cored so as to avoid centimetric-sized clasts. Although these sample dimensions do not conform to standard guidelines, such as those outlined by the International Society for Rock Mechanics and Rock Engineering (ISRM; [START_REF] Ulusay | The ISRM suggested methods for rock characterization, testing and monitoring[END_REF], we highlight that (1) the grain size of our materials is significantly smaller than our sample diameters and (2) a recent study has shown that the uniaxial compressive strength (UCS) of small (diameter = 8-9 mm) samples was very similar to that determined from standard-sized samples [START_REF] Van Stappen | Uniaxial compressive strength measurements of limestone plugs and cores: a size comparison and X-ray CT study[END_REF].

The connected porosity of each sample was then determined using the skeletal (connected) volume of the sample given by a helium pycnometer and the bulk volume calculated using the sample dimensions. The permeability of select samples was measured using a benchtop nitrogen-gas permeameter (see [START_REF] Farquharson | Evidence for the development of permeability anisotropy in lava domes and volcanic conduits[END_REF][START_REF] Heap | Exploring the scale-dependent permeability of fractured andesite[END_REF]. Permeability measurements were performed at room temperature under a confining pressure of 1 MPa (for more information on such measurements, see [START_REF] Heap | Microstructural and petrophysical properties of the Permo-Triassic sandstones (Buntsandstein) from the Soultz-sous-Forêts geothermal site (France)[END_REF].

Finally, the samples were deformed in a uniaxial loadframe at a strain rate of 1.0 × 10 -5 s -1 until macroscopic failure. The UCS of a material is a commonly used strength metric in engineering geoscience. Half of each sample suite was deformed "wet" (vacuum-saturated in deionised water and deformed in a water bath) and the other half deformed "dry" (dried in a vacuum-oven for at least 48 hours prior to deformation).

The water saturation procedure for the samples deformed wet consisted of two steps.

First, the vacuum-dried samples were placed inside a belljar that was then evacuated for at least 12 hours. Second, degassed deionised water was introduced into the bell jar whilst under vacuum. This procedure ensured that the void space within the rock was completely water-saturated.

The mineral content of the studied materials was quantified using X-ray powder diffraction (XRPD). Crushed sample pieces were ground in isopropanol with a McCrone micronizing mill using agate cylinders, and 10 wt.% ZnO was added (internal standard). The analyses were performed on side-loaded powder mounts using a PW 1800 X-ray diffractometer (Cu-Ka, 40 kV, 30 mA, automatic divergence slit, irradiated length 10 mm, 2-70° 2q, step width 0.02°, counting time 1 s per step, graphite monochromator). The mineral phases were quantified using the Rietveld program BGMN [START_REF] Bergmann | BGMN-a new fundamental parameters based Rietveld program for laboratory X-ray sources, its use in quantitative analysis and structure investigations[END_REF]. To identify the clay minerals, we also separated < 2 µm fractions by gravitational settling and prepared oriented mounts that were X-rayed in an air-dried and ethylene-glycolated state. The carbonate contents of the suevite samples were additionally quantified using a Hecatech EURO elemental analyser.

To complement our XRPD analyses, thermo-gravimetric (TG) analyses were performed using a Netzsch STA 449 C thermobalance apparatus. Powdered samples were heated in an argon atmosphere at a rate of 10 °C/min up to a temperature of 1000 °C and then cooled back to room temperature at the same rate.

A thin section (~30 µm-thick) of each the suevite blocks was prepared and their microstructure was examined using a TESCAN VEGA II XMU scanning electron microscope (SEM) (using a beam voltage of 150 kW and a working distance of 23 mm).

Minerals were identified during our SEM analyses using energy-dispersive X-ray spectroscopy.

Results

Mineral content and microstructure

Schönbrunn sandstone is a feldspathic sandstone that contains small quantities of muscovite and chlorite (Table 1). The suevites from Seelbronn and Altenbürg are both principally composed of amorphous phases, smectite, and plagioclase, with minor quantities of quartz, coesite, K-feldspar, calcite, biotite, and hematite (Table 1). The only noteworthy differences are that Seelbronn suevite contains more plagioclase than Altenbürg suevite (27 and 18 wt.%, respectively; Table 1) and that Altenbürg suevite contains more calcite (13 ± 1 and 2.1 ± 0.2 wt.%, respectively; Table 1). The calcite content of both suevite samples was determined by the elemental analyser to be 14.0 ± 0.1 and 2.11 ± 0.06 wt.%, respectively, confirming the results of the XRPD analyses.

The mineralogical composition of our suevites is in agreement with previous studies [START_REF] Engelhardt | Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts[END_REF][START_REF] Osinski | Hydrothermal activity associated with the Ries impact event, Germany[END_REF][START_REF] Sapers | Evidence for a spatially extensive hydrothermal system at the Ries impact structure, Germany[END_REF].

The results of the TG analysis (Fig. 4a) corroborate our XRPD data. The mass loss seen at ~700-750 °C in both suevites, associated with the decomposition of calcite [START_REF] Samtani | Comparison of dolomite decomposition kinetics with related carbonates and the effect of procedural variables on its kinetic parameters[END_REF], is much more pronounced for the Altenbürg suevite than for the Seelbronn suevite. The mass loss < 700 °C in both suevites is associated with the dehydration (~100-150 °C) and dehydroxylation (~400-600 °C) of smectite [START_REF] Girgis | Dehydration kinetics of some smectites: a thermogravimetric study[END_REF], abundant in these materials (Table 1). The mass loss in Schönbrunn sandstone between ~500-550 °C is associated with the dehydroxylation of chlorite [START_REF] Ross | Experimental alteration of a chlorite into a regularly interstratified chlorite-vermiculite by chemical oxidation[END_REF].

Backscattered SEM images of the thin sections prepared from the two suevite blocks are provided in Fig. 3. The suevite blocks are microstructurally complex, containing poorly sorted angular fragments (of quartz, calcite, K-feldspar, plagioclase) within a fine-grained matrix. The images also show that both suevites are pervasively microcracked.

Petrophysical data

The average connected porosities of Altenbürg suevite, Seelbronn suevite, and Schönbrunn sandstone are 0.238, 0.245, and 0.163, respectively (Table 2). The average permeability of both suevites is ~2 × 10 -15 m 2 , and the permeability of a sample of Schönbrunn sandstone was measured to be 4.69 × 10 -16 m 2 (Table 2).

Our mechanical data show that the suevite from the Seelbronn quarry is stronger than that from Altenbürg, and that the sandstone is much stronger (about twice as strong) than both suevites (Fig. 4b; Table 2). The average dry and wet strengths of the suevites from Seelbronn and Altenbürg are 34.2 (standard deviation, s. d., 11.2 MPa) and 29.2 (s. d. 5.8 MPa) MPa and 24.2 (s. d. 6.2 MPa) and 16.3 (s. d. 4.1 MPa) MPa, respectively (Fig. 4b). We consider the difference in strength between samples cored from the same block (standard deviations shown in Fig. 4b) to be a result of natural sample-to-sample variability. The average dry and wet strength of the sandstone is 59.3 (s. d. 2.4 MPa) and 30.6 (s. d. 0.5 MPa) MPa, respectively. The ratio of wet to dry strength is 0.71, 0.56, and 0.52, respectively, for Seelbronn suevite, Altenbürg suevite, and Schönbrunn sandstone.

Discussion

Weathering of the suevite at St. George's church

The study of [START_REF] Heap | Rock mass strength and elastic modulus of the Buntsandstein: an important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben[END_REF] compiles the available data for water-weakening in sandstone and shows that the ratio of wet to dry strength in sandstone decreases as clay content is increased. The water-weakening of sandstone was considered by these authors to be the result of a reduction in fracture surface free energy in the presence of water and that, due to the ability of clays to adsorb water, higher clay contents ensured that more water was adsorbed on the surface of growing microcracks. We similarly conclude here that the presence of clays within the rock matrix (~25 wt.%; Table 1) is largely responsible for the observed water-weakening in the studied suevites. We additionally conclude that the lower ratio of wet to dry strength in the Altenbürg suevite, compared to the Seelbronn suevite, is a result of the slightly higher wt.% of smectite (Table 1) and the higher wt.% of calcite (Table 1), another mineral considered to weaken in the presence of water [START_REF] Baud | Compaction and failure in high porosity carbonates: Mechanical data and microstructural observations[END_REF][START_REF] Baud | Impact of stylolites on the mechanical strength of limestone[END_REF]) (although we anticipate that clay contributes more to the observed water-weakening than calcite). Due to the absence of clay, the very low ratio of wet to dry strength of 0.52 for Schönbrunn sandstone is unexpected (Fig. 4c). Possible reasons for the observed water-weakening include: (1) the presence of minor (4 wt.%; Table 1) chlorite (a hydrated phyllosilicate mineral), (2) a reduction in the fracture toughness of feldspar, which is relatively abundant (35 wt.%; Table 1), and (3) a decrease in capillary pressure or an increase in pore pressure (e.g., [START_REF] Verstrynge | Multi-scale analysis on the influence of moisture on the mechanical behavior of ferruginous sandstone[END_REF]; however, we highlight that a permeability of ~5 × 10 -16 m 2 (Table 2) is likely high enough to ensure drained deformation. More data are required to forward a definitive reason for the high water-weakening observed in Schönbrunn sandstone.

The suevite from Seelbronn has a higher ratio of wet to dry strength than Altenbürg suevite (0.71 and 0.56, respectively; Fig. 4b; Table 2) and could help explain the observed weathering susceptibility (Fig. 2). Some of the outer walls however, such as the wall on the east side, are noticeably less weathered and likely reflect variations in stone alteration and environmental factors. Variation in the alteration of the stones used is not only visible by the different colours of the blocks (Fig. 2), but also expected due the preferential selection of medium-sized (typically 2-3 m in size) blocks ("bollen"), regardless of their degree of alteration, by medieval stonemasons. In the absence of modern cutting tools, these 2-3 m-sized blocks were easier to manage for medieval stonemasons. It is possible therefore that the blocks forming the east side of the church were sourced from a part of the quarry that was less altered (or even that they were sourced, or partly sourced, from another quarry). Nowadays, cuboids are cut from larger bollen (as large as 5-10 m), allowing stonemasons to assess the extent of alteration and therefore stone quality. Environmental factors could have also influenced the variation in weathering seen on different church faces. Swelling clays (Table 1) and micropores (Fig. 3), for example, can lead to building stone deterioration during wetting/drying cycles [START_REF] Wangler | Clay swelling mechanism in clay-bearing sandstones[END_REF]Wrangler et al., 2011;[START_REF] Ruedrich | Moisture expansion as a deterioration factor for sandstone used in buildings[END_REF][START_REF] Colas | Influence of the clay coating properties on the dilation behavior of sandstones[END_REF]. In the case of St. George's church, it is the south-facing side that will, on average, receive the most sunlight and is therefore most prone to solarinduced rock weathering [START_REF] Eppes | Deciphering the role of solar-induced thermal stresses in rock weathering[END_REF]. Since the prevailing wind direction is from west, as deduced from yearly weather reports for Nördlingen, it is the west-facing side that should receive more wind-driven rainfall. Freeze-thaw damage (average minimum temperatures are below 0 °C in the winter months) should also affect the sides of the church that receive the most precipitation and the least sunlight. We note that the tensile strength of materials, often invoked as the principal metric in, for example, freeze-thaw weathering [START_REF] Nicholson | Physical deterioration of sedimentary rocks subjected to experimental freeze-thaw weathering[END_REF], is typically a tenth or a twelfth of the UCS [START_REF] Paterson | Experimental rock deformation-the brittle field[END_REF], the strength metric reported herein.

5.2 The use of replacement stones at St. George's church Some of the original suevite blocks forming St. George's church (Fig. 1c) have been recently replaced with synthetic and natural suevite and sandstone (Fig. 2). It is recommended that damaged or weathered stones be replaced by new stones sourced from the same quarry or, if this is not possible, using stone with similar petrophysical properties is strongly advised [START_REF] Přikryl | Understanding the earth scientist's role in the pre-restoration research of monuments: an overview[END_REF]. It is further recommended, from an aesthetic and cultural point of view, that the replacement stones have a similar appearance (colour and texture) to the original stones [START_REF] Přikryl | Understanding the earth scientist's role in the pre-restoration research of monuments: an overview[END_REF]. To this end, there are many studies that have investigated the physical properties (e.g., porosity, saturation and capillary absorption coefficients, elastic wave velocity, compressive and flexural strengths) of replacement stones for monuments and buildings of historical significance [START_REF] Nijland | Evaluation of three Italian tuffs (Neapolitan Yellow Tuff, Tufo Romano and Tufo Etrusco) as compatible replacement stone for Römer tuff in Dutch built cultural heritage[END_REF][START_REF] Graue | Quality assessment of replacement stones for the Cologne Cathedral: mineralogical and petrophysical requirements[END_REF][START_REF] Quist | Replacement of Eocene white sandy limestone in historical buildings: over 100 years of practice in the Netherlands[END_REF][START_REF] De Kock | Replacement stones for Lede stone in Belgian historical monuments[END_REF][START_REF] De Kock | Characterization, performance and replacement stone compatibility of building stone in the 12th century tower of Dudzele (Belgium)[END_REF].

Our petrophysical assessment shows that the Schönbrunn sandstone is less porous, about an order of magnitude less permeable, and more than twice as strong (in dry conditions; Fig. 4b) than the original suevite used in the construction of the church (Table 2). We also note that the Schönbrunn sandstone is very different in appearance to the original suevite (Fig. 2). By contrast, the physical properties and appearance of the original suevite and the natural suevite used in the restoration of the church are very similar (Table 2). Based on our petrophysical analyses, and a visual assessment of the appearance of the stones, we recommend, where possible, that the original damaged or weathered suevite stones be replaced with natural suevite from the currently active Seelbronn quarry.

Conclusions

We provide herein experimental data to better understand (1) the observed weathering susceptibility of suevite and (2) the suitability of the replacement stones used in the restoration of St. George's church in Nördlingen. We find that the strength of the suevite originally used to build the church is reduced by about half when watersaturated, considered here as a result of its high smectite content. This helps explain the prevalent weathering of the stones forming the outer walls of the church. Due to its low ratio of wet to dry strength, large differences between its physical properties and those of the original suevite dimension stone, and visual differences, Schönbrunn sandstone is not considered here as an ideal replacement stone. By contrast, the suevite replacement stone from the quarry at Seelbronn is more resistant to weathering when water-saturated and has a similar appearance and similar physical properties to the original suevite. We therefore recommend that, where possible, natural suevite stones be used to replace the weathered blocks of original suevite at St. George's church and at buildings built using suevite within the area. George's church (Nördlingen, Germany). Wet-vacuum-saturated in deionised water (see text for details). Dry-dried in a vacuum oven at 40 °C for at least 48 h. UCSwet-wet uniaxial compressive strength.

UCSdry-dry uniaxial compressive strength.

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig. 2. (a) Photograph of the exterior wall of St. George's church (Nördlingen,

Fig. 3 .

 3 Fig. 3. (a) Photograph of cylindrical sample of Seelbronn suevite alongside a

Fig. 4 .

 4 Fig. 4. (a) Relative mass as a function of temperature, measured using a Netzsch STA

Table 1 .

 1 Quantitative bulk mineralogical composition, determined using X-ray powder diffraction (XRPD), for the three materials used in this study. The error associated with the estimation of the content of amorphous phases is a conservative estimate (it has not been calculated).

	Mineral	Altenbürg suevite	Seelbronn suevite	Schönbrunn
		(values in wt.%)	(values in wt.%)	sandstone (values in
				wt.%)
	Quartz	9 ± 1	10 ± 1	57 ± 2
	Coesite	1.7 ± 0.3	1.4 ± 0.3	-
	K-feldspar	3.1 ± 0.5	1.7 ± 0.5	18 ± 2
	Plagioclase	18 ± 2	27 ± 3	17 ± 2
	Calcite	13 ± 1	2.1 ± 0.2	-
	Smectite	25 ± 3	23 ± 3	-
	Muscovite	-	-	4 ± 1
	Chlorite	-	-	4 ± 1
	Biotite	2.0 ± 0.5	1.9 ± 0.5	-
	Hematite	0.2 ± 0.1	0.2 ± 0.1	-
	Amorphous phases	28 ± 5	33 ± 5	-

Table 2 .

 2 Experimental summary. SBS-Schönbrunn sandstone. ALT-Altenbürg

Acknowledgements

The first author acknowledges a Prix d'Espoirs grant, awarded by the University of Strasbourg. We warmly thank Olivier Lengliné, who chauffeured the first author to and from Nördlingen. We thank Bertrand Renaudié, Alexandra Kushnir, Matthias Wittner, Werner Häusler, and Adrian Jones. The constructive comments of the associate editor and two anonymous reviewers helped improve this manuscript.