Robert Collongues

Bernadette Bensaude-Vincent

Pierre Teissier

Pierre Teissier Robert

Collongues 2008 Halshs

Keywords: state chemistry, materials science, high temperature devices, metal oxides crystalgrowth

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Chaudron and the LCTC.

After World War II, Chaudron was a major figure in the French chemical community, both as a university professor and an industrial adviser. Chaudron had been trained in metallurgy and inorganic chemistry in Le Chatelier's laboratory from 1913 to 1921. In 1939 he became head of the Laboratoire central des traitements chimiques (LCTC, Central laboratory for chemical treatments), which had been founded two years before by Georges Urbain in Vitry-sur-Seine, a southern suburb of Paris. Between 1950 and1962 Chaudron was the director of the most famous French chemical engineering school, the École nationale supérieure de chimie de Paris (ENSCP; National superior school of chemistry at Paris), which allowed him to recruit students. From 1951 on, he also worked as a techno-scientific adviser for the Commissariat à l'énergie atomique (CEA, Atomic Energy Commission) at Saclay. In 1954 he was elected to the French Académie des sciences in the chemistry section and became president of the Académie for the year 1971-1972. In 1947, when Collongues joined Chaudron's laboratory, the LCTC was dedicated to the synthesis and study of metals, metallic alloys, and inorganic solid compounds, especially metal oxides. One major feature of this research center was the hybridization of solid-state physics and physical chemistry methods used to study crystal structures-particularly X-ray diffraction-with metallurgical methods used to characterize inorganic compounds, particularly thermal analysis and magnetism measurements. Chaudron helped to popularize among chemists the notion of "solid solution" commonly used by metallurgists. He thus promoted an interdisciplinary style of research without claiming that he was founding a new discipline.

A second feature of Chaudron's laboratory was its connections with industrial companies. In a period when French scientific research was mostly supported by a state agency created in the 1930s, the Centre national de la recherche scientifique (CNRS, National Center for Scientific Research), which encouraged fundamental research, Chaudron was eager to solve technical issues encountered in steel industries, aeronautics, mechanics, and refractory materials companies. In return he received money from these companies, although the LCTC was also the recipient of generous state funding from the CNRS.

Finally, as one of the leaders of the French metallurgy community, Chaudron had international relations with European metallurgists and solid-state physicists. He met many times with Nevill Mott from the University of Bristol in England. He was in touch with the solid-state reactions community, especially the Swedish school of thought headed by Johan Hedvall. Through inorganic chemistry, he was also connected with European chemists and organized a famous international CNRS conference in Paris during 1948 on chemical "reactions in solid state". Thus, Chaudron's laboratory was one of the rare places in France that intimately combined academic and applied researches and was open to foreign influences.

Early Career.

Collongues was assigned a rather fundamental subject for his PhD, dealing with defects in iron monoxide. This research lasted for seven years, the usual lapse of time for a PhD project in France at that time. In 1954 he defended his thesis, titled "Contribution à l'étude des propriétés et des modes de décomposition de la phase protoxyde de fer" (Contribution to the Study of Properties and the Decomposing Ways of the Iron Monoxide Phase). Iron monoxide was known to be a non-stoichiometric compound: instead of following the FeO stoichiometry, its general formula was Fe 1-x O, with 0<x<0.1. Below 570°C, the protoxide is decomposed and gives metallic iron and magnetite:

4 FeO → Fe + Fe 3 O 4 (I)
Collongues's aims were threefold: (1) understanding the mechanisms of (I) by analysis of the kinetics of the reaction;

(2) drawing a parallel between the iron monoxide and the steel phase diagrams in order to extend the definition of "solid solution" from intermetallic to ionic compounds; and (3) understanding the physical properties of the solid solution in terms of structure defects. Up to the 1960s, the metal oxides were usually prepared as solid polycrystalline powders, that is, ceramics made up of many grains randomly packed together.

Each grain was a single crystal with a typical size of one micrometer or less. Using metallography (metallurgical optical reflection microscopy), Collongues observed the micrometric domains and growing surfaces, and with X-ray diffraction (the Debye-Scherrer powder method) he determined the atomic structure of polycrystals. The same year he organized an international conference entitled "Les mécanismes de la fusion et de solidification" (Mechanisms of fusion and solidification).

Solid State Ionics.

In 1972 Collongues left the niche where he had "grown up" professionally to create his own laboratory, the Laboratoire de chimie appliquée de l'état solide (Laboratory of chemistry applied to the solid state), established within the ENSCP in Paris, where Bénard had succeeded Chaudron as a director. This institution, located in the Quartier Latin not far from the Sorbonne and other research centers, provided better opportunities for creating links and recruiting students. This strategic move was synchronized with a shift of research interests from refractory to electrical materials, prompted by one single and very promising material: beta-alumina. This ordinary ternary oxide was an inexpensive material largely available in the form (Al 2 O 3 ) 11 (Na 2 O) x , the structure of which had been determined by X-ray diffraction in the 1930s.

During the 1960s it was of interest only to a few crystallographers and ceramists because its good refractory properties qualified it for high temperature furnaces. In 1962, two members of Collongues's group, Jeanine Théry and Daniel Briançon, identified and described a new form of β-alumina, named β''-alumina. The discovery was hardly noticed until suddenly, in 1967, three scientists from the Ford Motor Company at Dearborn, Michigan, reinvented β-alumina. Their two first articles revealed that β-alumina's structural peculiarities led to a high ionic conductivity and that, consequently, β-alumina could be used to make sodium-sulfur batteries for electrical vehicles. This new technology was quite the opposite of the traditional liquid batteries since the electrodes (sodium and sulfur) had to be liquid whereas the electrolyte were solid. Furthermore, in order to liquefy sodium and sulfur and reach high enough conductivity, the cell had to work at high temperatures (350-400°C). βalumina and other compounds of its family first appeared to be ideal for solid-state electrolytes because they were high ionic conductivity refractory materials. Despite technological problems (reactivity of liquid sodium at 400°C, electrodes' leakage), solid-state batteries have some distinct advantages over traditional electrolyte: a longer life-time; an easier way of miniaturizing, for example in a thin-film form; a usefulness both in low-pover and high-energy forms; a combination of two functions: separating the electrodes and conducting the ions.

When the energy storage issue became crucial in the early 1970s during the oil crisis, β-alumina became a star material, intensely investigated by hundreds of chemists and physicists from both industry and academic laboratories from mainly the US and Europe. It theories. In the series of eight international conferences on fast ion transportation that took place from 1972 to 1992, Collongues and his group always had significant results to present.

However in the mid-1980s, without having reached an commercial application, the intense exploratory activity on β-alumina came to an end for several reasons: the concern of the companies and states for energy storage declined after the two oil crises, some technical problems still appeared insuperable for application, new high ionic conductors became more promising than β-alumina…. For a few years, Collongues's group went hunting other families of ionic solid conductors, such as the NASICON (Na superionic conductor) family, but it was a swan's song.

Optical Materials.

The group had to move on to a new research topic. An opportunity had opened up in 1979 when a team in the laboratory observed that a laser effect could be obtained with a family of compounds produced by inserting rare earths inside a β-alumina single crystal matrix. It was labeled LNA (Nd-doped lanthanum-hexaluminate) which chemical composition was:

  even prompted the emergence of a new subdiscipline named solid-state ionics dealing with many new "superionic" conducting materials, as it provided a model-material for developing methods of investigation and understanding the physical laws of high ionic conductivity. In the US, Whittingham and Huggins (Stanford University), William Roth, Bruce Dunn (General Electrics) gave particularly striking contributions to the field. Since Collongues's group already had an expertise in β-alumina, big French companies like Compagnie générale d'electricité (later renamed Alcatel), Renault and Electricité de France contracted the group to study its ionic conductivity. New spectroscopic and crystallographic techniques had to be used to detect atomic movements. Therefore, Collongues started a fruitful partnership with solid-state physics and spectroscopy laboratories, the members of his group providing their know-how in the synthesis and the structural characterizations of β-alumina single crystals and the physicists performing more technical characterizations and dealing with physical

  LnMgAl 11 O 19 (Ln = La 1-x , Nd x [La = Lanthanum; Nd = Neodymium]). The discovery of its potential laser effect resulted from the use of Electron Paramagnetic Resonance (EPR), a technique introduced in the laboratory in 1972 by a newcomer, Jacques Livage, on his return from a postdoctoral stay at Oxford University in England. For optical materials Collongues let the scientific leadership to others, especially Daniel Vivien, because he did not feel comfortable enough with the instrumentation (EPR) and related theories. Nevertheless he supervised the transition from ionic conductors to optical materials as a laboratory manager in charge of research strategies. He did not hesitate to raise funds from the French Ministère de la défense (Ministry of defense) as well as from medical companies, he bought new equipments and favored the shift toward quantum chemistry. In the last few years of his directorship, from 1988 to 1993, Collongues worked out a smooth transition in which he allowed more and more autonomy to Vivien, who scientifically was at the heart of the laser project and who became his successor at the head of the laboratory.The French Research Style.Collongues's career exemplifies the French way of doing materials research. In the 1960s there was a strong governmental incentive in the United States to create interdisciplinary laboratories investigating all kinds of materials from metals to ceramics to semiconductors and intimately combining science and engineering, but in France the shift from metallurgy to new materials was carried out by a few individuals working in CNRS laboratories under disciplinary labels such as chemistry. A new discipline named solid-state chemistry that embraced all inorganic solid compounds emerged out of the mainstream of inorganic chemistry at the intersection of metallurgy, ceramics, glass materials, and high temperature production. Interdisciplinarity and collaboration between physics and chemistry were the key words. However, most of Collongues's investigations did not require quantum mechanics (except for optical materials). In keeping with a long tradition of applied chemistry initiated by Le Chatelier, Collongues developed physical techniques to study materials structures, and he often used industrial opportunities to start new research programs. Over his long career, he investigated a wide range of materials: single crystal-growth in the 1960s, solid-state ionics during the 1970s, and optical materials after 1980. Each research cycle reused a part of the expertise acquired during the previous one, while new techniques were introduced in the group by the newcomers. Thus, Collongues exemplifies the strengths and weaknesses of the French research system. While the CNRS secured job positions and research funds, his group could also rely on industrial contracts but was not encouraged to patent. Researchers were not prone to mobility and tended to stay in the laboratory where they started their career. Thus through teaching and supervising from one generation to the other, they gradually formed a local research school. According to his former students, Collongues was an outstanding teacher, always including new research results in his courses and engaging in discussions with his students. In the laboratory, he was extremely demanding and very strict about clean and wellcontrolled syntheses, but open-minded enough to accept proposals from his collaborators even if he did not fully understand them. Among his seventy former PhD students, half went into industrial careers. A few of them stayed in the group, whereas others were encouraged to create new independent laboratories of solid-state chemistry: Jean-Claude Gilles became professor at the École supérieure de physique et de chimie industrielle de Paris (the Paris Superior School of industrial physics and chemistry) in 1972, Alexandre Revcolevschi at the University of Orsay near Paris in 1977, and Jean-Pierre Boilot at the École Polytechnique (Polytechnical school, Palaiseau) in the early 1980s. Jacques Livage created a laboratory at the University of Paris 6 and became one of the world leaders of a new branch of materials chemistry that he himself labeled "chimie douce" (soft chemistry) in 1978. By using ambient temperature and low energy bio-inspired chemical reactions, soft chemistry allows the synthesis of a wide range of new materials.Despite his huge influence on a generation, Collongues never enjoyed international recognition. He did not travel much and was not eager to speak English. His name can hardly be associated with a discovery or a new theory, but he was one of the major cogwheels of the solid-state chemistry machinery in France. Collongues's research school competed-both scientifically and institutionally-with other French groups in the field of materials chemistry, such as the research school headed by Paul Hagenmuller in Bordeaux, the group founded by Félix Trombe at Meudon-Bellevue, and the group around Jean Flahaut at the University of pharmacy in Paris. However, the rivalry did not prevent cooperation among the groups, and so a French community of solid-state chemistry emerged with a common language and specific methods. These were characterized by close connections between chemistry and physics; between fundamental and applied research; the synthesis and characterization of new solid compounds; and focus on the linear model linking atomic structure to physical properties and properties to applications. For Collongues's retirement in 1993, the members of the LCAESmost of them being his former studentsorganized a week-end in the countryside and played several cheerful spectacles to express their admiration and affection to the old master. Soon after, in 1994, Collongues discovered that he had a lung cancer. During four years, he fought with strong energy against the disease, analyzing the medical results like he used to do with chemical ones, giving the impression while speaking with the doctors that he was not the patient but their colleague. Nevertheless he died at the hospital of Villejuif near Paris on 10 May 1998.BibliographyWorks by Collongues Unpublished documents of Collongues can be found in the archives of the Académie des sciences in Paris. Particularly a Notice des titres et travaux scientifiques (1979) written by Collongues himself gives his complete bibliography up to 1979. "Les composés non stoechiométriques." In Quelques problèmes de chimie minérale, edited by Instituts Solvay. Bruxelles : R. Steeps, 1956. "Argent." In Nouveau traité de chimie minérale, edited by Paul Pascal. Vol. 3. Paris: Masson, 1957. "Alliages de cuivre et d'argent." In Nouveau traité de chimie minérale, edited by Paul Pascal.

  The techniques he used to draw phase diagrams and study crystal structures were taken from early twentieth century metallurgy and crystallography. In addition, his approach-based on notions such as defects and structure-dependent properties-followed the path opened by solid-state physics during the 1930s. And yet Collongues opened an avenue of research because he extended the techniques and methods traditionally used to study alloys or 050°C that made it useless for technological purposes. Nevertheless, the CEA, considering zirconia materials as potentially heat resistant, contracted Collongues to study them. Zirconium oxide was finally stabilized by the addition of rare or alkaline earth elements. Up to the 1990s, zirconium and its alloys were massively used to produce tubes in nuclear power plants.Thus, until 1973, Collongues's research group responded to the demand on high performance materials from atomic and military programs.From 1962 to 1973, Collongues developed a new expertise in single crystal-growth. Here again the innovative strategy was based on the transposition of traditional chemical metallurgy techniques to metal oxides. The group performed the synthesis of single crystals of metal oxides measuring a few centimeters, the first one being calcium oxide, synthesized in 1964 by using the flame fusion method (also called Verneuil's method) related to a plasma furnace. Whereas the physical measurements usually performed on powder crystals could only give average numbers in space, which hid the possible anisotropic properties, with measure new anisotropic properties. Moreover, Collongues performed this kind of determinations in a systematic way on a wide variety of metal oxides (zirconium, aluminium, germanium, titanium, and lanthanides).In 1966 Collongues's research group included 20 people (out of 111 people for the whole CECM). Although all members and Collongues himself still had to pay allegiance to the supreme reigning master, Chaudron, Collongues gained personal recognition. He became a professor at the Université de Paris (without a chair in 1964, and with a chair in 1967), was awarded several national science prizes in the 1960s, and in 1966 was elected a member of the National Comity of CNRS (e.g., the ruling body of CNRS) in the inorganic chemistry section.

	"defects"-investigated by the American physicist Frederick Seitz, building on earlier work
	by the Russian investigator Yakov Frenkel and the Germans Wilhelm Jost, Carl Wagner, and
	Walter Schottky-and ventured a comparison between the defect crystal structure and a solid
	solution, which led him to describe the crystal as a dynamic (diffusion and reactions), non-
	ideal (non-stoichiometric), and disordered (order-disorder phenomena) structure.
	Collongues's PhD research was not really innovative in terms of methods and
	concepts. In the 1960s, Collongues himself considered designing refractory materials for
	spacecrafts. But in order to meet industrial demands for heat-resistant materials, he had to
	produce higher temperatures in controlled conditions. With the help of new PhD students, he
	designed a number of sophisticated furnaces: an "image furnace" in 1960; a high-frequency
	plasma furnace in 1961 (following the pioneer work of the American, Thomas B. Reed, in
	Collongues was not the
	first one to study iron monoxide in LCTC. In 1939 Jacques Bénard had defended his PhD
	thesis, "Etude de la décomposition du protoxyde de fer et de ses solutions solides" (Study of
	the Decomposition of Iron Monoxide and its Solid Solutions). Bénard had used the same
	techniques to show that (I) occurred without structural changes and without matter transport
	(i.e., without diffusion). Collongues, however, went further: he tried to link the physical
	properties (density and electrical conductivity) to the chemical composition and experimental
	conditions (temperature, quantity of impurities). For this purpose, he used the notion of

model crystals to the investigation of ionic compounds such as non-stoichiometric metal oxides. Thus, he and others-such as Ferdinand A. Kröger at Royal Philips Electronics, at Eindhoven in the Netherlands or John Goodenough at Lincoln Laboratory (MIT)-paved the way towards the science of new materials.

High-temperature Research.

Early in 1953, Chaudron's laboratory was renamed Centre d'étude de chimie métallurgique (CECM; Center for the study of metallurgical chemistry), a name emphasizing the close connection between chemistry and metallurgy without, however, mentioning solid-state physics. In fact, two research programs were developed at the same time. On the metallurgy side, the aim was to obtain metals in their purest state; for instance, aluminium was purified by using the "zone melting" method developed by William Pfann at the Bell Laboratories in New Jersey for germanium in 1952. On the chemistry side, the investigative effort concerned solid-state metal oxides. The laboratory was divided into four different groups.

When Collongues completes his

PhD in 1954 at the age of thirty and gets a permanent position at the CNRS, Chaudron created a fifth research group: the High Temperature Section. Devoted to the study of metal oxides, it was placed under Collongues's supervision. His first two PhD students were women, and later on his group included several female researchers, which was quite unusual in those times. Jeanine Thery continued previous research on iron oxides and ferrites at still higher temperatures than before while Monique Perez y Jorba tackled a new material, zirconia (ZrO 2 ). With a melting point around 2,680°C, this zirconium oxide was extremely resistant to high temperature, but it went through a dramatic structural transition around 1,1960); and a direct induction furnace during 1962-1964. These new devices were built and operated thanks to the recruitment of skillful collaborators by the CECM and to the financial support of industrial firms interested in the development of such furnaces. During the 1960s, the group thus built up an expertise in temperatures ranging from 2,000 to 20,000°C. Its reputation was reinforced by the creation of an international journal, the Revue Internationale des Hautes Températures et des Réfractaires (International review of high temperatures and refractory materials) edited by Collongues from 1963 to 1976 and dedicated to high temperatures production and refractory crystal compounds. However, the audience of the journal, written in French, was limited to the French-speaking European solid-state chemistry communities.

Crystal-growth.

centimeter-size single crystals the crystal was large enough to allow measurements on one single-oriented domain. Moreover, single crystals provided further information on atomic structures. Collongues's group was thus able to make more precise structure determinations and to


Takashi Murakami
email: murakami@eps.s.u-tokyo.ac.jp

Atsuyuki Inoue

Bruno Lanson

Alain Meunier

Daniel Beaufort

ILLITE-SMECTITE MIXED-LAYER MINERALS IN HYDROTHERMAL ALTERATION OF VOLCANIC ROCKS: II. ONE-DIMENSIONAL HRTEM STRUCTURE IMAGES AND FORMATION MECHANISMS

Keywords: Alkylammonium-solvation, HRTEM, hydrothermal system, illite-smectite mixed layer minerals, structure image

Smectite illitization was investigated in felsic volcaniclastic rocks from a drill core near the Kakkonda active geothermal system, Japan, using high resolution transmission electron microscopy (HRTEM) that provided one-dimensional structure images of mixed-layer illite-smectite (I-S) minerals normal to [hk0]. Simulated images of a rectorite-like structure revealed that smectite can be distinguished from illite in mixed-layer I-S by HRTEM if the basal spacing of smectite is larger than that of illite. The larger basal spacing of smectite, 1.3 nm under HRTEM, was obtained by intercalation of dodecylammonium ions into smectitic interlayers. In simulated and observed images normal to [hk0], tetrahedral (T) and octahedral (O) cation-layers are imaged as dark lines, an illitic interlayer a bright line, and a smectitic interlayer a dark line sandwiched by two bright lines. Samples are from 435 m (5% I; R0), 635 m (35% I; R0), 656 m (62% I; R1), and 756 m (85% I; R3) depths where % I is the percent of illite layers in a sample and R is the Reichweite parameter. Sample 435 consisted mostly of smectite, and illite layers occurred, though small in amount, as M1 units (module of type 1, defined as one consisting of two polar T-O-T silicate layers with one central illitic interlayer and two, half smectitic interlayers at the outermost surface; the number corresponds to that of central illitic interlayers). M1 units were dominant and isolated or consecutive smectite layers of > two were present in sample 635. Sample 656 consisted mostly of packets of M1 units of 1 to 5 containing M2 to M5 units occasionally. Isolated or consecutive smectite layers of > two were not present in 656. Illite layers occurred almost as M1

3 units in samples 435, 635 and 656, and the number of M1 units increased with increase in % I. Sample 756 was characterized by the presence of M2 to M10 units accompanied by smectitic interlayers at the external surface and the absence of M1 units and isolated smectite layers. HRTEM data strongly suggest that illitization in a hydrothermal system occurs by precipitation of M1 units for mixed-layer I-S minerals up to 60% I. This does not require the presence of precursor smectite. Illitization of I-S minerals with more than 60% I proceeds by precipitation of various types of Mn(n≥2) units. Illite occurs only as Mn(n≥1) units throughout illitization.

INTRODUCTION

The transformation of smectite to illite via mixed-layer illite-smectite (I-S) minerals is controlled by physico-chemical environments, and the structures of mixed-layer I-S minerals can give a deep insight into the transformation mechanisms [START_REF] Altaner | Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization[END_REF]. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) methods have been utilized to determine the structures of mixed-layer I-S minerals (e.g., references in [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF] issue for XRD and references below for HRTEM). The former gives information on average layer stacking within crystallites based on Markovian statistics, while the latter gives information on layer stacking in local regions within a crystallite. Thus the two methods are commonly complementary to one another in studies of mixed-layer I-S minerals.

The results of one-dimensional XRD structure analysis and characterization of component layers in I-S samples by chemical treatments such as alkylammonium ion and Li-saturation test were described in [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF], this issue. This HRTEM investigation of one-dimensional structure images of mixed-layer I-S minerals in combination with the previous XRD study [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF] provides a better understanding of processes and mechanisms of smectite illitization during hydrothermal alteration of volcanic rocks.

Numerous HRTEM studies of I-S minerals involve one-dimensional lattice imaging (references in [START_REF] Altaner | Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization[END_REF][START_REF] Altaner | Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization[END_REF][START_REF] Bauluz | Transmission electron microscopy study of smectite illitization during hydrothermal alteration of a rhyolitic hyaloclastite from Ponza, Italy[END_REF] for recent studies). An essential problem in HRTEM investigation of mixed-layer I-S minerals is the collapse of expandable interlayers under high vacuum. This makes distinction between smectitic and illitic interlayers, and thus, between smectite and illite in I-S mixed-layers very difficult, if not impossible. Recent improvements in imaging and sample preparation techniques have addressed this problem. Permanent expansion of smectitic interlayers using n-alkylammonium ions for identification of smectitic and illitic interlayers has been accomplished [START_REF] Bell | Microstructure in mixed-layer illite/smectite and its relationship to the reaction of smectite to illite[END_REF][START_REF] Vali | Expanding behaviour, structural disorder, regular and random irregular interstratification of 2:1 layer-silicates studied by high-resolution images of transmission electron microscopy[END_REF][START_REF] Vali | Alkylammonium ion treatment of clay minerals in ultrathin section: A new method for HRTEM examination of expandable layers[END_REF]. Ultramicrotomed, transmission electron microscopy (TEM) specimens are usually used for I-S minerals with n-alkylammonium ions. However, ultramicrotomed TEM specimens are thick (about 50 nm) and the thickness varies area by area, which changes image contrast significantly. Guthrie andVeblen (1989, 1990) have shown that smectitic and illitic interlayers can be distinguished from one another in overfocused images. However, they indicated that ambiguities are still present in the interpretation of such images due to specimen orientation and microscope focus, and that structural information can be obtained near the Scherzer focus. Ion-milled TEM specimens are usually used for observations under overfocus conditions. [START_REF] Stucki | Effects of iron oxidation state on the texture and structural order of Na-nontronite gels[END_REF] have found LR White resin is useful to avoid complete collapse of smectite under high vacuum of TEM. This method has been applied to many HRTEM observations (e.g., [START_REF] Krekeler | A microtexture study of palygorskite-rich sediments from the Hawthorne Formation, southern Georgia, by transmission electron microscopy and atomic force microscopy[END_REF]. It was recommended that LR White resin impregnated, I-S minerals are imaged under overfocus conditions [START_REF] Kim | A technique for maintaining texture and permanent expansion of smectite interlayers for TEM observations[END_REF]. To keep expansion of smectitic interlayers under high vacuum of TEM, a freezing technique with liquid nitrogen also has been proposed [START_REF] Murakami | Microstructure of interstratified illite/smectite at 123 K: A new method for HRTEM examination[END_REF]. [START_REF] Dong | Phase relations among smectite, R1 illite-smectite, and illite[END_REF] concluded through their HRTEM observations that smectite illitization proceeds: (1) pure smectite, (2) smectite with small proportions of discrete R1 I-S and illite, (3) R1 I-S with small proportions of smectite and illite, (4) illite with some smectite and R1 I-S, and (5) illite where R denotes the Reichweite parameter.

Subsequent observations have supported this conclusion [START_REF] Tillick | Genesis of dioctahedral phyllosilicates during hydrothermal alteration of volcanic rocks: I. The Golden Cross epithermal deposit[END_REF][START_REF] Yan | Genesis of dioctahedral phyllosilicates during hydrothermal alteration of volcanic rocks: II. The Broadlands-Ohaaki hydrothermal system, New Zealand[END_REF][START_REF] Bauluz | Transmission electron microscopy study of smectite illitization during hydrothermal alteration of a rhyolitic hyaloclastite from Ponza, Italy[END_REF]. However, because these authors employed one-dimensional lattice imaging under overfocus conditions, the obtained contrast does not necessarily correspond to the projected potential of mixed-layer I-S minerals, which leads to some ambiguities in the interpretation of TEM images [START_REF] Guthrie | Interpreting one-dimensional high-resolution transmission electron micrographs of sheet silicates by computer simulation[END_REF]. In the present study, I-S minerals formed by hydrothermal alteration were intercalated with C 12 -alkylammonium ions to ensure expansion of smectitic interlayers under high vacuum of TEM. Then, we examined the structural information by HRTEM imaging each tetrahedral (T) and octahedral (O) tetrahedral (T) cation-layer in the T-O-T silicate layers, which distinguishes smectite from illite in the mixed-layer I-S minerals. For the structural information we employed one-dimensional structure imaging instead of conventional, one-dimensional lattice imaging.

EXPERIMENTAL

Samples

Samples used for the present TEM study were a smectite-to-illite series in felsic volcaniclastic rocks from a drill core (IT-2) near the Kakkonda active geothermal system, Japan [START_REF] Inoue | Illite-smectite mixed-layer minerals in felsic volcaniclastic rocks from drill cores, Kakkonda, Japan[END_REF]. Clay fractions (< 2 µm) were obtained by separating them from rock samples using ultrasonic vibration and centrifugation, and used for the present TEM study. The samples were from 435 m (5% I; Reichweite R0), 635 m (35% I; R0), 656 m (62% I; R1), and 756 m (85% I; R3) depths. The depths are hereafter used for sample names. The percent of illite layers (% I) data in parentheses are from [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF] except for that of sample 656 which are from [START_REF] Inoue | Illite-smectite mixed-layer minerals in felsic volcaniclastic rocks from drill cores, Kakkonda, Japan[END_REF]. The I-S samples were intercalated with C 12 -alkylammonium (dodecylammonium) ions to permanently expand smectitic interlayers under high vacuum of TEM. Sample 435 was also intercalated with C 18 -alkylammonium (octadecylammonium) ions to compare expandability to that with C 12 -alkylammonium ions. Rectorite (RAr-1, Arkansas) of Clay Mineral Society standard was also used without intercalation to compare the electron diffraction pattern to those of the I-S minerals.

Transmission electron microscopy

A JEOL JEM 2010 microscope was used for the HRTEM observation. The TEM had a point to point resolution of 0.2 nm with a spherical aberration coefficient of 0.5 mm and was operated at 200 kV. The TEM specimens were prepared by impregnating the I-S crystals in resin, pushing them between two glass slides, and slicing and polishing them mechanically. This procedure made it easy to obtain TEM images normal to [hk0]. The polished mixed-layer crystals were sandwiched by two Mo TEM grids with adhesive, and thinned to electron transparency by Ar ion milling (Dual Ion Mill, Gatan Ltd.) with the grids on a cold stage cooled by liquid nitrogen. The TEM specimens were thinned at 4 kV of operation voltage, 30 µA of specimen current and 20˚ of specimen tilt angle. HRTEM images were observed near the Scherzer focus (-42 nm), and taken at x120,000 or x150,000 of magnification because higher magnification caused significant damage on the crystals. Some of the HRTEM negatives were digitized and processed to remove noise and the image of amorphous material by rotational filtering [START_REF] Kilaas | Optical and near-optical filters in high-resolution electron microscopy[END_REF]) implemented within Digital Micrograph V. 2.5, Gatan Ltd. The use of rotationally filtered (RF) images has been described in detail by [START_REF] Banfield | Atomic-resolution transmission electron microscope evidence for the mechanism by which chlorite weathers to 1:1 semi-regular chlorite-vermiculite[END_REF]. Selected area electron diffraction (SAED) was employed to measure d values of mixed-layer I-S minerals. The d values were originally calibrated by using metal Au single crystals, recalibrated by fringes of mineral species with known d values, such as biotite and chlorite, and directly measured on images. The errors of the d values were estimated to be + 0.05 nm.

HRTEM imaging alone cannot directly distinguish a T-O-T silicate layer of illite from that of smectite. We first assumed a polar model (e.g., [START_REF] Altaner | Comparison of structural models of mixed-layer illite/smectite and reaction mechanisms of smectite illitization[END_REF] to describe the 2:1 layers of mixed-layer I-S minerals: one illitic interlayer is sandwiched by a pair of tetrahedral sheets and a pair of half octahedral sheets of illite, and one smectitic interlayer by those of smectite. The former is hereafter referred to as an illitic layer of mixed-layer I-S minerals, and the latter a smectitic layer (Figure 1). An M1, module of type 1, unit is defined as consisting of two polar T-O-T silicate layers with a central illitic interlayer and two, half smectitic interlayers at the outermost surface, which is denoted as -SIS-in [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF], this issue. In a sequence of -SISISIS-, for instance, I and S denote the illitic and smectitic interlayers respectively, and -S and S-represent a half part of expandable interlayer at the outermost surface. An M2 unit, denoted as -SIIS-, consists of three T-O-T silicate layers and three interlayers: one homogeneous, non-polar T-O-T silicate layer of illite is located at the center sandwiched by two polar T-O-T silicate layers and two illitic interlayers are in the middle and two, half smectitic interlayers at the outermost surface. Thus, an increase in n of an Mn unit increases the number of non-polar T-O-T silicate layers of illite. Similar classification was applied to the interpretation of TEM images of I-S minerals by [START_REF] Bauluz | Transmission electron microscopy study of illitization in pelites from the Iberian Range, Spain: layer-by-layer replacement?[END_REF].

Simulation

Simulated images calculated using MacTempas software (Total Resolution Co.) were compared to observed HRTEM images to provide information on the mixed-layer I-S structures. The crystal structure of 1M illite for the simulation was based on 1M muscovite by [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF] who transformed the coordinates of 2M 1 muscovite [START_REF] Richardson | Crystal structure of a pink muscovite from Archer's Post, Kenya: Implications for reverse pleochroism in dioctahedral micas[END_REF] to those of 1M. Simulations were made for a rectorite-like, perfectly ordered I-S (hereafter, rectorite-like I-S) derived from 1M illite because the crystal structures of mixed-layer I-S minerals consist of illitic and smectitic interlayers and T-O-T silicate layers (e.g., [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF]. We assumed a polar structure with a periodic repetition of an illitic and smectitic interlayer between T-O-T silicate layers, and chemical formulas of K 0.8 Al(Si 3.2 Al 0.8 )O 10 (OH) 2 and Na 0.3 Al(Si 3.7 Al 0.3 )O 10 (OH) 2 for illitic and smectitic layers, respectively. Hydrogen atoms were ignored for the calculations. Because mixed-layer I-S minerals should be imaged with an electron beam normal to the c*-axis or along [hk0] to distinguish illitic from smectitic interlayers (e.g., [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF], the simulations were made along [530]. The simulated images of rectorite-like I-S with d 001 =2.3 nm were examined because the SAED patterns of sample 435 (5% I) had d 001 =1.3 nm as mentioned below, i.e., smectite with C 12 -alkylammonium ions collapsed incompletely to show a 1.3 nm basal-spacing under high vacuum of TEM. The basal-spacing of the illitic layer of rectorite-like I-S was assumed to be 1.0 nm in all simulations. The objective aperture radius was 3.5 nm -1 . Calculations were made changing a defocus amount from -82 to -22 nm with an increment of 20 nm and changing a specimen thickness from 2 to 10 nm with an increment of 2 nm.

To examine the effects of other factors (e.g., cation occupancies) on HRTEM contrast, we made further calculations at a defocus -42 nm (the Scherzer focus) and a specimen thickness of 4 nm where we obtained a contrast that corresponds to the real structure (Figure 1). The further calculations were made for the following conditions:

(1) The d 001 values of the rectorite-like I-S were 2.0, 3.0 and 6.0 nm, assuming that the basal-spacings of a smectitic layer of the rectorite-like I-S were 1.0, 2.0 and 5.0 nm, respectively. (2) The interlayer occupancies of K and Na were 0.0, 0.25, 0.5, 0.75, and 1.0 for the illitic and smectitic interlayers, respectively. ( 3 [010]/[310]/[3-10] images were not calculated because they are two-dimensional structure images (e.g., [START_REF] Kogure | New insights into the biotite chloritization mechanism via polytype analysis[END_REF].

RESULTS

Simulated TEM images

A simulated image of rectorite-like I-S with d 001 =2.3 nm normal to [530] is compared to the crystal structure in Figure 1. Each of the tetrahedral and octahedral cation-layers corresponded to one dark line, and the dark line of the tetrahedral cation-layer was slightly darker than that of the octahedral cation-layer. Thus, each T-O-T silicate layer resulted in one set of three dark lines in the simulated image. The illitic interlayer made one bright line and the smectitic interlayer one dark line sandwiched by bright lines. The simulated image indicates that we can distinguish illitic interlayers from smectitic ones in any mixed-layer I-S minerals when smectite does not collapse completely. Simulated images similar to that in Figure 1 were obtained when a specimen thickness was < 10 nm at a defocus -42 nm, and when it was < 4 nm at a defocus -22 nm. However, simulated images did not correspond to the projected potential of the T-O-T silicate layers when defocus amounts were < -62 nm or > -2 nm.

Our simulated, structure image is essentially different from lattice images of which contrast does not necessarily correspond to the projected potential of mixed-layer I-S minerals (e.g., [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF]. In their calculations, [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF] showed that one T-O-T silicate layer is imaged as a pair of dark fringes at the Scherzer focus and with a specimen thickness of 5.2 nm. They suggested that the sequence of illite and smectite is determined practically under overfocus conditions where heavy dark fringes are present near the position of the smectitic interlayers and weaker dark fringes near the illitic interlayers. However, the layer thickness varies with factors such as defocus and specimen tilt if it is measured from images [START_REF] Guthrie | High-resolution transmission electron microscopy of mixed-layer illite/smectite: Computer simulations[END_REF]. In contrast, we can measure the layer thickness that is a distance from one O cation layer to an adjacent one directly from an HRTEM image in the present simulation as we demonstrate with 1.0 and 1.3 nm in Figure 1.

The variation of simulated images with a function of the basal-spacing of the smectitic layer of rectorite-like I-S is shown in Figure 2. Bright lines appear at the smectitic interlayers for the basal-spacing of 1.0 nm (Figure 2a). If smectite collapses completely to a 1.0-nm basal-spacing, the illitic interlayers cannot be distinguished from the smectitic interlayers in mixed-layer I-S minerals. The dark lines or fringes at the smectitic interlayers were increased in number with increasing the basal-spacing of the smectitic layer (short, vertical bars in Figure 2). Figures 2c and2d suggest that mere superimposition of a few rectorite-like I-S layers generates some additional dark fringes in the smectitic interlayers. In any mixed-layer I-S minerals with expanded interlayers, bright lines adjacent the dark lines of the tetrahedral cation-layers appear at the smectitic interlayers (e.g., arrows in Figure 2).

The dark and bright fringes at the smectitic interlayers occur due to the effect of truncation of a Fourier series, essentially a result of the diameter of the objective aperture (e.g., [START_REF] Buseck | Imaging and diffraction[END_REF]. Although the dark fringes at the smectitic interlayers appear due to the truncation of a Fourier series, the basal-spacings of the smectitic layers (1.0, 1.3, 2.0 and 5.0 in Figures 2a, b, c andd, respectively) are real.

The occupancies in the range of 0.0 to 1.0 of K and Na at the interlayers did not affect the contrast at both the illitic and smectitic interlayers. This indicates that one-dimensional structure imaging cannot distinguish vermiculite from smectite.

Although the content of vermiculite in the present samples is small, 5 % at maximum by ethylene glycol solvation, vermiculite does not significantly affect the smectite illitization at Kakkonda [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. Therefore we simply assumed that all of the 1.3-nm layers are smectitic. We did not calculate the images of rectorite-like I-S with C 12 -alkylammonium ions in the smectitic interlayers because the collapsed structure is not known. We infer little effects of C 12 -alkylammonium ions that contain only light elements on the HRTEM contrast because there are no effects of the interlayer occupancies on the HRTEM contrast as mentioned above.

A simulated image assuming 0.5 Al and 0.5 Fe at the octahedral sites was the same as that in Figure 1 except that the contrast at the octahedral cation layer was slightly thicker and darker than that in Figure 1. There was no difference in contrast between the polar and non-polar models. [hk0] simulated images varying 1 ≤ h ≤ 7 and 1 ≤ k ≤ 5 were the same as that in Figure 1

Selected area electron diffraction patterns

Figure 3 gives typical SAED patterns of the present samples as well as Arkansas rectorite. Smectitic interlayers of Arkansas rectorite without C 12 -alkylammonium ions collapsed almost completely to give a d 002 value of 1.00 nm (Figure 3a). Smectite intercalated with C 12 -alkylammonium ions (sample 435) collapsed incompletely and gave a d 001 value of 1.28 nm (arrow in Figure 3b) compared to that of 1.71 nm by XRD under air [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF], which suggests that an M1 unit has d 001 and d 002 values of 2.28 and 1.14 nm, respectively, under high vacuum of TEM. Smectite intercalated with C 18 -alkylammonium ions had a d 001 value of 1.35 nm (arrow in Figure 3c) that was near that of smectite intercalated with C 12 -alkylammonium ions within error. We therefore only used C 12 -alkylammonium ions for further HRTEM study. Mixed-layer I-S minerals of samples 635 and 656 had a same d value of 1.14 nm (arrows in Figures 3d ande, respectively), suggesting that M1 units of a d 001 value of 2.28 nm are dominant in those samples. The d value of 1.00 nm indicates that sample 756 has packets of illite (arrow in Figure 3f). In sample 756, less ordered I-S minerals than illite such as M3 units were also present as shown by the diffraction spots and diffuse toward the center from the arrow head in Figure 3f.

Observed TEM images

Sample 435. Mixed-layer I-S of sample 435 consisted mostly of smectite packets with a few to ten T-O-T silicate layers (Figure 4a). A thick packet may be composed of a few sub-packets. The basal spacings of the smectitic layers were 1.3 nm (S's in Figures 4b andc), which is consistent with the result of the SAED pattern (Figure 3b). Figures 4b andc indicate that illitic interlayers are distinguished from smectitic interlayers as was predicted by the above simulation (Figure 1): an illitic interlayer corresponds to one bright line between two, adjacent T-O-T silicate layers (indicated by a bracket in Figure 4) and a smectitic interlayer corresponds to one dark line and two bright lines sandwiched by two, adjacent T-O-T silicate layers. Figures 4b andc also indicate that the smectitic layers were homogeneously intercalated with C 12 -alkylammonium ions in general. Although smectitic layers were predominant in sample 435, illitic layers occurred as M1 (-SIS-) units as shown in Figures 4b andc. Figures 4b andc display two M1 units and one M1 unit, respectively, both on smectitic layers. A stack of M1 units of more than three was not observed. We found only one example of two consecutive non-polar illitic layers within smectite packets.

Sample 635. Sample 635 consisted of packets with a few to ten T-O-T silicate layers (Figure 5a) and the packets were up to several micrometers long. M1 (-SIS-) units were dominant (Figure 5b), which is consistent with the d value of 1.14 nm (Figure 3d). Smectite layers were observed as being both isolated from and attached to M1 units (Figure 5c). The XRD examination indicated that sample 635 contains M1 units with dominant smectite layers using a C 12 -alkylammonium saturated specimen and is described as a structure of Reichweite=0 (e.g., the occurrence probabilities of layer-doublets and layer-triplets ; W SS = 0.42, W IS = 0.23, W SSS = 0.27, W SIS = 0.15) using Ca-, ethylene glycol-saturated specimens [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. The apparent discrepancy in frequency of M1 units may be due to the thickness of packets observed between HRTEM and XRD [START_REF] Drits | Mixed-layer minerals: diffraction methods and structural features[END_REF]. The thinner the packets observed in TEM, the more the heterogeneity in layer sequence is facilitated so that M1 units become visible in thinner packets. M2 (-SIIS-) or M3 (-SIIIS-) units were rarely observed (Figure 5c).

There are two dark fringes (indicated by two pairs of two facing arrows in Figure 5b) between two M1 units in the bottom of Figure 5b. The two M1 units are clearly separated in the left of Figure 5b. However, they are attached to one another through a smectitic layer of a d value of 1.7 nm in the center and right. It is not known whether the two individual M1 units coalesce into one crystal (i.e., a sequence of -SISIS-) in the center and right or they are attached to one another only physically. An arrow in Figure 5c shows a 1.5-nm smectitic layer although the other part of the same layer has a 1.3-nm smectitic layer. This may show heterogeneous collapse of a smectitic interlayer, or show the presence of vermiculite, although the occurrence of vermiculite layers was rare as suggested by [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF].

Sample 656. Sample 656 consisted of packets with a few to ten T-O-T silicate layers (Figure 6a). M1 units were predominant; the observed packets were a stack of M1 units of 1 to 5. Figure 6b shows a perfectly ordered stack of five M1-units though partly damaged. In addition, M2-to M5 units occurred occasionally, but isolated smectite layers or packets were not observed. Sample 756. Sample 756 consisted of packets with a few to ten T-O-T silicate layers (Figure 9). M2 to M10 units were present in sample 756, which is consistent with the presence of the diffuse spot near the arrow in Figure 3f. Although M3 units (arrows in Figure 9) were most abundant in number, the abundance of M3 units in sample 756 was less than one-third. The inset of Figure 9 demonstrates a typical example of an M3 unit; between the four T-O-T silicate layers there exist three illitic interlayers, which is compared to the simulated image in Figure 1. Isolated smectite layers were not found in 756 by HRTEM investigation although XRD indicates it contained 12.5% smectite layers [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. These suggest that Mn (n≥2) units are smectitic at the external surface as is an M1 unit mentioned above.

Figure 10 shows changes in smectitic and illitic nature at the boundaries of packets 1, 2 and 3. In area b (arrow with b Figure 10a) the boundary between packets 1 and 2 is smectitic, and that between packets 2 and 3 is illitic (arrows with S and I in Figure 10b, respectively). In area c (arrow with c Figure 10a) the boundary between packets 2 and 3 is changed to smectitic while that between packets 1 and 2 stays smectitic (arrows with S in Figure 10c). In area d (arrow with d Figure 10a) the boundary between packets 1 and 2 becomes illitic (arrow with I in Figure 10d). Figure 10 indicates that the outermost interlayers of Mn units can change their nature from smectitic to illitic.

DISCUSSION

The T-O-T silicate layers were imaged as sets of three dark lines for both illite and smectite for the present HRTEM study. If smectitic interlayers collapse completely, the one-dimensional structure images do not provide any information on the sequence of mixed-layer I-S minerals (Figure 2a). However, the expansion of smectitic interlayers was maintained under high vacuum of TEM by the intercalation of C 12 -alkylammonium ions. No apparent indication of C 12 -alkylammonium expulsion was observed with a few, possible exceptions (e.g., Figure 8). Consequently, the one-dimensional structure imaging and the expansion of smectitic interlayers led to unambiguous identification of illitic and smectitic interlayers, which can elucidate the structures of I-S minerals, and provide detailed insight into the processes and mechanisms of smectite illitization with more accuracy than previously reported using lattice fringe images [START_REF] Dong | Phase relations among smectite, R1 illite-smectite, and illite[END_REF]Bauluz et al., 2002, and references therein).

HRTEM investigation indicates that sample 435 is composed mostly of smectite and is consistent with XRD results by [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. Illitic layers occur, though small in amount, as M1 units (Figures 4b and4c). Packets of M1 units of 1 to 5 are predominant in samples 635 (35% I) and 656 (60% I) (Figures 5a and6b, respectively).

These indicate that the increase in illite content in the Kakkonda I-S minerals with up to 60% I results from the formation of M1 units but not from discrete illite layers independent of M1 units. The difference in illite content between samples 635 (35% I) and 656 (60% I) is due to the difference in abundance of isolated smectite and Mn(1≤n≤5) units in the two samples. The observations strongly suggest that illitization in the range of smectite to I-S with 60% I occurs by precipitation of M1 units. This mechanism does not require the presence of precursor smectite. The observations also suggest that M1 units are somewhat stable by their distinct occurrence in samples with up to 60% I. The uniqueness and stability of M1 units have been discussed for long time (Dong et al., 1997 and references therein) based on the data of rectorite-like I-S. [START_REF] Dong | Phase relations among smectite, R1 illite-smectite, and illite[END_REF] concluded that M1 I-S mineral is unique based on the chemical composition and common layer spacing, which is recently confirmed by a first-principles study [START_REF] Stixrude | First-principles study of illite-smectite and implications for clay mineral systems[END_REF]. Our HRTEM observations further confirm the uniqueness and stability of M1 units.

There can be a few mechanisms for illitization of I-S minerals of more than 60 % I or formation of Mn(n≥2) units: they are Mn + S = M(n+1), two M1s = M3, and Mn + Mm = M(n+m+1) by apparent layer-by-layer formation, and direct precipitation of Mn(n≥2). Fundamental constraints of the mechanisms are based on HRTEM observations of samples 635, 656 and 756: predominant M1 units with few Mn(n≥2) units in sample 635, predominant M1 units with some Mn(2≤n≤5) units and without isolated smectite in sample 656, and Mn(2≤n≤10) units without M1 units or isolated smectite in sample 756. The progressive formation of illitic interlayers in a smectite packet is not consistent with the above HRTEM observations, and thus, mere layer-by-layer formation is excluded from the main mechanism of illitization.

In theory, one Mn unit with one homogeneous smectite layer within a packet can make one M(n+1) unit by apparent layer-by-layer formation at the outermost interface, as suggested by an example of the formation of one M2 unit in Figure 8. However, there are few isolated smectite layers in 656. Therefore, the mechanism of M(n+1) unit formation from a combination of Mn units and smectite should be excluded from the main mechanism of illitization.

A pair of M1 units can be transformed to one M3 unit by replacement of a smectitic interlayer by an illitic interlayer. Our HRTEM observations are: there a few possible examples suggesting this transformation in sample 656 (e.g., arrows with I and S in Figure 7), the abundance of M3 units in sample 756 is less than one-third, and M2 units already grow in sample 656 (Figures 7 and8). In addition, a stack of M1 units can make only Mn(odd) units whereas various Mn(even) units are also present in sample 756. Therefore, layer-by-layer formation of Mn(n≥3) units using precursor M1 units could be a possible mechanism for illitization, but HRTEM data suggest this is not a major contributor.

Because the diameter of the I-S particles is up to several µm and the thickness is usually less than 10 nm, it is almost impossible to observe a whole packet of I-S by HRTEM. However, Figure 7 clearly indicates the formation of an M2 unit. If this formation is interpreted as a growth of a T-O-T silicate layer on the M1 unit, we should have observed a growth of a T-O-T silicate layer on a stack of M1 units that are dominant in sample 656 (for instance, a sequence of -SIISISISIS-). Because we do not observe such overgrowth, the formation of the M2 unit in Figure 7 should be interpreted as formation of M2 units themselves by precipitation and growth (e.g., [START_REF] Inoue | Mechanism of illite formation during smectite-to-illite conversion in a hydrothermal system[END_REF]. A growth mechanism does not require the presence of precursor M1 units but rather requires direct precipitation from solution. The precipitation and growth mechanism can also make any Mn(n≥2) units and is consistent with the occurrence of Mn(n≥2) units in sample 756. Thus, direct precipitation of Mn(n≥2) units is a possible, main mechanism for illitization of mixed-layer I-S minerals of more than 60 % I.

For illitization at the final stage, e.g., formation of Mn(n≥10) units, it is possible that one Mn unit stacked by one Mm unit forms one M(n+m+1) unit by layer-by-layer replacement at the boundary. Figure 10 demonstrates such examples. The external surface of an Mn unit must be smectitic to explain the illite contents of the samples revealed by X-ray diffraction analysis [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. The external surfaces of packets 1, 2 and 3 in Figure 10a should be smectitic originally as shown in Figure 10c.

The smectitic nature changes to illitic as shown in Figures 10b and10d, which results in an increase in n of Mn units.

The present HRTEM observations of mixed-layer I-S minerals in a hydrothermal system clearly show that illitization occurs by precipitation of M1 units for mixed-layer I-S minerals with up to 60 % I. This conclusion is consistent with the XRD data indicating that the precipitation of I-S with Reichweite (R) = 0 (e.g., sample 635) and R1 (e.g., sample 656) structures occurs in a narrow temperature range from 154 to 158 o C and an intermediate product of the two structures is a physical mixture of R0 and R1 phases (e.g., sample 645) in Kakkonda [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF]. With increasing temperature, the illitization takes place apparently continuously via various types of orderings in I-S structures. The illitization of I-S structures with more than 60% I proceeds by precipitation of various types of Mn(n≥2) units and further illitization can occur by the formation of an illitic interlayer at the external interface of two Mn units at temperatures between 160-200 o C. The predominant mechanism of precipitation over layer-by-layer transformation was previously proposed (e.g., [START_REF] Dong | Phase relations among smectite, R1 illite-smectite, and illite[END_REF], and the present structure images provide distinct confirmation of the precipitation mechanism.

The present study indicates that the precipitation of M1 units for I-S with < 60 % I and that of Mn(n≥2) units with > 60 % I are the main illitization mechanisms and illite occurs only as Mn(n≥1) units throughout illitization, which is different from previously reported.

CONCLUSIONS

The observations of one-dimensional HRTEM structure images of mixed-layer I-S in a hydrothermal system give us conclusions below:

1. By one-dimensional HRTEM structure imaging, we can distinguish one tetrahedral or octahedral sheet from the others in a T-O-T silicate layer, and therefore, an illitic layer from a smectitic layer if smectite does not collapse completely.

2. Mixed-layer I-S consists of stacks of sub-units of Mn(n≥0). Here, for example, an M1 unit is defined as one consisting of two polar T-O-T silicate layers with one central illitic interlayer and two, half smectitic interlayers at the outermost surface; an increase in n of an Mn unit increases the number of non-polar T-O-T silicate layers of illite at the center.

3. Smectite illitization occurs in a manner that M1 units increase in number in M0 units for mixed-layer I-S with up to 60 % I, and then, Mn(n≥2) units increase in M1 units for I-S with > 60 % I. The increases in M1 and Mn(n≥2) units correspond to narrow temperature ranges, 154-158 and 160-200 o C, respectively, in Kakkonda.

Illite occurs only as Mn(n≥1) units throughout illitization.

4. The dominant mechanism of illitization is precipitation of various types of Mn(n≥1) units from solution, and this mechanism does not require the presence of precursor smectite or M1 units. The consecutive change of the R parameter from R0 to R3 revealed by XRD [START_REF] Inoue | Illite-smectite mixed-layer minerals in hydrothermal alteration of volcanic rocks: I. One-dimensional XRD structure analysis and characterization of component layers[END_REF] is well explained by the precipitation of M1 units followed by Mn(n≥2) units.

  ) The octahedral sites were occupied by 0.5 Al and 0.5 Fe. (4) The rectorite-like I-S had a non-polar structure. (5) An electron beam was along [hk0] where 1 ≤ h ≤ 7 and 1 ≤ k ≤ 5. [100]/[110]/[1-10] and

  . This occurs because rectorite-like I-S is a sheet silicate. [100]/[110]/[1-10] and [010]/[310]/[3-10] images were not calculated as mentioned above.

Figure 7

 7 Figure7shows isolated and stacked M1 units together with isolated M2 units.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Simulated images of rectorite-like I-S with d 001 = 2.0 (a), 2.3 (b), 3.0 (c) and

Figure 3 .

 3 Figure 3. Selected area electron diffraction patterns of rectorite (RAr-1, Arkansas) (a),

Figure 4 .

 4 Figure 4. TEM images of sample 435 at low magnification (a) and at high

Figure 5 .

 5 Figure 5. TEM images of sample 635 at low magnification (a) and at high

Figure 6 .

 6 Figure 6. TEM images of sample 656 at low magnification (a) and at high magnification

Figure 7 .

 7 Figure 7. TEM image of sample 656 showing an illitic to smectitic change at one

Figure 8 .

 8 Figure 8. TEM image of sample 656 showing a possible layer-by-layer transformation.

Figure 9 .

 9 Figure 9. TEM images of sample 756 at low magnification and at high magnification in

Figure 10 .

 10 Figure 10. TEM images of sample 756 showing changes in and illitic nature at

  Figure1

Figure 10

 10 Figure10

ACKNOWLEDGMENTS

The authors are grateful to T. Tachikawa and T. Takeshige for the technical assistance. This manuscript was greatly improved by the comments of H. Dong, M. Krekeler, D. McCarty and one anonymous reviewer. The electron microscopy was performed in the Electron Microbeam Analysis Facility for Mineralogy at the Department of Earth and Planetary Science, the University of Tokyo. This work was supported by the Science Grant of the Ministry of Education, Science and Culture.