X-Ray Determination of Superstructure Of

Home , Pyrophyllite

Clay Science 10, 15-35 (1996)

X-RAY DETERMINATION OF SUPERSTRUCTURE OF

PYROPHYLLITE FROM YANO-SHOKOZAN MINE, HIROSHIMA,

JAPAN

ANDRZEJ WIEWIORA1 AND TADATAKA HIDA2

Institute of Geological Sciences, Polish Academy of Sciences, 02-089 1 Warsaw, Al. Zwirki i Wigury

93, Poland 2 Shokozan Research Laboratory Co . Ltd., Nishihon-machi, Shobara-shi, Hiroshima, 727 Japan

(Accepted August 28, 1996)

ABSTRACT

Observation of the pyrophyllites from Nakamuraguchi and Takinotani under SEM showed typical particles morphology and sizes in the range of 0.1-1ƒÊm in thickness and several ƒÊm in diameter. Chemical analyses conducted with the use of WDS spectrometers proved classical pyrophyllite composition, with minor admixture of Fe. By IR method location of Fe3+ in octahedra has been confirmed. Based on the diffraction patterns it was demonstrated that pyrophyllite from Nakamuraguchi mainly consists of the monoclinic form whereas pyrophyllite from Takinotani consists of mixture up to 20% of the triclinic form. X-ray diffraction patterns from powdered samples, displayed sharp basal and subfamily reflections and strongly blurred polytypic reflections indicating a significant structural disorder. Transmission diffraction patterns from non-powdered needle- like aggregate displayed in addition the basal reflections of a series d00l•El=18.4A. They are due to the superstructure where d001 spacing is double of the spacing of

pyrophyllite. Simulated pattern for the superstructure displayed d00l•El=18.4A series of basal reflections also a reflection 111 at 4.16A of similar intensity to the one in experimental pattern. A mixture of 70% superstructure, 25% of monoclinic and 5% of triclinic structure gives the best match to the experimental diffractogram of

pyrophyllite from Nakamuraguchi and Takinotani. Evidently these pyrophyllites have complex composition in which apart of domains built up of triclinic one layer

per unit cell arrangement the important role plays layer pairing. Two-layer domains are created in which all layers have similar composition or every second layer has different electron density. In that latter case superstructure is observed.

Key words: Pyrophyllite, Polytype, Superstructure, Shokozan.

INTRODUCTION

In the Yano-Shokozan area of the Hiroshima Prefecture there are several pyrophyllite orebodies related to volcanic tuffs (Fig. 1). Previously they had been largely studied by

Kinosaki (1963) and Matsumoto (1968, 1979). Geology and alteration of this area are known from Watanabe et al. (1994) and Hida et al. (1996). Nevertheless, the structure of pyrophyllite itself seemed not to be recognized in full. Although the detailed structure of pyrophyllite is known from several X-ray studies 16A. Wiewiora and T. Hida X-Ray Determination of Superstructure 17

(Gruner, 1934, Rayner and Brown, 1965, Brindley and Wardle, 1970, Wardle and

Brindley, 1972, Lee and Guggenheim, 1981), from electron diffraction (Zvyagin et al ., 1979, Sidorenko et al., 1981) and HRTEM (Hillebrand et al ., 1983), practical determination of polytype in natural samples met with difficulties because frequently polytypic diffractions were completely or severely blurred, due to partly random translations induced in natural

(Chukhrov et al., 1975) and/or in laboratory conditions (Eberl, 1979, Wiewiora et al., 1993).

Naturally occurring pyrophyllite is known to have one-layer triclinic , two-layer monoclinic and/or a mixture of these two forms. Brindley and Wardle (1970) compared natural

pyrophyllites with mixtures of monoclinic pyrophyllite with 10% and 20% triclinic pyrophyllite. We used their results to quantify monoclinic and triclinic pyrophyllite in the studied samples. However the diffraction effects observed in the X-ray patterns of the Nakamuraguchi and Takinotani pyrophyllites are similar to those described by Wiewioira et al. (1993) for ground pyrophyllite from Zalamea in Spain. They indicated certain type

of layer arrangement marked by a peak at 4.16A, which was not satisfactorily explained . In addition, in a non ground sample a series of basal reflections d00l•El=18 .4A was recorded indicating superstructure in pyrophyllite from Nakamuraguchi and Takinotani . We bring the first information about the superstructure of the pyrophyllite , determined by XRD technique. To our knowledge, no such structure was determined in pyrophyllite of any other provenance so far.

MATERIALS AND METHODS

Pyrophyllites from Nakamuraguchi and Takinotani in this area have been studied . Generally, deposits of this area mainly consists of pyrophyllite and quartz accompanied

with kaolinite, sericite, alunite, diaspore , and corundum. The deposits of the area are divided into seven zones as follows: sericite , alunite, kaolin, pyrophyllite, diaspore, corundum and andalusite zones. Locations of sampling points for principal analyses are A and B of Figs. 1, 5 and 6, belonging to pyrophyllite and diaspore zone respectively . X-ray diffraction patterns made on the pyrophyllite from the Nakamuraguchi showed the content of quartz is lesser. Pyrophyllite from the Takinotani significantly contain kaolinite , diaspore and quartz. Pyrophyllite morphology was studied under scanning electron microscope JSM 840A by Jeol. Combined with Link Analytical AN 10000/85S system it has been used to

determine chemical composition in several points of the studied hand-crushed material . Both of them, the elemental profiles determined by EDS for qualitative estimation and

point quantitative analyses by WDS spectrometres (Table 1) were used for chemical determinations. The infrared spectra (IR) were recorded for sample in KBr pellet from (2mg sample and 300mg KBr) using a FTIR apparatus Nicolet , model 510P. The X-ray diffraction powder patterns were recorded using a RAD-III diffractometer of Rigaku-denki for collecting peak intensities more than 2000 cps in 2ƒÆ range 18-32•‹ CuKƒ¿ for determination of polytypes ratio according to Brindley and Wardle (1970) , and a transmission focusing diffractometer of CGR with position sensitive detector PSD -120 18A. Wiewiora and T. Hida

TABLE 1. Microprobe chemical analyses and cystallochemical formulae of pyrophyllite from Nakamuraguchi.

of Inel, France for determination of the superstructure. More detailed study of the polytypic modification of the Nakamuraguchi and Takinotani pyrophyllites was done by comparison of their diffraction patterns with simulated for the triclinic, 1AA-II, 1 and for two-layer monoclinic 2MA-V, 1 (notice indicative notation of Weiss and Durovic, 1984) polytypic modifications. X-ray diffraction powder patterns for the ideal, MDO (maximum degree of order) pyrophyllite structures were calculated using DIFK computer program (Weiss, Durovic, 1984, Wiewiora et al., 1985). The modified structure data of Lee and Guggenheim (1981) and Gruner (1934) were used. Unit cell parameters were refined independently using 23 reflections for the triclinic and 21 for monoclinic symmetries from the powder data measured on the sample from Nakamuraguchi. They are presented in Table 2 jointly with the unit cell parameters of pyrophyllite, from literature. For simulation of the triclinic pattern, the anisotropic thermal coefficients of Lee and Guggenheim (1981) and symmetry of CI space group, and for monoclinic pattern the data of Gruner (1934) with the overall isotropic temperature factor and symmetry of the C2/c space group were applied. Experimental diffractograms of the triclinic pyrophyllite from Zalamea (Badajoz, Spain) and this of 6min dry ground sample were also used for the comparison with those of the studied samples. X-Ray Determination of Superstructure 19

TABLE2. Unit cell parameters of pyrophyllite polytypes.

RESULTS

Scanning Electron Microscopy and Chemical Composition The morphology of the Nakamuraguchi pyrophyllite studied by SEM is presented in Fig. 2A. The plates have different thickness from below 0, 1ƒÊm to 1ƒÊm , rarely above this value, and several micrometers in diameter. Their shape is irregular , surfaces are flat and free of impurities. No intergrowths were observed. Most results proved Si , Al and O as the major components of the platy type material. In most cases , the chemical composition controlled by EDS profile analyses is as presented in Fig. 2B . The point quantitative

analyses (Table 1) revealed in pyrophyllite Al:Si ratio near 1:2 , some Fe and/or Ti ranging 0.012-0.023 and 0-0.023 respectively, more rarely MnO-0 .04, Cr2O3-0.002 and traces of Na, K, Ca. In some cases Al to Si ratio was found 2:1 indicating clearly andalusite type composition. Sometimes composition was close to 1:1 characteristic for kaolinite minerals. In the vast majority of cases chemical composition is limited to Al and Si cations and oxygen. According to Evans and Guggenheim (1988) in the system Al2O3- SiO2-H2O pyrophyllite may have phase boundaries with the andalusite , kaolinite and with quartz and diaspore (the both were identified in the Takinotani pyrophyllite) in temperature range 70-130•Ž. High concentrations of Ti and/or Fe have been traced , indicating rutile and/or pseudorutile , the minerals derived from ilmenite. The occurrences of impurities were not the goal of this study, therefore we limited our report to those incidentally observed.

Infrared Spectra The infrared (IR) spectra of the pyrophyllite from Nakamuraguchi are presented in 20A. Wiewiora and T. Hida

FIG. 2. A-SEM photograph of the Nakamuraguchi pyrophyllitc plates; magnification 5000•~, B-chemical composition on a central plate. X-Ray Determination of Superstructure 21

FIG. 3. IR spectra of the Nakamuraguchi pyrophyllite. Stretching and lattice vibrations are marked according with Farmer (1974). 22A. Wiewioraand T. Hida

Fig. 3. The main IR bands are assigned as shown in the figure. They are characteristic of the chemical composition presented in Table 1. All frequencies are in agreement with those presented for pyrophyllite by Farmer (1974). They prove nearly ideal composition, with very small octahedral substitution of aluminum by the trivalent iron and possibly by tetravalent titanium as shown by very little band at 3646cm-1 (Fig. 3A) assigned to AlFe3+-OH and/or AlTi4-OH stretching vibration. Both the chemical elements were determined by EDS analyses under scanning electron microscopy (Table 1).

X-ray powder diffractometry X-ray diffraction patterns from the Nakamuraguchi and Takinotani HF solution (25 wt%) treated pyrophyllite are shown in Fig. 4. It was estimated from the figure that pyrophyllite from the Nakamuraguchi mainly consists of the monoclinic form whereas pyrophyllite from the Takinotani consists of mixture with up to 20% of the triclinic form. Distribution of the alteration zones with different ratios of monoclinic to triclinic pyro-

FIG. 4. Expanded diffraction patterns of monoclinic and triclinic pyrophyllites, recorded with CuKa radiation and goniometer speed 1/4•‹2ƒÆ/min: A-pyro-

phyllite from the Takinotani with more than 20% triclinic form, B-pyrophyllite from the Takinotani with 10% to 20% triclinic form and C-nearly pure monoclinic pyrophyllite from the Nakamuraguchi. Q-quartz. X-Ray Determination of Superstructure 23 24A. Wiewiora and T. Hida X-Ray Determination of Superstructure 25

phyllite in the Nakamuraguchi and Nishiyama-higashi, and Takinotani orebodies is shown in Figs. 5 and 6 respectively. Most of pyrophyllite from the Nakamuraguchi , Nishiyama- higashi is the monoclinic form, although a few pyrophyllites show the admixtures of 10-20% triclinic form. In the Takinotani pyrophyllite of diaspore zone the triclinic form is up to and more than 20%. The content of triclinic form is higher than in the Nakamuraguchi and Nishiyama-higashi orebodies. According to phase relationship of Al2O3-SiO2-H2O system by Hemley et al . (1980), corundum and andalusite are formed in the conditions of relatively high temperature . Based on this we estimated that the Nakamuraguchi and Nishiyama-higashi orebodies were formed in the conditions of higher temperature than the Takinotani one. Therefore , it is considered that the triclinic pyrophyllite was formed in the conditions of lower temperature as compared with the formation temperature of the monoclinic form. The current identification of pyrophyllite polytypes is based upon the use of JCPDS 12-0203 card for monoclinic pyrophyllite and JCPDS 25-0022 card for the triclinic one. The identification of the triclinic pyrophyllite by using X-ray powder diffraction is unquestionable. Triclinic (1Tc=1AA-II, 1) structure was successfully determined by the powder method (Wardle and Brindley, 1972) and fully refined by the single crystal method (Lee and Guggenheim, 1981). With the structure data it may be easily calculated the powder diagram to be compared with the experimental . This makes polytype identification reliable. The identification of the monoclinic polytype seems less realistic as it is based on application of the JCPDS 12-0203 data which comes from the identification of natural pyrophyllite from Honami, Nagano Prefecture, Japan. Apart from Gruner's (1934) data and partly successful analysis of Rayner and Brown (1966), no newer X-ray powder data on monoclinic pyrophyllite is available. Contrary to this some doubts relating to monoclinic symmetry of the Honami pyrophyllite were evoked by Wiewiora et al. (1993) , who demonstrated that 6 minutes of dry grinding of the triclinic Spanish pyrophyllite from Zalamea (Badajoz area) induced layers displacement along b unit cell edge. Subsequently this treatment changed the X-ray powder diagram and made it identical with the pattern of the Honami monoclinic pyrophyllite. The studied powdered sample of pyrophyllite from the Nakamuraguchi (Fig. 7C) gives X-ray pattern similar to that of the 6min ground Zalamea pyrophyllite (Fig. 7B) and to this of the Honami (Brindley, Wardle, 1970, Wiewiora et al., 1993). Direct comparison of the expanded spectra of the pyrophyllite from Nakamuraguchi (Fig. 7C) and Takinotani (Fig. 8C) with that of the natural triclinic pyrophyllite from Zalamea (Fig.7A) proved presence of the weak, residual reflections (e.g. 0.21, d=4.046; 022, d=3.17; 112, d=2.958) indicating presence of the domains characterized by the stacking order common for 1AA-II, 1 polytype. Apart from reflections belonging to the extraneous phases such as diaspore, quartz and kaolinite present in the Takinotani sample-there was a broad band with the maximum at 4.16A (Fig. 8C), which cannot be attributed neither to the extraneous phases nor to triclinic pyrophyllite . This peak was evidently considered to be due to monoclinic pyrophyllite (Fig. 4). To verify such identification, expanded X-ray diffractogram of the Nakamuraguchi pyrophyllite were compared with the simulated diffractograms of the triclinic (1AA-II , 1) and monoclinic 26A. Wiewiora and T. Hida

FIG. 7. Transmission diffraction patterns recorded with CoKƒ¿1 radiation and position sensitive detector PSD-120 (Inel, France) of the powder samples: A-natural triclinic pyrophyllite from Zalamea (Badajoz, Spain), B-6' ground triclinic pyrophyllite from Zalamea, C- Nakamuraguchi pyrophyllite. Q-quartz, R-rutile.

(2MA-V, 1) polytypes of pyrophyllite (Fig. 9). This comparison showed that the peak at 4.16A is not common for the triclinic structures of the pyrophyllite and it has low intensity for the monoclinic.

X-ray Study of the Non powdered Samples X-ray diffractograms from natural non-grounded Nakamuraguchi and Takinotani

pyrophyllite samples, prepared in a form of a needle 0.5mm in diameter cut directly from the rock-body are presented in Fig. 10. These show reflections additional to those recorded from the powdered sample (Fig. 7C). The most suprising is the appearance of the reflections forming series with d00l•El=18.4A which is exactly the double value of d(001)=9.2A common for the most known pyrophyllites. Apart from reflections 001 at 18.4A, and 003 at 6.11A, all the other reflections including reflection at 4.17A were

present on the diffractograms obtained from the powdered samples (Figs 7, 8 and 9). Similar width of all the basal reflections, odd and even orders, proves clearly that the observed reflections are due to the same structure. Appearance of the reflections at 18.4A, 6.11A, i.e. odd series of 18.4A apart from even series similar to those which are common for the known diffraction pattern of pyrophyllite, indicates superstructure with the unit cell composed of two layers differing each with respect to their crystal-chemical

properties. The appropriate structural model must be found to explain superstructure. Three such models were considered, which may assure odd, in addition to even, order X-Ray Determination of Superstructure 27 28 A. Wiewiora and T. Hida X-Ray Determination of Superstructure 29

FIG. 10. Simulated diffraction patterns for the structural models: A-pyrophyllite superstructure

(see text for explanation), B-mixed diffractogram of 0.7 superstructure +0.25 monoclinic and 0.05 triclinic pyrophyllite polytypes,and experimental: C and D-transmission diffraction

patterns recorded with CoKƒ¿1 radiation and position sensitive detector PSD-120 (Inel, France) of non-ground, needle-like (0.5 in diameter) monolite of the Takinotani and Nakamuraguchi

pyrophyllites, respectively.

reflections with d001=18.4A. The first model incorporated regular interstratification of the 2:1 (pyrophyllite type) layers with two bayerite layers and the second one-the regular interstratification of the donbassite (di-dioctahedral chlorite) type layer with one bayerite type layer. Chemically the both models were equivalent. The unit cell parameters of donbassite and pyrophyllite were very close to each other: pyrophyllite: a=5.171 b=8.952 ƒÀ=99.93•‹ donbassite: a=5.174 b= 8.956 ƒÀ=97.83•‹(Alexandrova et al., 1972)

Assuming the overall triclinic symmetry and refined unit cell parameters from the experimental diffractogram may be easily simulated the diffraction patterns showing series of the basal reflections with d001=18.4A. The relative distance between tetrahedral oxygens and neighbouring hydroxyls equal 0.15 in pyrophyllite, smaller than in donbassite, equaling

0.195, translated into identical value of 2.79A in absolute scale for the both species. This is within the limits characteristic for hydrogen bonds . These two models require Al: Si ratio equal 1.5:2, higher than 1:2 characteristic for pyrophyllite composition. Such ratio was not encountered in our chemical determinations. Also , simulated diffractogram do not match well to the experimental one.

The third model assumes interlayering of the essentially pyrophyllite type (2:1) layers 30A. Wiewiora and T. Hida

of differing crystal-chemical properties. The differences are in the electron densities of the structural sheets. These were achieved by introducing 0.1 Fe3+ into octahedra of the every second layer to obtain the effect of the superstructure. The simulated diffraction pattern is presented in Fig.10A. In the same figure the experimental diffraction patterns of the Takinotani (Fig. 10C) and Nakamuraguchi (Fig. 10D) pyrophyllites can be found. The two diffraction patterns are characterized by the presence of the series of reflections with d(001)=18.4A. The reflection at 4.16A and several other reflections have similar intensities in both the diffractograms. Some reflections, namely 022, 024, 113, 113, 133 are present only in the simulated pattern. It was possible that structural order in natural samples was not fair enough to give rise to this series of reflections. Some weak reflections present in the diffraction pattern of natural samples can. be explained by admixture of normal monoclinic and triclinic polytypes. The best match between diffractograms of natural and simulated structures was obtained assuming mixed diffractogram composed of 70% superstructure +25% 2MA-V, 1 and 5% 1AA-II, 1 polytypes (Fig. 10B and Table 3).

DISCUSSION AND CONCLUSIONS

Experimental observation of Eberl (1979) showed the triclinic pyrophyllite polytype formed in temperature over 375•Ž, pressure of 2kb and varying Al/Si ratio. On the other hand, Kitagawa (1992) suggested that pyrophyllite crystals grew from hydrothermal solution below 375•Ž rather than from vapor phase, on the bases of TEM observation of

green pyrophyllite from Nishiyama. Data of Evans and Guggenheim (1988) also indicated temperature lower than 375•Ž. Generally, it was known that the triclinic polytypes of clay minerals, e.g. such as kaolinite and sericite crystallized in lower temperature than their monoclinic analogues. Based on mineral assemblages which accompany pyrophyllite in the Nakamuraguchi and Takinotani orebodies we found pyrophyllite to stick to this pattern. Namely, the Nakamuraguchi and Nishiyama-higashi orebodies containing mostly monoclinic pyrophyllite were formed in the conditions of higher temperature than the Takinotani one. The pyrophyllites from Nakamuraguchi and Takinotani present X-ray reflections due to superstructure (d00l•El_??_18.4A and d(111)_??_4.17A), reflections characteristic for 2MA- V, 1 and 1AA-II, 1 polytypes, and bear symptoms of disordering. Polytypic reflections are blurred or smeared, subfamily reflections (20/, 13/) are sharp and strong. Although matching with mixed diffractogram (Fig. 10) is not ideal it is reasonably good, especially in the regions non sensitive to structural disorder. It is believed that the pyrophyllites from Nakamuraguchi and Takinotani have structure composed of triclinic one-layer stacking, domains with paired layers of the same electron densities (two-layer monoclinic

polytype), and domains with paired layers of non completely equal electron densities. Such a mixed structure gives the diffraction pattern characteristic to statistical averaging of the diffraction effects. It should be noted that coexistence of one-layer triclinic and two-layer monoclinic sequences were observed by high resolution transmission electron microscopy HRTEM in pyrophyllite from Wallis (Switzerland) (Hillebrand et al., 1983). This complex structure reflects varying environmental conditions, possibly marked by X-Ray Determination of Superstructure 31

TABLE3. X-ray data for non powdered samples of pyrophyllite . Reflections of extraneous phases (kaolinite, gibbsite, ilmenite) are not included 32A. Wiewiora and T. Hida X-Ray Determination of Superstructure 33

*Triclinic polytype 1AA-II, 1; **monoclinic polytype 2MA-V,1; d(P), interlayer spacing from peak position; Irel (P), peak relative intensity; and Ss, superstructure. 34A. Wiewiora and T. Hida varying chemical composition of the polysomatic solutions which were responsible for the crystallization of the pyrophyllite in the Nakamuraguchi and Takinotani orebodies.

ACKNOWLEDGEMENTS

Dr. Daizo Ishiyama of Akita University, and Dr. Ryuji Kitagawa of Hiroshima University made valuable comments on earlier drafts of this paper. The authors wish to thank Dr. Kanzo Matsumoto for his helpful advice. Special thanks are due to Mrs. Dorota Grabska and Mr. Michal Kuzniarski of the Institute of Geological Sciences PAS in Warszawa for technical assistance.

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