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 mono- clinic 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 pyro- phyllite. 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 mono- clinic 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