X-ray determination of superstructure of pyrophyllite from Yano-Shokozan mine, Hiroshima, Japan

Abstract

Observation of the pyrophyllites from Nakamuraguchi and Takinotani under SEM showed typical particles morphology and sizes in the range of 0.1-1μm in thickness and several μm 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 Fe 3+ 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 d 001 .l=18.4A. They are due to the superstructure where d 001 spacing is double of the spacing of pyrophyllite. Simulated pattern for the superstructure displayed d 001 .l= 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 onc 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 densitv. In that latter case superstructure is observed.