High temperature x-ray study on a single crystal of humite, Mg7Si3O12(OH,F)2 up to 1023 K

Department of Mineralogy, Petrology and Economic Geology, graduate school of Science, Tohoku University
â—‹Keiko Sumioka Takahiro Kuribayashi Yasuhiro Kudoh

The specimen used in this study was a natural humite from Tilley Foster mine, U. S. A.. Chemical composition determined by electron micro probe analyzer (JEOL, JXA-8800M) yeilds the formula, (Mg6.46Fe0.37Ti0.03Mn0.02)6.88Si3.00O12(F0.99OH0.95O0.06)2.00. The unit cell pasrameters and x-ray diffraction intensities of a single crystal of 180x250x250 μm3 placed in a silica capillary at 296K, 423 K, 573 K, 723 K, 873 K and 1023 K were measured by an imaging plate x-ray diffractometer (RIGAKU, R-AXIS IV) equipped with rotating anode (MoKα, 50kVx89mA) and a U-shaped resistance heater (Huber: High temperature attachment Type 231). The thermal expansion coefficients along the crystal axes up to 1023 K are αa=9.4(3)x10-6 K-1, αb=12.9(2)x10-6 K-1 and αc=11.9(3)x10-6 K-1. The expansion coefficients are in the order αb c a indicating that the thermal expansion coefficient of the direction perpendicular to the closest packing oxygen layer is smallest (Figure). The structure refinement were carried out using SHELEXL 97 (Sheldrick, 1997). The residual idexes (%) at 296K, 423 K, 573 K, 723 K, 873 K and 1023 K were 2.06, 2.85, 2.90, 2.55, 2.78 and 3.37, respectively.