X-ray studies on protoenstatite

Abstract

Abstract Diffraction intensity data of well ordered protoenstatite, MgSiO3, inverted from orthoenstatite, were collected at 1260°C and 1360°C by means of four-circle diffractometry using a gas flame heater. The effect of temperature on the structure of protoenstatite was studied based on these data and those at 1080°C (Murakami, Takeuchi and Yamanaka, 1982). The results revealed the following notable features of the structure of protoenstatite at high temperatures up to near the incongruent melting point: (1) The variations of the b axis and the unit cell volume as a function of temperature were approximately linear. (2) The expansion of the c axis terminated at around 1300°C, whereas that of a markedly enhanced at temperatures above 1300°C. (3) The structure underwent a minor sudden change at around 1300°C; the O1 atoms were displaced approximately towards the Si atoms along the a axis perpendicular to the oxygen layers, and the O2 atoms, towards the Mg atoms along the same direction. The Si – O(1) and Si – O(2) bond distances uncorrected for thermal motion are 1.55(2) Å and 1.64(1) Å, respectively, at 1360°C.