Abstract
Using single crystals of synthetic well-ordered orthoenstatite, MgSiO3, the ortho-to-proto inversion has been studied with precession photography and the crystal structure of protoenstatite has been reinvestigated based on the diffraction-intensity data collected at 1080°C on a four-circle diffractometer. The results include: (1) A series of precession photographs, taken after heating crystals for various durations at 1000°C, have revealed that the inversion is preceded by the occurrence of an intermediate structure, presumably having the ortho type but with stretched silicate chains like those of protoenstatite. (2) A well-ordered protostructure can be readily obtained by heating the orthoenstatite crystals at 1300°C. (3) The crystal structure at the above temperature has cell dimensions a = 9.306(6) Å, b = 8.892(7) Å; c = 5.349(4) Å, and space group Pbcn. (4) The silicate tetrahedra are not distorted as are those of the reported structure (Smyth, Z. Kristallogr. 134, 262, 1971) giving a mean Si – O bond length of 1.626(6) Å and an O(3′)–O(3)–O(3”) angle of 167.5(6)°. (5) The mean Mg – O length, uncorrected for the thermal motion of the protostructure, is 1.9% larger than that of the orthostructure, while the mean Si – O is 0.5% smaller.
