THE POST-SPINEL PHASES OF TWELVE SILICATES AND GERMANATES

Abstract

With the exception only of Mn2SiO4, the orthosilicates and orthogermanates of Mg, Mn, Fe, Co, Ni, and Zn all possess the spinel structure (or a distorted spinel structure) at some P-T conditions. The post-spinel phases of these compounds have been investigated to loading pressures of about 250–320 kbar at 900–1800°C in a diamond-anvil cell heated by a laser. With the exceptions of Mn2SiO4 and Zn-compounds, two distinct groups of the post-spinel phases have been found: (1) Mg2SiO4, Mg2GeO4, and Mn2GeO4 transform ultimately into an assemblage of orthorhombic perovskite plus rocksalt phases without the intervention of the assemblage of mixed oxides; and (2) Fe, Co, and Ni compounds, on the other hand, transform into their component oxides with the rocksalt and rutile structures without transforming further into the assemblage perovskite plus rocksalt phases (the olivine form of Mn2SiO4 has also been found to react to the mixed oxides). The β-Zn2SiO4 transforms to ZnSiO3 (ilmenite) plus ZnO (rocksalt) and ultimately to its component oxides with the rocksalt and rutile structures. The distorted spinel form of Zn2GeO4 transforms to the assemblage of an as yet unidentified orthorhombic phase of ZnGeO3 plus ZnO (rocksalt), and disproportionates into its component oxides at still higher pressure.