SILICATE ILMENITES AND THE POST-SPINEL TRANSFORMATIONS

Abstract High-pressure phase relations of several ferromagnesian silicates were investigated, using a double-stage split-sphere type apparatus. Clinoenstatite disproportionates at about 170 kbar and 1000°C into stishovite plus β-Mg 2 SiO 4 , which is replaced by γ-Mg 2 SiO 4 at 190 kbar. The γ-Mg 2 SiO 4 reacts with stishovite to form the MgSiO 3 ilmenite at about 260 kbar. The hexagonal unit cell dimensions of MgSiO 3 ilmenite were determined to be a = 4.7284 (4) a and c = 13.5591 (16) a. At 1000°C the ZnSiO 3 clinopyroxene transforms directly into ilmenite at 120 kbar, and s-Zn 2 SiO 4 disproportionates into ZnSiO 3 (ilmenite) plus ZnO (rocksalt) at 130 kbar. The hexagonal unit cell dimensions of ZnSiO 3 ilmenite were determined to be a = 4.7462 (3) a and c = 13.7569 (10) a. ZnSiO 3 (ilmenite) was found to decompose into ZnO (rocksalt) plus stishovite at about 200 kbar. It is interesting to note that silicate ilmenite is nonexistent in transition-metal compounds. The γ phases of Ni 2 SiO 4 , Co 2 SiO 4 , and Fe 2 SiO 4 decompose directly into their oxide mixture (rocksalt plus stishovite). Neither s- nor γ-Mn 2 SiO 4 exists, and α-Mn 2 SiO 4 disproportionates into MnO (rocksalt) plus MnSiO 3 (garnet), which decomposes into MnO (rocksalt) plus stishovite at about 220 kbar. These experimental results indicate the possibility that ilmenite and related minerals are significant constituents in the lower-half part of the transition zone, provided the Mg/Fe value is high.