High-pressure phases of Co 2GeO 4, Ni 2GeO 4, Mn 2GeO 4 and MnGeO 3: implications for the germanate-silicate modeling scheme and the Earth's mantle

Abstract

In a diamond-anvil press coupled with YAG laser heating, the spinels of Co 2GeO 4 and Ni 2GeO 4 have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO 3 transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a 0 = 5.084 ± 0.002 , b 0 = 5.214 ± 0.002 , and c 0 = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO 3 is −6.6%. Mn 2GeO 4 disproportionates into a mixture of the perovskite phase of MnGeO 3 plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.