High-pressure phases of Co2GeO4, Ni2GeO4, Mn2GeO4 and MnGeO3: implications for the germanate-silicate modeling scheme and the Earth's mantle |
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Authors: | Lin-Gun Liu |
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Institution: | Research School of Earth Sciences, Australian National University, Canberra, A.C.T. Australia |
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Abstract: | In a diamond-anvil press coupled with YAG laser heating, the spinels of Co2GeO4 and Ni2GeO4 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 MnGeO3 transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a0 = 5.084 ± 0.002, b0 = 5.214 ± 0.002, and c0 = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO3 is ?6.6%. Mn2GeO4 disproportionates into a mixture of the perovskite phase of MnGeO3 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. |
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