Spinelloid phases in the system MgGa2O4-Mg2GeO4 |
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| Authors: | J Barbier B G Hyde |
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| Institution: | 1. Research School of Chemistry, The Australian National University, G.P.O. Box 4, 2601, Canberra, A.C.T., Australia
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| Abstract: | Spinelloid phases have been observed and characterized by powder X-ray diffraction and high-resolution electron microscopy. Mg3Ga2GeO3(III), with a narrow composition range of approximately 3 mole percent Mg2GeO4, is stable at atmospheric pressure up to about 1,420° C, and is isostructural with β-Mg2SiO4 and the spinelloid Phase III of the NiAl2O4-Ni2SiO4 system. This represents the first occurrence of a β-phase structure stable at 1 atm pressure. Above 1,420° C (1 atm) Mg3Ga2GeO8 (III) decomposes reversibly into a mixture of spinel and olivine. At high pressure (around 30 kbar at 1,100° C) it transforms into another spinelloid phase, Mg3Ga2GeO8 (IV), isostructural with Phase IV of the NiAl2O4-Ni2SiO4 system. In terms of crystal structures and phase relations therefore there exists a close analogy between the magnesium gallo-germanate and nickel alumino-silicate systems, the former being a lower-pressure analogue of the latter. Our investigation of a number of other pseudo-binary spinel-olivine oxide systems suggests that the formation of spinelloid phases can be associated with the inverse character of the spinel component. |
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