Silicate and Germanate Garnets, Ilmenites and Perovskites: Thermochemistry, Lattice Vibrations, and Spectroscopy
- Murli H. Manghnani and
- Yasuhiko Syono
Published Online: 21 MAR 2013
Copyright © 1987 by Terra Scientific Publishing Company (TERRAPUB), Tokyo.
High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto
How to Cite
Navrotsky, A. (1987) Silicate and Germanate Garnets, Ilmenites and Perovskites: Thermochemistry, Lattice Vibrations, and Spectroscopy, in High-Pressure Research in Mineral Physics: A Volume in Honor of Syun-iti Akimoto (eds M. H. Manghnani and Y. Syono), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM039p0261
- Published Online: 21 MAR 2013
- Published Print: 1 JAN 1987
Print ISBN: 9780875900667
Online ISBN: 9781118664124
- Mineralogy and Crystal Chemistry;
- Phase transformations;
- High Pressure-High Temperature Research
Thermochemical studies by high-temperature calorimetry have been performed on the high-pressure polymorphs of CaGeO3, CdGeO3, MgGeO3, MnSiO3, and on MgSiO3 ilmenite. The enthalpies of transition show systematic trends. The enthalpies of transition among pyroxene and pyroxenoid polymorphs are uniformly small. The pyroxenoid (or pyroxene) to garnet transition has a ΔH° of less than 10 kJ mol−1 in magnitude for CaGeO3 and CdGeO3, 36 kJ mol−1 in MnSiO3 and (estimated) 50–60 kJ mol ' in MgSiO3. The entropies of these transitions, obtained both by combining calorimetry and phase studies and from lattice-vibrational modelling using Kieffer's approach, are small and negative, confirming shallow positive P-T slopes. The transition from pyroxene to ilmenite occurs stably in MgGeO3, with ΔH°=6 kJ and metastably (with β+stishovite or γ+stishovite as intervening phases) in MgSiO3 with ΔH°=70 kJ. It has small negative ΔS° values but MgSiO3 ilmenite may have a slightly higher entropy than a mixture of spinel and stishovite. The transition pyroxenoid (or pyroxene) to perovskite has ΔH° near 40 kJ mol−1 for CaGeO3, CdGeO3, and MgGeO3. The garnet to perovskite transition has a positive ΔS° for CaGeO3. The ilmenite to perovskite transition in CdGeO3 has a positive ΔS°, as is probably also the case for the ilmenite to perovskite transition in MgSiO3. The vibrational modelling supports this conclusion and the idea that perovskite-forming transitions have negative P-T slopes.