The enthalpies of formation from the oxides of Mg2SnO4 and Co2SnO4 were found by oxide melt solution calorimetry to be +1.13 ± 0.48 kcal/mol and -2.31 ± 0.28 kcal/mol, respectively. Using these data, the slopes, ∂P/∂T, for disproportionation of these spinels to the component oxides at high pressure were calculated to be +30.4 ± 4.2 bar/K for Mg2SnO4 and -10.3 ± 2.4 bar/K for Co2SnO4, in general agreement with the data of Jackson et al. (1974a,b). Using thermochemical data for the formation of olivines, for olivine-spinel transitions and for the transformation of quartz to stishovite, we calculate pressures for the disproportionation of silicate spinels to be in the range 150-200 kbar. Calculated slopes ∂P/∂T for the disproportionation reactions are -10.7, -24.9, -11.2, and +7.6 bar/K for Mg2SiO4, Fe2SiO4, Co2SiO4, and Ni2SiO4. The large negative slope calculated for Fe2SiO4 results from a surprisingly large positive slope reported for the olivine-spinel transition in that compound (Akimoto et al., 1969). Further consideration of the systematic trends in the thermodynamics of spinel formation from the oxides suggests that the silicate spinels should have entropies of formation close to zero, resulting in values of ∂P/∂T which are zero or at most only slightly negative. This confirms the conclusion of Jackson, Liebermann, and Ringwood that values of ∂P/∂T for spinel disproportionation are unlikely to be more negative than -10 bar/K and may well be slightly positive. Reaction of spinels to form other post-spinel phases, particularly ilmenite and perovskite, are discussed in terms of available thermochemical data.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science