Spinel disproportionation at high pressure: calorimetric determination of enthalpy of formation of Mg₂SnO₄ and Co₂SnO₄ and some implications for silicates

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.