Elasticity and phase equilibria of spinel disproportionation reactions

Abstract

Summary Ultrasonic data for the velocities of the low- and high-pressure polymorphs of stannate and titanate compounds undergoing the spinel rocksalt + rutile (Mg2SnO4, Co2SnO4) and spinel rocksalt + ilmenite (Mg2TiO4, Co2TiO4) phase transformations have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 60 kbar. The inverse stannate and titanate spinels have significantly lower velocities (elastic moduli) for a given density (molar volume) than the normal A2+2B4+O4 spinels. The observed elasticity moduli for the mixed phase assemblages are in good agreement with theoretical calculations using the Hashin—Shtrikman scheme and the known properties of the individual phases. The velocity (n) - density (ρ) relationships across the spinel disproportionation reactions are critically dependent on the nature and extent of the changes in cation–anion coordination and nearest-neighbour interionic distances which accompany these transformations. For both the stannate and titanate spinels, the coordination of the divalent cation increases on disproportionation. However, in the stannate case the mean interionic distances are unchanged and the n–ρ slopes (linear or logarithmic) are similar to those for the quartz–coesite transition in SiO2 and the olivine–spinel and olivine–beta phase transformations in silicates and germanates. In contrast, the mean interionic distances increase on disproportionation for the inverse titanate and normal silicate, germanate, aluminate and ferrite spinels so that their n–ρ slopes are low and comparable to those observed for the coesiterutile, pyroxene–garnet, pyroxene–ilmenite and ilmenite–perovskite transformations. Experimental studies of the phase boundaries for the disproportionation of the spinels Mg2 SnO4, Co2 SnO4 and Li2 NiF4 into mixtures of their cornponent oxides (fluorides) reveal slopes dP/dT of 40 ° 10, 0 ± 7 and ± bar/K, respectively. These non-negative slopes are consistent with a considerable body of thermodynamic data for spinels. Available thermodynamic data do not support the existence of a distinctly negative slope for the proposed disproportionation of (normal) Mg2, SiO4, spinel. Evidence from X-ray and phase equilibrium studies suggest the possibility that Si4+, Mg2+ and Fe2+ share the octahedral sites in silicate spinels under mantle conditions. The consequences of such cation disorder would be a positive slope accompanied by anomalous elasticity–density behaviour within the transition zone if the 650-km discontinuity were caused by disproportionation of spinel into mixed oxides. The implications for the thermomechanical regime of descending lithospheric plates are also examined.