On the basis of thermochemical data for olivine (α), modified spinel (β), and spinel (&ggr;) polymorphs of Mg2SiO4,Fe2SiO4, and Co2SiO4, the binary Mg2SiO4-Fe2SiO4 and Mg2SiO4-Co2SiO4 phase diagrams are calculated. From Mg/Fe partitioning data between olivine-garnet β-phase-garnet, olivine-pyroxene, and β-phase-pyroxene, thermochemical parameters for β-Fe2SiO4 are estimated. The β-Fe2SiO4 is highly metastable with respect to &ggr;-Fe2SiO4. The calculated binary Mg2SiO4-Fe2SiO4 phase diagram shows a narrow field of α+β and a broad field of β+&ggr;, in reasonable agreement with experimental phase relations. The composition range over which β phase solid solution is stable expands with increasing temperature but is generally limited to Mg/(Fe+Mg)>0.8. In the Mg2SiO4-Co2SiO4 system, the β phase is calculated to occur only at Mg/(Co+Mg)>0.05 and Mg/(Co+Mg)>0.6, with α and &ggr; coexisting at intermediate compositions. Using the above phase relations in Mg2SiO4-Fe2SiO4 and assuming an adiabatic geotherm, temperature are calculated to range from about 1573 to 1700 K at depths of 400-520 km for a mantle of lherzolite composition.<