High-pressure phase relations and crystal chemistry of calcium ferrite-type solid solutions in the system MgAl2O4-Mg2SiO4

Abstract

To map the stability field of calcium ferrite-type MgAl2O4-Mg2SiO4 solid solutions, high-pressure phase relations in the system MgAl2O4-Mg2SiO4 were studied in the compositional range of 0 to 50 mol% Mg2SiO4. The calcium ferrite solid solutions are stable above 23 GPa at 1600 °C, and the maximum solubility of Mg2SiO4 component in MgAl2O4 calcium ferrite is 34 mol%. Lattice parameters and unit-cell volume of calcium ferrite-type MgAl2O4 (space group Pbnm) determined by Rietveld analysis are a = 9.9498(6) Å, b = 8.6468(6) Å, c = 2.7901(2) Å, and V = 240.02(2) Å3. Lattice parameters for the MgAl2O4-Mg2SiO4 solid solutions with the compositions of 14, 24, and 34 mol% Mg2SiO4 indicated the following compositional dependency of lattice parameters: a (Å) = 9.9498 + 0.1947·XMg₂SiO₄, b (Å) = 8.6468 . 0.1097·XMg₂SiO₄, and c (Å) = 2.7901 + 0.0086·XMg₂SiO₄, where XMg2SiO4 is the mole fraction of Mg₂SiO₄ component. A linear extrapolation of the composition-molar volume relationship gave an estimated volume of 36.49(2) cm3/mol for the hypothetical calcium ferrite-type Mg2SiO4. This value is larger than that of the isochemical mixture of MgSiO3 perovskite and MgO, 35.72(1) cm3/mol. This implies that the mixture of MgSiO3 perovskite and MgO is more stable than the hypothetical calcium ferrite-type Mg2SiO4 under the lower mantle conditions.