High-pressure and high-temperature phase relations in CaSiO3 and CaMgSi2O6 and elasticity of perovskite-type CaSiO3

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Phase transformations in CaSiO3 and CaMgSi2O6 were studied up to ~ 50 GPa using both high-pressure in situ X-ray diffraction study and quench experiments. A new intermediate high-pressure and high-temperature phase named CaSiO3-III was found in the pressure range from 10 to 13.8 GPa, above which a perovskite-type CaSiO3 was stable. Both CaSiO3-III and perovskite-type CaSiO3 are unquenchable. The compression curve of perovskite-type CaSiO3 was determined up to 31.5 GPa at room temperature. Least-squares fitting of the Birch-Murnaghan equation of state gave the following values for the bulk modulations and a zero-pressure volume: K0 = 325 +/- 10 GPa and V0 = 45.58 +/- 0.07 Å3 where dK0/dP was assumed to be 4. This bulk modulus, however, is very sensitive to the choice of the zero-pressure volume and has large uncertainty. CaMgSi2O6 was found to break down into CaSiO3 and MgSiO3 at 17 GPa. CaSiO3 crystallizes into cubic perovskite-type structure while MgSiO3 breaks down further into Mg2SiO4 plus SiO2. With increasing pressure, the last transforms into ilmenite, and then into perovskite structure, while the coexisting perovskite-type CaSiO3 remains unchanged. Solubilities of both Ca and Mg in MgSiO3 and CaSiO3 perovskites are found to be very low.

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