High-pressure phase relations in the system CaAl 4Si 2O 11–NaAl 3Si 3O 11 with implication for Na-rich CAS phase in shocked Martian meteorites

Abstract

High-pressure phase relations in the system NaAl 3Si 3O 11–CaAl 4Si 2O 11 were examined at 13–23 GPa and 1600–1900 °C, using a multianvil apparatus. A Ca-aluminosilicate with CaAl 4Si 2O 11 composition, designated CAS phase, is stable above about 13 GPa at 1600 °C. In the system NaAl 3Si 3O 11–CaAl 4Si 2O 11, the CAS phase dissolving NaAl 3Si 3O 11 component coexists with jadeite, corundum and stishovite below 22 GPa, above which the CAS phase coexists with Na-rich calcium ferrite, corundum and stishovite. At 1600 °C, the solubility of NaAl 3Si 3O 11 component in the CAS solid solution increases with increasing pressure up to about 50 mol% at about 22 GPa, above which the solubility decreases with pressure. The maximum solubility of NaAl 3Si 3O 11 component in the CAS phase increases with temperature up to around 70 mol% at 1900 °C at 22 GPa. The dissociation of NaAlSi 2O 6 jadeite to NaAlSiO 4 calcium ferrite plus stishovite occurs at about 22 GPa. Lattice parameters of the CAS phase with the hexagonal Ba-ferrite structure change with increase of the NaAl 3Si 3O 11 component: a-axis decreases and c-axis slightly increases, resulting in decrease of molar volume. Enthalpies of the CAS solid solutions were measured by high-temperature drop-solution calorimetry techniques. The results show that enthalpy of hypothetical NaAl 3Si 3O 11 CAS phase is much higher than the mixture of NaAlSi 2O 6 jadeite, corundum and stishovite and is close to that of the mixture of NaAlSiO 4 calcium ferrite, corundum and stishovite. When we adopt the Na:Ca ratio of 75:25 of the natural Na-rich CAS phase in a shocked Martian meteorite, Zagami, the phase relations determined above suggest that the natural CAS phase crystallized from melt at pressure around 22 GPa and temperature close to or higher than 2000–2200 °C. The inferred P, T conditions are consistent with those estimated using other high-pressure minerals in the shocked meteorite.