Formation of hexagonal NaAl3Si3O11 (NAS) phase, the Na end-member of hexagonal CaAl4Si2O11 (CAS) phase, near 23 GPa above 2373 K in the compositions of NaAl3Si3O11 and NaAlSi3O8

Abstract

Phase relations of NaAl3Si3O11 and NaAlSi3O8 were explored at pressures of 20.5–23.5 GPa and temperatures of 2373 and 2573 K by multianvil experiments. For NaAl3Si3O11, a new phase, which is tentatively named NAS phase, was obtained at 22.5 GPa, 2373 and 2573 K and also at 23.5 GPa, 2573 K. NAS phase was found to be isostructural to CaAl4Si2O11 (CAS) phase, but have smaller lattice parameters. The other phase assemblages obtained for NaAl3Si3O11 at 2373 K are jadeite + corundum + stishovite and NaAlSiO4 calcium ferrite-type (CF) phase + corundum + stishovite. Our results suggest that NAS phase is stable around 23 GPa above 2373 K, while its stability region is much narrower than that of CAS phase in both pressure and temperature. Based on our results, the CAS–NAS solid solution with a Na/Ca ratio close to that of the Na-rich CAS phase (74:26) found in the shocked Martian meteorites can be stabilized around 23 GPa at temperatures below 2373-2573 K. For NaAlSi3O8, the phase assemblages of jadeite + stishovite and CF phase + stishovite were obtained at 20.5-23.5 GPa and 2373 K, while partial melting occurred at 2573 K, accompanied by the liquidus phase assemblages of stishovite + NAS phase and stishovite + corundum at 22.5 and 23.5 GPa, respectively. Stishovite and NAS phase are the residual solid phases in the partial melting of NaAlSi3O8. This result is supportive to the previous inference that the phase assemblage of stishovite + Na-rich CAS phase observed in the melt pockets of the shocked Martian meteorites was formed by a rapid crystallization of a high-pressure labradorite melt.