HIGH-PRESSURE TRANSITIONS IN THE SYSTEM KALSI3O8-NAALSI3O8

Phase relations in the system KAlSi3O8-NaAlSi3O8 have been examined at pressures of 5-23 GPa and temperatures of 700-1200-degrees-C. KAlSi3O8 sanidine first dissociates into a mixture of wadeite-type K2Si4O9, kyanite and coesite at 6-7 GPa, which further recombines into KAlSi3O8 hollandite at 9-10 GPa. In contrast, NaAlSi3O8 hollandite is not stable at 800-1200-degrees-C near 23 GPa, where the mixture of jadeite plus stishovite directly changes into the assemblage of calcium ferrite-type NaAlSiO4 plus stishovite. Phase relations in the system KAlSi3O8-NaAlSi3O8 at 1000-degrees-C show that NaAlSi3O8 component gradually dissolves into hollandite with increasing pressure. The maximum solubility of NaAlSi3O8 in hollandite at 1000-degrees-C was about 40 mol% at 22.5 GPa, above which it decreases with pressure. Unit cell volume of the hollandite solid solution decreases with increasing NaAlSi3O8 component. The hollandite solid solution in this system may be an important candidate as a host mineral of K and Na in the uppermost lower mantle