MELTING REACTIONS IN SYSTEM KA1S1308-S102-H2O TO 18.5 KILOBARS

Synthetic sanidine with composition Or95Ab5 was crystallized from gel, and a portion was mixed with 15 wt percent natural quartz. The sanidine and sanidine-quartz mixture were reacted with 30 to 40 wt percent water in sealed platinum capsules, using piston-cylinder apparatus. The melting relationships for these two crystalline mixtures, in the presence of excess water, were determined to 18.5 kb pressure. The divariant melting intervals located for each sample, along with the results of previous studies in the system KAlSi3O8-SiO2-H2O, provided good estimates of the positions of univariant curves for the two reactions: (a) Or + V ⇌ L, and (b) Or + Qz + V ⇌ L. Values for reaction (a) are 5 kb-875°C; 10 kb-825°C; 15 kb-795°C; 20-775°C. Values for reaction (b) are 5 kb-740°C; 10 kb-710°C; 15 kb-700°C; 20 kb-697°C. Muscovite was present in trace amounts in all runs, including those quenched from conditions well above its maximum temperature stability limit, but it was not found enclosed in glass. We therefore conclude that muscovite was a deposit from the vapor phase. Schematic phase diagrams for the system K2O-Al2O3-SiO2-H2O indicate that muscovite would not be expected as a stable liquidus phase for mixtures on the join KAlSi3O8-SiO2-H2O at pressures less than 10 kb.