The system K₂O-Al₂O₃-SiO₂; Part 1, Phases on the KAlSiO₄-KAlO₂ join

High-temperature experiments have yielded information about the part of the tectosilicate join between KAlSiO₄ and KA1O₂. Orthorhombic KAlSiO₄ synthesized at 950°C [a = 9.057(2), b = 15.642(2), с = 8.582(2)A], space group P2₁2₁2 transforms upon heating above 1450-1485°C to another orthorhombic phase having a larger unit cell [a = 18.110(3), b = 15.600(3), с = 8.560(2)А]. The space group of the latter is shown from single-crystal precession photographs to be one of three possibilities: P2₁am, Pma2, or Pmam.

A body-centered tetragonal phase, K_1+xАl_1+xSi_1-xO₄ (x ≈ 0.1), with unit-cell dimensions a = 8.943(1), с = 5.221(1)A is stable at the expense of orthorhombic KAlSiO₄ and silica-saturated KAlO₂ solid solution in the range 1400-1600°C. Five space groups within Laue group 4/mmm are possibilities.

The compound previously reported to be K₂Al₂SiO_e is most likely a member of the f.c.c. solid solution K_1-xAl_1-xSi_xO₂. The SiO₂-rich end-member of this solid solution has a composition with x ≈ 0.25 at 1600°C, and apparently belongs to Laue group m3m