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, K1+xАl1+xSi1-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 K2Al2SiOe is most likely a member of the f.c.c. solid solution K1-xAl1-xSixO2. 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