Revised Phase Diagram for the System Al 2 O 3 —SiO 2

On the basis of 190 runs made up to 1860°C in sealed noble‐metal containers the following revisions have been made in the equilibrium diagram for the system A1 2 O 3 –SiO 2 . Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al 2 O 3 (3/2 ratio) to 63 mole % A1 2 O 3 . Metastable solid solutions can be prepared up to about 67 mole % Al 2 O 3 . There is no evidence for stable solubility of excess SiO 2 beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al 2 O 3 and from them the composition of the eutectic is confirmed at 5 mole % SiO 2 . The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al‐Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al 2 SiO 5 composition with ortho‐rhombic unit‐cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.