Subsolidus Reactions in Oxide Systems in the Presence of Water at High Pressures

Phase equilibrium studies at subsolidus temperatures, made in the presence of an aqueous phase at high pressures, are briefly reviewed for the systems eucryptite–silica, Al 2 O 3 –Ga 2 O 3 , MgAl 2 O 4 –Al 2 O 3 , and TiO 2 . Low‐temperature equilibrium relations have been largely clarified for the first three systems because of the greatly increased reaction rate in the presence of a fluid phase. For the system TiO 2 , however, hydrothermal techniques appear to be inadequate for the determination of stability relations. The hydrothermal syntheses of forsterite at 500° to 700°C. from magnesia and silica or from dolomite and silica, and of mullite at 500°C. from clay minerals, are mentioned as reactions of possible industrial significance. A further acceleration of reaction rate at low or moderate temperatures is being achieved by grinding at elevated temperatures and pressures.