The high-temperature and mediuy-pressure part of the PTdiagram of the system SiOrHxO has been investigated experimentally. The equilibrium diagram is discussed in the light of Schreinemakers general theory of PTdiagrams. The triple invariant point cristobalite + tridymite + quartz lies at 1190Β°C and 1430 atm. Neither cristobalite nor tridymite are stable at high pressure. Quartz may precipitate from the melt at a very high temperature (1360Β°C and higher), if the pressure is great enough, and if the water content is low. Using new experimental and published data a PT-diagram of the system SiO,-HxO in the large PT-region is given.
In 1913, Fenner published his classical paper in which general features ofthem-diagram of Si02, optical constants of the phases of silica, and the exact temperatures of phase transformations of silica were discussed.
Later, in 1930, Smits gave a theoretical scheme for a PT-diagram of the system silicawater. He suggested that the system should contain two critical points ''P" and "Q" on the critical curve. The scale for the pressure was not given at all.
In 1955, Tuttle and England published their preliminary diagram of the system silicawater. They found a great drop in the melting temperature of silica due to the solution of water in the melt. Tuttle and England determined the position of the "three-phase" curve solid-liquid-gas and estimated the quantity of water solved in the silica-melt under