Studies in the System CaO‐Al ₂ O ₃ ‐SiO ₂ H ₂ O: III, New Data on the Polymorphism of Ca ₂ SiO ₂ and Its Stability in the System CaO‐SiO ₂ ‐ H ₂ O

The nature of the low‐temperature inversions γ‐α' and α'‐β was investigated by various techniques: hydrothermal and “dry” quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X‐ray diffractometer patterns obtained at elevated temperatures, “static” pressure techniques, and infrared absorption spectrometry. A revised energy‐temperature diagram is presented for Ca 2 SiO 4 , with the transition γ' to α' taking place at about 725°C. and the α'‐β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ‐α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat‐of‐transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO 2 ‐3H 2 O (X), 6CaO 3SiO 2 .H 2 O (Y), and 9CaO‐6SiO 2 H 2 O (Z), were found to be stable above 700°C. at H 2 O pressures greater than 7500 lb. per sq. in.