Summary: Hydrothermal studies in the silica-rich region of the CaO-SiO₂-H₂O system have given information on the stability field of tobermorite. At saturated steam pressures tobermorite is unstable relative to xonotlite (C₆S₆H) above 140 °C but between 10 000 and 40 000 lb/in.² (69 and 276 N/mm²), the decomposition temperature is raised to 285±5 °C. Tobermorite is stable at all compositions between C/S = 0·67 and 1·0 up to this temperature and has a formula close to C₅S₆H₅. Variable compositions reported earlier are thought to be mixtures rather than lime-rich or silica-rich tobermorites.

Data have also been collected on the equilibrium assemblages below C/S = 0·67 and between C/S = 1·0 and 1·5 involving truscottite, gyrolite, xonotlite, foshagite, and hillebrandite.