Summary: Since its discovery by Tilley in 1930, scawtite has been recognized in several locations in regions of metamorphosed siliceous limestones. Scawtite is believed to be the same as CSH(A) and, as such, has been synthesized sporadically many times during runs in the CaO-SiO2-H2O system where slight contamination from CO2 has occurred. Attempts to synthesize scawtite were successfully carried out at low partial pressures of CO2 in an excess of water. The stability of selected scawtite compositions lies between 140 and 300° C, and above saturated steam pressures is not pressure-dependent, at least up to 50 000 psi. The scawtite facies (2) lies between the tobermorite-calcite facies (3) and the xonotlite-calcite facies (1). A hydration series exists from (1) to (2) to (3).