Summary: Z-phase was obtained hydrothermally at 120 °C by decomposition of Al-substituted tobermorite and by reaction of lime and colloidal silica. X-ray and electron diffraction show that the structural element is hexagonal, with a 9·65, c 15·3 Å, and good {0001} cleavage. Reversible water loss and lattice shrinkage occur on heating, the layer thickness (c) decreasing to 12.1 Å at 400 °C. For material in equilibrium with air of normal humidity, the composition is probably between CaO. ₂SiO₂. 1·₇H₂O and CaO. 2SiO₂. 2H₂O; Z = 8 for the structural element. New t.g. and infrared absorption data are presented; the infra-red spectrum closely resembles that of gyrolite, but OH ions attached only to Ca appear to be absent. Conditions of formation are discussed; if Z-phase has any stability field, it is below 120 °C. Crystal structures for Z-phase, gyrolite, and truscottite are suggested, based on the known structure of reyerite.