A hydrated silicate and member of the phyllosilicates (sheet silicates) with composition Ca₂Al[Si₃AlO₁₀]OH₂; sp. gr. 2.9; hardness 6.0; colourless, white, or green; tabular or in granular aggregates; has a layered structure similar to the micas; occurs in association with the zeolites infilling cavities in basic igneous rocks and in contact metamorphosed (see thermal metamorphism) limestones. See also prehnite-pumpellyite facies.