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prehnite 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.