apophyllite

apophyllite, potassium-calcium fluoride-silicate mineral that is related structurally to the zeolite family of aluminosilicates. Like the zeolites, it has a high water content, although apophyllite has no aluminum in its chemical composition, which is approximately represented by the formula Ca₄KFSi₈O₂₀·8H₂O. In many ways it is more like the micas. It is usually found as glassy, white to grayish crystals with zeolite minerals in basalt, granite, and gneiss, as in Trentino, Italy; Belfast, N.Ire.; the Faroe Islands; Kimberley, S.Af.; and Guanajuato, Mex. For detailed physical properties, see silicate mineral (table).