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 Ca4KFSi8O20ยท8H2O. 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).