A mineral belonging to the zeolite family of silicates and crystallizing in the monoclinic system. It usually occurs in crystals with prominent side pinacoid, often having a diamond shape. There is perfect side pinacoid cleavage on which the luster is pearly; elsewhere the luster is vitreous. The crystals often have undulating faces, and are made up of subindividuals in nearly parallel position (see illus.). In polarized light they show optical anomalies of a sectoral nature. The hardness is 3½ to 4 on Mohs scale; specific gravity is 2.18–2.20. The mineral is usually white or colorless but may be yellow or red.