silicate mineral, calcium and aluminum silicate, Ca₂Al₃(SiO₄)₃OH, characteristic of regional metamorphism and of hydrothermal alteration of igneous rocks. A member of the epidote (q.v.) group of nesosilicates, zoisite occurs as white, green-brown, or gray crystals or masses in crystalline schists, often with amphibole minerals; in metamorphosed calcareous shales; very commonly in argillaceous (clayey) calcareous sandstones; and less commonly in thermally metamorphosed limestone. Occurrences include Italy, Germany, Switzerland, Mexico, Scotland, and the United States (Massachusetts). thulite, a manganous variety from Telemark, Nor., and Piedmont, Italy, is pink; tanzanite, a gem variety from Tanzania, is vivid blue. Zoisite has the same chemical formula as clinozoisite but has a different crystal structure. For detailed physical properties, see silicate mineral (table).