ZOISITE  , a

rock-forming mineral, consisting of basic calcium and aluminium silicate, Ca2(AlOH)Al2(SiO4)3, crystallizing in the orthorhombic system . It is closely related to epidote (q.v.) both in the angles of the crystals and in chemical composition: a zoisite containing some iron replacing aluminium may be identical in composition with an epidote (" clinozoisite ") poor in iron . The crystals are prismatic in habit and are deeply furrowed parallel to their length; terminal planes are rare; there is a perfect cleavage parallel to the brachy-pinacoid . Columnar and compact masses are more common . The hardness is 61 and the specific gravity 3'25-3.37 . The colour is often grey; a rose-red variety, known as " thulite," occurs with sky-blue vesuvianite at Telemarken in Norway, and has been used to a limited extent as an ornamental stone . According to differences in the optical characters, two kinds of zoisite have been distinguished . Zoisite is a product of dynamo-metamorphism, and occurs as a constituent of some crystalline schists, such as amphibolite and eclogite . It was first observed by Baron Zois (after whom it was named) in the eclogite of Sau-Alpe in Carinthia; other localities are the Ducktown copper mines in Tennessee, where it occurs embedded in chalcopyrite; Loch Garve in Ross-shire, &c . The " saussurite " of the Alps and elsewhere, which has resulted from the alteration of the plagioclase felspar of gabbro, consists largely of zoisite with epidote . (L . J .