1911 Encyclopædia Britannica/Zoisite

by Leonard James Spencer

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 61/2 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. S.)