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(after the Slovenian enlightener S. Zois [1747–1819]), a mineral of the class of nesosilicates, with a chemical composition of Ca2Al3 [SiO4] [Si2O7](OH). The typical impurities are Fe3+, Fe2+, Mn, Mg, and, less frequently, Cr, V3+, Na, and K. The structure of zoisite is based on single and twinned tetrahedrons of [SiO4]4– radicals. Zoisite crystallizes in the orthorhombic system, forming prismatic crystals with striations on the faces. It is usually found in granular, columnar, fibrous, and occasionally radiated aggregates. The color is gray or greenish. Manganese-containing zoisite, called thulite, is pink; the chromium-containing variety is bright green. The vanadium-containing zoisite, called tanzanite, exhibits dichroism between dark blue and red violet. Zoisite has a hardness of 6–6.5 on Mohs’ scale and a density of 3,250–3,370 kg/m3.
Zoisite is a rock-forming mineral for several metamorphosed shales and hydrothermally altered basic igneous rocks (it is the chief component of saussurite). It is also found in crystal-bearing veins, skarns, corundum plagioclasites (anorthosites), and other formations. Certain transparent colored varieties are used in jewelry-making.