EPIDOTE, a mineral species consisting of basic calcium, aluminium and iron orthosilicate, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence : they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. Many of the char acters of the mineral vary with the amount of iron present 5-17%), for instance, the colour, the optical constants, and the specific gravity The hardness is 62. The colour is green, grey, brown or nearly black, but usually yellowish- or pistachio green.
Epidote is an abundant rock-forming mineral, but one of sec ondary origin. It occurs in crystalline limestones and schistose rocks of metamorphic origin, and is also a product of weathering of various minerals composing igneous rocks; combined with quartz it is known as epidosite (q.v.). Well-developed crystals are found at many localities, the best being from Knappenwand, in the Untersulzbachthal in Salzburg, and Prince of Wales island, Alaska. The perfectly transparent, dark-green crystals from the Knappen wand and Brazil have occasionally been cut as gem-stones.
Belonging to the same isomorphous group with epidote are the species piedmontite and allanite, which may be described as man ganese and cerium epidotes respectively. Piedmontite occurs as small, reddish-black, monoclinic crystals in the manganese mines at San Marcel, Piedmont, and in crystalline schists at several places in Japan. The purple colour of the Egyptian porfido rosso antico is due to the presence of this mineral. Allanite contains ferric iron and metals of the cerium group; externally it differs widely from epidote, being black or dark brown in colour, pitchy in lustre, and opaque in the mass. Although not a common min eral it is of fairly wide distribution as a primary accessory constit uent of many crystalline rocks. Orthite, from 6pO6s, "straight," is a hydrated form found as slender prismatic crystals, sometimes 'ft. in length, at Finbo, Sweden. (L. J. S.)