EPIDOTE  , a

mineral species consisting of basic calcium, aluminium and iron orthosilicate, Ca2(A1OH)(A1,Fe)2(SiO4)3, 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 . The faces lettered M, T and r in the figure are often deeply striated in the same direction: M is a direction of perfect cleavage, and T of imperfect cleavage: crystals are often twinned on the face T . Many of the characters of the mineral vary with the amount of iron present (Fe2O3,5-17%), for instance, the colour, the optical constants, and the specific gravity (3.3-3'5)• The hardness is 62 . The colour is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green . The pleochroism is strong, the pleochroic colours being usually green, yellow and brown . The names thallite (from OaXXos, " a young shoot ") and pistacite (from 1rcvreucca, " pistachio nut ") have reference to the colour . The name epidote is one of R . J . Haiiy's crystallographic names, and is derived from i rLSoves, "increase," because the base of the primitive prism has one side longer than the other . Several other names (achmatite, bucklandite, escherite, puschkinite, &c.) have been applied to this species . Withamite is a carmine-red to straw-yellow, strongly pleochroic variety from Glencoe in Scotland .

Fouqueite and clinozoisite are

white or pale rose-red varieties containing very little iron, thus having the same chemical composition as the orthorhombic mineral zoisite (q.v.) . Epidote is an abundant rock-forming mineral, but one of689 secondary origin . It occurs in crystalline limestones and schistose rocks of metamorphic origin; and is also a product of weathering of various minerals (felspars, micas, pyroxenes, amphiboles, garnets, &c.) composing igneous rocks . A rock composed of quartz and epidote is known as epidosite . Well-developed crystals are found at many localities, of which the following may be specially mentioned: Knappenwand, near the Gross-Venediger in the Untersulzbachthal in Salzburg, as magnificent, dark green crystals of long prismatic habit in cavities in epidoteschist, with asbestos, adularia, calcite, and apatite; the Ala valley and Traversella in Piedmont; Arendal in Norway (arendalite); Le Bourg d'Oisans in Dauphine (oisanite and delphinite); Haddam in Connecticut; Prince of Wales Island in Alaska, here as large, dark green, tabular crystals with copper ores in metamorphosed limestone . The perfectly transparent, dark green crystals from the Knappenwand and from 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 manganese and cerium epidotes respectively . Piedmontite has the composition Ca2(A10H)(Fe,Mn)2(SiO4)3; it occurs as small, reddish-black, monoclinic crystals in the manganese mines at San Marcel, near Ivrea in 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 has the same general formula R2"(R"'OH)R21"(SiO4)3, where R" represents calcium and ferrous iron, and R"' aluminium, ferric iron and metals of the cerium group . In external appearance it differs widely from epidote, being black or dark brown in colour, pitchy in lustre, and opaque in the mass; further, there is little or no cleavage, and well-developed crystals are rarely met with . The crystallographic and optical characters are similar to those of epidote; the pleochroism is strong with reddish-, yellowish-, and greenish-brown colours .

Although not a

common mineral, allanite is of fairly wide distribution as a primary accessory constituent of many crystalline rocks, e.g. gneiss, granite, syenite, rhyolite, andesite, &c . It was first found in the granite of east Greenland and described by Thomas Allan in 18o8, after whom the species was named . Allanite is a mineral readily altered by hydration, becoming optically isotropic and amorphous: for this reason several varieties have been distinguished, and many different names applied . Orthite, from opOos, " straight," was the name given by J . J . Berzelius in 1818 to a hydrated form found as slender prismatic crystals, sometimes a foot in length, at Finbo, near Falun in Sweden . (L . J .