Epidote, a mineral species consisting of basic calcium, aluminium and iron orthosilicate, Ca2(AlOH)(Al, 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 6½. 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 θαλλός, “a young shoot”) and pistacite (from πιστάκια, “pistachio nut”) have reference to the colour. The name epidote is one of R.J. Haüy’s crystallographic names, and is derived from ἐπίδοσις, “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 of 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 epidote-schist, with asbestos, adularia, calcite, and apatite; the Ala valley and Traversella in Piedmont; Arendal in Norway (arendalite); Le Bourg d’Oisans in Dauphiné (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(AlOH)(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)R2″′(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 1808, 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 ὀρθός, “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.
