EPIDIORITE, in petrology, a typical member of a family of rocks consisting essentially of hornblende and felspar, often with epidote, garnet, sphene, biotite, or quartz, and having usually a foliated structure. The term is to some extent synonymous with “amphibolite” and “hornblende-schist.” These rocks are metamorphic, and though having a mineral constitution somewhat similar to that of diorite, they have been produced really from rocks of more basic character, such as diabase, dolerite and gabbro. They occur principally among the schists, slates and gneisses of such districts as the Scottish Highlands, the north-west of Ireland, Brittany, the Harz, the Alps, and the crystalline ranges of eastern N. America. Their hornblende in microscopic section is usually dark green, rarely brownish; their felspar may be clear and recrystallized, but more frequently is converted into a turbid aggregate of epidote, zoisite, quartz, sericite and albite. In the less complete stages of alteration, ophitic structure may persist, and the original augite of the rock may not have been entirely replaced by hornblende. Pink or brownish garnets are common and may be an inch or two in diameter. The iron oxides, originally ilmenite, are usually altered to sphene. Biotite, if present, is brown; epidote is yellow or colourless; rutile, apatite and quartz all occur with some frequency. The essential minerals, hornblende and felspar, rarely show crystalline outlines, and this is generally true also of the others. The rocks may be fine grained, so that their constituents are hardly visible to the unaided eye; or may show crystals of hornblende an inch in length. Their prevalent colour is dark green and they weather with brown surfaces. In many parts of the world epidiorites and the quartz veins which sometimes occur in them have proved to be auriferous. As they are tough, hard rocks, when fresh, they are well suited for use as road-mending stones.
(J. S. F.)
EPIDOSITE, in petrology, a typical member of a family of metamorphic rocks composed mainly of epidote and quartz. In colour they are pale yellow or greenish yellow, and they are hard and somewhat brittle. They may occur in more than one way and are derived from several kinds of rock. Some have been epidotic grits and sandstones; others are limestones which have undergone contact-alteration; probably the majority, however, are allied to epidiorite and amphibolite, and are local modifications of rocks which were primarily basic intrusions or lavas. The sedimentary epidosites occur with mica-schists, sheared grits and granulitic gneisses; they often show, on minute examination, the remains of clastic structures. The epidosites derived from limestones may contain a great variety of minerals such as calcite, augite, garnet, scapolite, &c., but their source may usually be inferred from their close association with calc-silicate rocks in the field. The third group of epidosites may form bands, veins, or irregular streaks and nodules in masses of epidiorite and hornblende-schist. In microscopic section they are often merely a granular mosaic of quartz and epidote with some iron oxides and chlorite, but in other cases they retain much of the structure of the original rock though there has been a complete replacement of the former minerals by new ones. Epidosites when streaked and variegated have been cut and polished as ornamental stones. They are translucent and hard, and hence serve for brooch stones, and the simpler kinds of jewelry. These rocks occasionally carry gold in visible yellow specks.
(J. S. F.)
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.