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.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)