EPIDIORITE, in petrology, an altered basic igneous rock, consisting essentially of hornblende and plagioclase felspar, used in road-mending. The term is now restricted to those metamor phosed igneous rocks of gabbroic or doleritic composition in which either relict minerals or relict textures, or both, are pre served. The epidiorites thus constitute a connecting link in the transformation of basic igneous rocks to amphibolites (q.v.) and hornblende-schists in which recrystallization is typically com plete. Though having the same general mineral composition as diorites, the epidiorites are distinctly more basic rocks. The usual mineralogical changes in the passage of dolerite or gabbro to epidiorite are the partial or complete conversion of augite to the fibrous hornblende known as uralite, with simultaneous pro duction of sphene, and the rearrangement of the felspar molecule to form limpid grains of albite and a hydroxyl-bearing calcium aluminium silicate, zoisite or epidote, this assemblage being known as saussurite. Other minerals such as prehnite and grossu lar are sometimes constituents of this saussuritic aggregate. The original ophitic or porphyritic textures of the igneous rock are usually not completely destroyed, and are strictly referred to as blastophitic and blastoporphyritic textures.
With advancing metamorphism the chemical changes are more fundamental. The albite felspar gradually becomes richer in anorthite at the expense of the epidote, but the development of the red iron-bearing garnet does not usually ensue till recrystalliza tion is complete, when the rock is more strictly referred to as amphibolite or hornblende-schist. Epidiorites are the common products of basic igneous rocks subject to dynamic metamorphism, and as such appear in regions of comparatively low grades of metamorphism. Their chief development is in the form of small bosses, dikes and sills. Within the limits of one exposure all stages in the gradual transition from dolerite to amphibolite, through epidiorite, may frequently be studied, the cores of dikes and sills consisting of unaltered igneous rock while the margin is completely recrystallized to amphibolite or hornblende-schist.
Epidiorites are of widespread distribution occurring among slates, phyllites and schists, as in the Scottish Highlands, the Alps, the Harz, Brittany and the crystalline ranges of eastern North America, etc. Many so-called greenstones (q.v.) are in reality epidiorites. In many parts of the world they form the matrices of auriferous quartz veins, e.g., in the Precambrian shield of western Australia. (C. E. T.)