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EPIDIORITE

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Originally appearing in Volume V09, Page 689 of the 1911 Encyclopedia Britannica.
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EPIDIORITE , in See also:

petrology, a typical member of a See also:family of rocks consisting essentially of See also:hornblende and See also:felspar, often with See also:epidote, See also:garnet, See also:sphene, See also:biotite, or See also:quartz, and having usually a foliated structure. The See also:term is to some extent synonymous with " See also:amphibolite " and " hornblende-schist." These rocks are metamorphic, and though having a See also:mineral constitution somewhat similar to that of See also:diorite, they have been produced really from rocks of more basic See also:character, such as See also:diabase, See also:dolerite and See also:gabbro. They occur principally among the See also:schists, slates and gneisses of such districts as the Scottish See also:Highlands, the See also:north-See also:west of See also:Ireland, See also:Brittany, the Harz, the See also:Alps, and the crystalline ranges of eastern N. See also:America. Their hornblende in microscopic See also:section is usually dark See also:green, rarely brownish; their felspar may be clear and recrystallized, but more frequently is converted into a turbid aggregate of epidote, See also:zoisite, quartz, sericite and See also:albite. In the less See also:complete stages of alteration, ophitic structure may persist, and the See also:original See also:augite of the See also:rock may not have been entirely replaced by hornblende. See also:Pink or brownish garnets are See also:common and may be an See also:inch or two in See also:diameter. The See also:iron oxides, originally See also:ilmenite, are usually altered to sphene. Biotite, if See also:present, is See also:brown; epidote is yellow or colourless; See also:rutile, See also: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 See also:fine grained, so that their constituents are hardly visible to the unaided See also:eye; or may show crystals of hornblende an inch in length. Their prevalent See also:colour is dark green and they See also:weather with brown surfaces.

In many parts of the See also:

world epidiorites and the quartz See also: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.

End of Article: EPIDIORITE

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