Online Encyclopedia

EPIDOSITE

Online Encyclopedia
Originally appearing in Volume V09, Page 689 of the 1911 Encyclopedia Britannica.
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EPIDOSITE  , in

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petrology, a typical member of a
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family of metamorphic rocks composed mainly of
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epidote and
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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
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epidiorite and
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amphibolite, and are
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local modifications of rocks which were primarily basic intrusions or lavas . The sedimentary epidosites occur with
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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
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great variety of minerals such as
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calcite,
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augite, garnet, scapolite, &c., but their source may usually be inferred from their close association with
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talc-silicate rocks in the field . The third
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group of epidosites may form bands,
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veins, or irregular streaks and nodules in masses of epidiorite and
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hornblende-schist . In microscopic section they are often merely a granular mosaic of quartz and epidote with some iron oxides and
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chlorite, but in other cases they retain much of the structure of the originar rock though there has been a
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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 .

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