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PREHNITE

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Originally appearing in Volume V22, Page 277 of the 1911 Encyclopedia Britannica.
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PREHNITE , a See also:

mineral consisting of See also:calcium See also:hydrogen orthosilicate, H2Ca2Al2(SiO4)3. It crystallizes in the See also:hemimorphic class of the orthorhombic See also:system, but the hemimorphic See also:character is usually obscured by twinning. Crystals are generally platy in See also:habit, but they rarely occur singly and distinctly shaped; almost invariably they are closely aggregated together to See also:form See also:barrel-shaped or globular See also:groups with a crystalline See also:surface. This form, together with the See also:pale oil-See also:green See also:colour, gives the mineral a very characteristic See also:appearance. It is translucent and has a vitreous lustre. The hardness is rather over 6 and the spec. See also:gray. 2.80–2.95. Crystals are pyro-electric. Prehnite is sometimes classed with the See also:zeolites, since it occurs under the same conditions as these minerals and often in association with them: the small amount of See also:water (4.4%) is, however, expelled only at a red See also:heat and is therefore not water of See also:crystallization. Prehnite occurs as a mineral of secondary origin in the amygdaloidal cavities of basic igneous rocks, such as See also:basalt and See also:diabase, and less often, in See also:veins in See also:granite and See also:gneiss. See also:Fine specimens are found with zeolites in the volcanic rocks of several places in the See also:south of See also:Scotland, e.g. Old See also:Kilpatrick in See also:Dumbartonshire, Bishopton in Renfrewshirc, Campsie Hills in See also:Stirlingshire and in the neighbourhood of See also:Edinburgh; also at See also:Paterson and See also:Bergen See also:Hill in New See also:Jersey, and with native See also:copper in the See also:trap-rocks of the See also:Lake See also:Superior region.

In the See also:

French (at Le Bourg d'Oisans) and Tyrolese See also:Alps it occurs with See also:axinite, See also:epidote, See also:felspar, &c., lining crevices in gneiss. Large masses have been found at Credock in Cape See also:Colony, from which locality it was brought in the 18th See also:century by See also:Colonel Prehn, the See also:governor of the colony; hence the names " Cape See also:chrysolite " and prehnite (of A. G. See also:Werner, 1789). Prehnite is sometimes cut and polished for small ornaments; it then somewhat resembles See also:chrysoprase in appearance.

End of Article: PREHNITE

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