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OLIVINE

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Originally appearing in Volume V20, Page 89 of the 1911 Encyclopedia Britannica.
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OLIVINE , a See also:

rock-forming See also:mineral composed of See also:magnesium and ferrous crthosilicate, the See also:formula being (Mg, Fe)sSiO4. The name olivine, proposed by A. G. See also:Werner in 1790, alludes to the See also:olive-See also:green See also:colour commonly shown by the mineral. The transparent varieties, or " See also:precious olivine " used in See also:jewelry, are known as See also:chrysolite (q.v.) and See also:peridot (q.v.). The See also:term olivine is often applied incorrectly by jewellers to various green stones. Olivine crystallizes in the orthorhombic See also:system, but distinctly See also:developed crystals are comparatively rare, the mineral more often occurring as compact or granular masses or as grains and blebs embedded in the igneous rocks of which it forms a constituent See also:part. There are indistinct cleavages parallel to the macropinacoid (M in the fig.) and the brachypinacoid. The hardness is 6,1; and the sp. gr. 3.27-3.37, but reaching 3.57 in the highly ferruginous variety known as hyalosiderite. The amount of ferrous See also:oxide varies from 5 (about 9 % in the See also:gem varieties to 30 % in hyalosiderite. The See also:depth of the green, or yellowish-See also:brown colour, also varies with the amount of See also:iron.

The lustre is vitreous. The indices of See also:

refraction ( 1.66 and 1.70) and the See also:double refraction are higher than in many other rock-forming minerals; and these characters, together with the indistinct cleavage, enable the mineral to be readily distinguished in thin rock-sections under the See also:microscope. The mineral is decomposed by hot hydrochloric See also:acid with separation of gelatinous See also:silica. Olivine often contains small amounts of See also:nickel and See also:titanium dioxide; the latter replaces silica, and in the variety known as titan-olivine reaches 5%. Olivine is a See also:common constituent of many basic and ultrabasic rocks, such as See also:basalt, See also:diabase, See also:gabbro and See also:peridotite: the dunite, of Dun See also:Mountain near See also:Nelson in New See also:Zealand, is an almost pure olivine-rock. In basalts it is often See also:present as small porphyritic crystals or as large granular aggregates. It also occurs as an See also:accessory constituent of some granular dolomitic limestones and crystalline See also:schists. With See also:enstatite it forms the bulk of the material of meteoric stones; and in another type of meteorites large blebs of glassy olivine fill spaces in a cellular See also:mass of metallic iron. Olivine is especially liable to alteration into See also:serpentine (hydrated magnesium silicate) ; the alteration proceeds from the outside of the crystals and grains or along irregular cracks in their interior, and gives rise to the separation of iron oxides and an irregular See also:net-See also:work of fibrous serpentine, which in rock-sections presents a very characteristic See also:appearance. Large greenish-yellow crystals from Snarum in Buskerud, See also:Norway, at one See also:time thought to be crystals of serpentine, really consist of serpentine pseudomorphous after olivine. Many of the large rock-masses of serpentine have been derived by the serpentinization of olivine-rocks. Olivine also sometimes alters, especially in crystalline schists, to a fibrous, colourless See also:amphibole, to which the name pilite has been given.

By See also:

ordinary weathering processes it alters to limop.ite and silica. Closely related to olivine are several other See also:species, which are included together in the olivine See also:group : they have the orthosilicate formula R"2SiO4, where R" represents See also:calcium, magnesium, iron, See also:manganese and rarely See also:zinc; they all crystallize in the orthorhombic system, and are isomorphous with olivine. The following may be mentioned Monticellite, CaMgSiOs, a rare mineral occurring as yellowish-See also:grey crystals and grains in granular See also:limestone at See also:Monte Solnma, See also:Vesuvius. Forsterite, Mg2SiOs, as colourless or yellowish grains embedded in many. crystalline limestones. Fayalte, Fe2SiO4, or iron olivine is dark brown or See also:black in colour. It occurs as nodules in a volcanic rock at See also:Fayal in the See also:Azores, and in See also:granite at the Mourne Mountains in See also:Ireland; and as small crystals in cavities in See also:rhyolite at the Yellowstone See also:Park, U.S.A. It is a common constituent of crystalline iron slags. Tephroite, Mn2SiO4, a grey (rm4p6s, ash-coloured), cleavable mineral occurring with other manganiferous minerals in See also:Sweden and New See also:Jersey. (L. J.

End of Article: OLIVINE

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