OLIVINE, a rock-forming mineral composed of magnesium and ferrous orthosilicate, the formula being The name alludes to its olive-green colour and is often applied incor rectly by jewellers to various green stones. The transparent varie ties, or "precious olivine" used in jewellery, are known as chryso lite (q.v.) and peridot.
Olivine crystallizes in the orthorhombic system, but distinctly 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 part. The hardness is 6i, the sp. gr. 3.27-3.37, but reaching 3.57 in the highly ferruginous variety known as hyalosiderite. The amount of ferrous oxide varies from 5% (about 9% in the gem varieties) to 30% in hyalosiderite. The depth of the green, or yellowish-brown colour, also varies with the amount of iron. The lustre is vitreous. The indices of refraction (1.66 and 1.7o) and the double refraction are higher than in many other rock-forming minerals; and these characters, together with the indistinct cleav age, enable it to be readily distinguished in thin rock-sections under the microscope. The mineral is decomposed by hot hydro chloric acid with separation of gelatinous silica ; it often contains small amounts of nickel and titanium dioxide ; the latter replaces silica, and in the variety known as titan-olivine reaches 5%.
Olivine is a common constituent of many basic and ultrabasic rocks, such as basalt, dolerite, gabbro and peridotite ; and the dun ite, of Dun mountain near Nelson in New Zealand, is an almost pure olivine-rock. It also occurs as an accessory constituent of some granular dolomitic limestones and crystalline schists. With 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 mass of metallic iron.
Olivine is especially liable to alteration into serpentine (hy drated magnesium silicate) ; the alteration proceeds from the out side of the crystals and grains or along irregular cracks in their interior, and gives rise to the separation of iron oxides and an irregular net-work of fibrous serpentine, which in rock-sections presents a very characteristic appearance. Large greenish-yellow crystals from Snarum in Buskerud, Norway, at one time thought to he crystals of serpentine, really consist of serpentine pseudo-. morphous after olivine. Many of the large rock-masses of serpen tine have been derived by the serpentinization of olivine-rocks. Olivine also sometimes alters, especially in crystalline schists, to a fibrous, colourless amphibole, to which the name pilite has been given. By weathering processes it alters to limonite and silica.
Closely related to olivine are several other species, which are included together in the olivine group : they have the orthosilicate formula where R" represents calcium, magnesium, iron, manganese and rarely zinc ; they all crystallize in the orthorhom bic system, and are isomorphous with olivine. These include: monticellite, a rare mineral occurring as yellowish grey crystals and grains in granular limestone at Monte Somma, Vesuvius; Forsterite, Mg2SiO4, as colourless or yellowish grains embedded in many crystalline limestones; Fayalite, Fe2SiO4, or iron-olivine, a dark brown or black variety occurring as nodules in volcanic rock; Tephroite, Mn2SiO4, a grey (reckpos, ash-coloured), mineral occurring in Sweden and New Jersey. (L. J. S.)