1911 Encyclopædia Britannica/Olivine

by Leonard James Spencer

OLIVINE, a rock-forming mineral composed of magnesium and ferrous orthosilicate, the formula being (Mg, Fe)₂SiO₄. The name olivine, proposed by A. G. Werner in 1790, alludes to the olive-green colour commonly shown by the mineral. The transparent varieties, or "precious olivine" used in jewelry, are known as chrysolite (q.v.) and peridot (q.v.). The term olivine is often applied incorrectly by jewellers to various green stones.

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. There are indistinct cleavages parallel to the macropinacoid (M in the fig.) and the brachypinacoid. The hardness is 6¾; and 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 1·70) and the 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 microscope. The mineral is decomposed by hot hydrochloric acid with separation of gelatinous silica. Olivine 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 ultra basic rocks, such as basalt, diabase, gabbro and peridotite: the dunite, of Dun Mountain near Nelson in New Zealand, is an almost pure olivine-rock. In basalts it is often present as small porphyritic crystals or as large granular aggregates. 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 (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 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 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 amphibole, to which the name pilite has been given. By ordinary 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 R′′₂SiO₄, where R′′ represents calcium, magnesium, iron, manganese and rarely zinc; they all crystallize in the orthorhombic system, and are isomorphous with olivine. The following may be mentioned:—

Monticellite, CaMgSiO₄, 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 is dark brown or black in colour. It occurs as nodules in a volcanic rock at Fayal in the Azores, and in granite at the Mourne Mountains in Ireland; and as small crystals in cavities in rhyolite at the Yellowstone Park, U.S.A. It is a common constituent of crystalline iron slags.

Tephroite, Mn₂SiO₄, a grey (τεφρός, ash-coloured), cleanable mineral occurring with other manganiferous minerals in Sweden and New Jersey.  (L. J. S.)