olivine(redirected from Olivine group)
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olivine(ŏlĭv`ēn), an iron-magnesium silicate mineral, (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. It is a common constituent of magnesium-rich, silica-poor igneous rocks; metamorphism of some high magnesium sediments also can form olivine. Dunite consists almost entirely of olivine. It also occurs in lunar rocks and meteorites. Olivine has a characteristic yellow-green to olive-green color, hence the name. Transparent olivine of good color can be cut into gemstones; the gem form is known as peridot. Sources of gem-quality olivine are St. John's Island in the Red Sea, Myanmar, and Arizona. Magnesium-rich olivine has a high melting point and is used in the manufacture of refractories. It was formerly called chrysolite.
(also peridot or chrysolite), a mineral of the neso-silicate class; the main representative of the olivine group. The olivine group includes forsterite, Mg2[SiO4]; olivine, (Mg,Fe)2[SiO4]; fayalite, Fe2[SiO4]; tephroite, Mn2[SiO4]; knebelite, (Fe,Mn)2[SiO4]; and monticellite, CaMg[SiO4].
The minerals of the olivine group differ from one another in both properties and composition. They crystallize in the ortho-rhombic system to form tabular or prismatic crystals. The olivine structure is composed of isolated tetrahedral SiO44- groups and of Mg2+ and Fe2+ cations surrounded by six oxygen ions. The structural distribution of magnesium and iron, as determined by Mössbauer spectra, serves as a geothermometer. Olivine has imperfect cleavage; its hardness on Mohs’ scale is 6.5–7.0, and its density is 3,200–4,400 kg/m3, depending on the number of heavy iron and manganese atoms per molecule. Its color varies from yellowish green to olive green; sometimes the mineral is colorless.
Olivine is widespread in nature as a rock-forming mineral in ultrabasic and basic rocks, such as dunites, peridotites, olivine gabbros, diabases, and basalts, including lunar basalt. The olivine structure rearranges to form a spinel-type lattice under high pressure (130–160 kilobars). The effect of hydrothermal solutions readily alters olivine into serpentine and sometimes also to talc. On the earth’s surface, olivine decomposes to yield magne-site, hydrous iron oxides, and opal. Transparent olivine crystals, or chrysolites, are precious stones.