olivine(redirected from olivinic)
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(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.