Olivine

The mineral olivine ( /ˈɒlɪˌviːn/) is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. The primary component of the earth's upper mantle, it is a common mineral in Earth's subsurface but weathers quickly on the surface. The mineral olivine ( /ˈɒlɪˌviːn/) is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. The primary component of the earth's upper mantle, it is a common mineral in Earth's subsurface but weathers quickly on the surface. The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: Mg2SiO4) and fayalite (Fe-endmember: Fe2SiO4). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo70Fa30). Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1,900 °C (3,450 °F), while fayalite's is much lower (about 1,200 °C ). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen, silicon, magnesium and iron. Manganese and nickel commonly are the additional elements present in highest concentrations. Olivine gives its name to the group of minerals with a related structure (the olivine group)—which includes tephroite (Mn2SiO4), monticellite (CaMgSiO4) and kirschsteinite (CaFeSiO4). Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica (SiO4) unit being joined by metal divalent cations with each oxygen in SiO4 bound to 3 metal ions. It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations. Olivine gemstones are called peridot and chrysolite. Olivine rock is usually harder than surrounding rock and stands out as distinct ridges in the terrain. These ridges are often dry with little soil. Drought resistant scots pine is one of few trees that thrive on olivine rock. Olivine pine forest is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Olivine rock is hard and base-rich. The habitat is endangered by mining and road construction. Olivine is named for its typically olive-green color (thought to be a result of traces of nickel), though it may alter to a reddish color from the oxidation of iron. Translucent olivine is sometimes used as a gemstone called peridot (péridot, the French word for olivine). It is also called chrysolite (or chrysolithe, from the Greek words for gold and stone). Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea. Olivine occurs in both mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica. That magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite can be residues left after extraction of magmas, and typically they are more enriched in olivine after extraction of partial melts. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and olivine is one of the Earth's most common minerals by volume. The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.