Formation of Olivine:
Olivine is the name given to a group of rock-forming minerals, typically found in certain types of igneous rocks including dunite, basalt, gabbro, and peridotite, among others. Such rocks are commonly found at divergent plate boundaries and in the centre of tectonic plates.
Its high crystallisation temperature makes it one of the first minerals to crystallize from magma. At the time of the slow cooling of the magma, crystals of olivine form and usually settle to the bottom of the magma chamber as they have a relatively high density. As a result, this concentrated accumulation of olivine creates olivine-rich rocks such as dunite in the bottom of the magma chamber.
Interestingly, magnesium rich olivine has been found in extra-terrestrial locations − in meteorites, an asteroid Itokawa, on the moon, and in planet Mars.
Name and composition:
The name “olivine” is derived from the characteristic olive green colour, though some less commonly found forms of iron-rich olivines (called fayalites) are brownish in colour.
This group of silicate minerals has a generalized chemical composition of A2SiO4, with “A” being usually Mg or Fe, but sometimes also Ca, Mn, or Ni. Typically, the chemical composition ranges between Mg2SiO4 and Fe2SiO4.
Some crystals of olivine are also formed during the metamorphism of a form of dolomitic limestone or dolomite. Magnesium from the dolomite and silica from the quartz and other impurities in the limestone gets transformed into olivine. This olivine further metamorphoses and gets transformed into serpentine( also used as a source of magnesium and asbestos, or as a decorative stone.)
The crystal system in Olivine is orthorhombic with a spinel structure.
Olivine is easily altered by weathering, and therefore is not a common mineral found in sedimentary rocks.
Olivine finds use in industry, as a refractory material and in various metallurgical processes as a slag conditioner. This mineral is used in furnaces as it has a very high crystallization temperature compared to other minerals. High-magnesium olivine, also called forsterite is added to blast furnaces as it causes impurities from steel to be removed by forming a slag.
Quite commonly, the magnesium-rich variety of olivine which has a formula approaching Mg2SiO4 is cut by jewellers to create the popular green gemstone known as peridot. This gemstone is popular as the birthstone for the zodiac sign Leo. This gemstone is rich in culture and history and in ancient times, peridot talismans were worn by the Pharoahs of Egypt. The most valued colours of peridot are dark olive green and a bright lime green. The higher the iron content, the more the colour tends to become brown and this is undesirable as a gemstone. Browner shades of olivine are made into the gemstone chrysolite.
Olivine has also in earlier times commonly been used as a refractory material, to make refractory bricks and also used to be deployed as a form of casting sand. Nowadays these uses are uncommon, since alternative materials are cheaper and easier to obtain.
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