Olivine – Formation and Uses
Olivine Formation:
Olivine is the name given to a group of rock-forming minerals, typically found in certain types of igneous rock, such as dunite, basalt, gabbro, and peridotite, among others. Such rocks are commonly found at divergent plate boundaries and at the center of tectonic plates.
Its high crystallization temperature makes it one of the first minerals to crystallize from magma. At the time of the slow cooling of the magma, olivine crystals are formed which normally 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, at the bottom of the magma chamber.
Interestingly, magnesium-rich olivine has been found in extraterrestrial places: in meteorites, an Itokawa asteroid, on the moon, and on the planet Mars.
Name and composition:
The name “olivine” is derived from the characteristic olive-green color, although some less common forms of iron-rich olivines (called fayalites) are brown in color.
This group of silicate minerals has a general chemical composition of A2SiO4, with “A” usually being Mg or Fe, but sometimes also Ca, Mn, or Ni. Typically, the chemical composition ranges from Mg2SiO4 to Fe2SiO4.
Some olivine crystals also form during metamorphism of a form of dolomitic or dolomite limestone. Magnesium from dolomite and silica from quartz and other impurities from limestone are transformed into olivine. This olivine metamorphoses further into serpentine (it is 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 weathered and therefore is not a common mineral found in sedimentary rocks.
Applications
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 in the steel to be removed forming a slag.
Most commonly, jewelers cut the magnesium-rich variety of olivine that is close in formula to Mg2SiO4 to create the popular green gemstone known as peridot. This gemstone is popular as the birthstone for the Leo zodiac sign. This gemstone is rich in culture and history and in ancient times the pharaohs of Egypt wore peridot talismans. The most prized colors of peridot are dark olive green and bright lime green. The higher the iron content, the more the color tends to turn brown and this is undesirable as a gemstone. The browner shades of olivine turn into the gemstone chrysolite.
Olivine has also been commonly used in earlier times as a refractory material, to make refractory bricks, and was also used as a form of sand casting. These uses are rare today, as alternative materials are cheaper and more readily available.