Olivine formation:

Olivine is the name given to a group of rock-forming minerals, typically found in certain types of igneous rocks, such as dunite, basalt, gabbro, and peridotite, among others. These rocks are commonly found at the boundaries of divergent plates and in the center of tectonic plates.

Its high crystallization temperature makes it one of the first minerals to crystallize from magma. At the time of slow cooling of magma, olivine crystals form and generally settle to the bottom of the magma chamber, as they have a relatively high density. As a result, this concentrated build-up of olivine creates olivine-rich rocks like dunite at the bottom of the magma chamber.

Interestingly, magnesium-rich olivine has been found in extraterrestrial locations: 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 generalized chemical composition of A2SiO4, with “A” generally being Mg or Fe, but sometimes also Ca, Mn or Ni. Normally, the chemical composition varies between Mg2SiO4 and Fe2SiO4.

Some olivine crystals are also formed during the metamorphism of a dolomitic or dolomite limestone form. Magnesium from dolomite and silica from quartz and other impurities from limestone are transformed into olivine. This olivine further metamorphoses into serpentine (it is also used as a source of magnesium and asbestos, or as a decorative stone).

Olivine’s crystal system is orthorhombic with a spinel structure.

Olivine is easily altered by weathering and is therefore 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 to form a slag.

Quite often, jewelers cut the magnesium-rich variety of olivine that has a formula close to Mg2SiO4 to create the popular green gemstone known as peridot. This gemstone is popular as the birthstone of the zodiac sign Leo. This gemstone is rich in culture and history, and in ancient times, the pharaohs of Egypt used 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 not desirable as a gemstone. The brown tones of olivine become the chrysolite gemstone.

Olivine has also been commonly used in earlier times as a refractory material, to make firebricks, and is also used to be used as a form of foundry sand. Today these uses are rare, as alternative materials are cheaper and easier to obtain.