Olivine

Olivine is a group of rock-constituting minerals, although the name is usually applied with special reference to the main solid solution of the group, which is between forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). Olivines are one of the most important minerals in the classification of rocks igneous. Magnesium-rich olivine is notable for being the main component of the Earth's upper mantle.

Regarding their chemistry and structure, olivines are neso-silicates that crystallize in the orthorhombic crystalline system. The chemical formula of olivines is A₂SiO₄ where A it can be iron, magnesium, manganese or nickel among others. The more iron it contains, the denser the olivine is. The color of olivine varies from yellowish green, to brown and olive green, the latter color being the reason for its name, which comes from Latin. Low iron contents (12-15%) give olivine green and light colours, while olivines rich in fayalite tend to be brown to black.

Olivine is a source of magnesium in mining, and is extracted from olivine-rich rocks such as dunite. Olivine is also used in the mining industry to purify iron producing slag. Apart from this, olivine is also used in the manufacture of materials resistant to low temperatures.

Olivine group minerals

The group of olivines includes the minerals:

Semi-precious stones

Peridot

Gem-quality forsterite is called peridot. Localities where peridot has been found include: Zeberged in Egypt, Canary Islands, Hawaii, Peridot Mesa in Arizona, Mogok in Burma, Ameklovdalen in Norway, Zhangjikou-Xuanhua in China, mountains Usambara in Tanzania and in certain parts of Antarctica.

Crysolite

A variety of peridot with a yellowish color is called chrysolite. Its name derives from the Greek where it means "golden stone". The term chrysolite is also used to designate gems of other minerals unrelated to olivine. The use of this term is discouraged.

Paragenesis

Image of picritical basalt with numerous olivin crystals.

Olivine rich in magnesium (forsterite) stands out as the main component of the Earth's upper mantle. It is considered that at depths of 350 to 450 km the olivine of the Earth's mantle changes its structure due to the enormous pressure forming a olivine polymorph with spinel structure. Magnesium-rich olivine is also common in mafic and ultramafic rocks such as basalt, gabbro, peridotite, and dunite. Lunar basalts and various meteorites also contain olivine. Olivine poor in magnesium but rich in iron, that is, fayalite, can appear in rocks such as granite and syenite. Olivine cannot be in chemical equilibrium with quartz in a magma since it reacts with its constituent, SiO2, forming enstatite This situation is expressed in the following chemical reaction:

Mg2SiO4(orlivinor)+SiO2→ → Mg2Si2O6(enstatita)- Yeah.

Monticellite is commonly formed in contact zones between dolomites and igneous intrusions. When this occurs, forsterite is also usually formed, although at a lower temperature and once decarbonation is more advanced than when montecellite forms. Knebelite and tefroite appear associated with skarn mineralizations.

Another environment where olivine is formed is during the metamorphism of iron-rich sediments, giving rise to olivines rich in fayalite.

Weathering and alteration

Of the common minerals on the earth's surface, olivine is one of the most easily altered or weathered. Leaching is an important weathering process in magnesium-rich olivines, this is observed as loss of magnesium and adding water and some iron to the crystal. When the olivine is rich in iron, leaching causes oxidation of the olivine and removal of the silica.

Olivine usually weathers readily into iddingsite (a combination of clay minerals, iron oxides, and ferrihydrites) in the presence of water. Iddingsite formation begins with the formation of tiny comb- or saw-shaped channels in the olivine It is in these channels, which are spaced ~200 Å apart, that the first clay minerals originate, forming bridges across the channels. When weathering is more advanced, iron oxidizes to form crystals of goethite from 10 to 30 nm. These crystals have the same crystallographic orientation as olivine. In this phase of weathering part of the magnesium dissolves and leaves the olivine crystal, and another part becomes part of smectite. It is after these transformations, olivine is no longer considered as such and becomes iddingsite.

Xenolito with olivin partially altered to iddingsita.

When the physical weathering of the olivine predominates over the chemical one, black or green sands can form due to the accumulation of olivine. This is the case of some beaches on the islands of Oahu and Hawaii.

Olivine can also be altered in the presence of water by metasomatism in serpentine, magnesite, or talc. The following chemical reactions illustrate such transformations:

5Mg2SiO4(orlivinor)+H2O→ → 2Mg3Si2O5(OH)4(serpentina)+(4MgO+SiO2)- Yeah.

2Mg2SiO4(orlivinor)+2H2O+CO2→ → Mg3Si2O5(OH)4(serpentina)+MgCO3(magnesita){displaystyle 2Mg_{2}SiO_{4}(olivin) + 2H_{2}O + CO_{2}rightarrow Mg_{3}Si_{2}O_{5}(OH)_{4}(serpentine) + MgCO_{3} (magnesite)}}}

4Mg2SiO4(orlivinor)+H2O+5CO2→ → Mg3Si4O10(OH)2(talcor)+5MgCO3(magnesita){displaystyle 4Mg_{2}SiO_{4}(olivin) + H_{2o} + 5CO_{2}rightarrow Mg_{3}Si_{4}O_{10}(OH)_{2}(talco) + 5MgCO_{3} (magnesita)}}}

The reaction of olivine with water and silica can be expressed simply as:

3Mg2SiO4(orlivinor)+H2O+5SiO2 2Mg3Si4O10(OH)2(talcor){displaystyle 3Mg_{2}SiO_{4}(olivin) + H_{2o} + 5SiO_{2}rightleftharpoons 2Mg_{3}Si_{4}O_{10}(OH)_{2}(talk)}