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Olivine

earth science Maturity 7-9 Next article →

Olivine is a magnesium iron silicate mineral that plays a vital role in our planet's structure. It is classified as a nesosilicate, which means it is an orthosilicate with a specific atomic arrangement.

Atomic structure of olivine 1
Atomic structure of olivine 1

This mineral is a primary component of the Earth's upper mantle. In fact, olivine and its high-pressure structural variants make up over 50% of the upper mantle by volume. Because it is so abundant, it is one of the most common minerals on Earth. Its presence is critical because its physical properties influence the rheology, or the way matter flows, within the mantle. This solid flow is what drives the movement of plate tectonics across the globe.

The chemical makeup of olivine exists on a spectrum between two different types, called endmembers. One endmember is forsterite, which is rich in magnesium (Mg2SiO4). The other is fayalite, which is rich in iron (Fe2SiO4). The ratio between these two determines the mineral's specific properties. For example, forsterite has an unusually high melting temperature of almost 1,890°C at atmospheric pressure. In contrast, fayalite has a much lower melting point of about 1,205°C. Scientists express these compositions using molar percentages, such as Fo70, which means the mineral is 70% forsterite.

At the atomic level, olivine has a very organized structure. It uses an orthorhombic P Bravais lattice. In this structure, each silica (SiO4) unit is joined by divalent metal cations. Every oxygen atom in the silica unit is bound to three metal ions. This creates a structure similar to spinel, which is seen in minerals like magnetite. However, olivine uses different types of ions to maintain its shape. While olivine is mostly made of magnesium, iron, silicon, and oxygen, it can also hold small amounts of manganese (Mn) and nickel (Ni). These trace elements are often responsible for the mineral's characteristic olive-green color.

Olivine forms in several different geological environments. It often crystallizes from magma that is rich in magnesium but low in silica. This process creates mafic rocks like basalt and gabbro. When a rock contains more than 40% olivine, it is classified as an ultramafic rock called peridotite. If the olivine content exceeds 90%, the rock is known as dunite.

Iddingsite
Iddingsite

Olivine can also form through metamorphism. This happens when impure dolomite or other sedimentary rocks with high magnesium and low silica undergo intense heat and pressure. While magnesium-rich olivine is very stable, iron-rich fayalite is much rarer in the Earth's crust.

Deep within the Earth, extreme pressure changes the very nature of olivine. As it sinks to greater depths, it undergoes phase transitions, where the atoms rearrange into new structures. Below depths of about 410 km, olivine transforms into wadsleyite through an exothermic process, which releases heat. At roughly 520 km, wadsleyite transforms into ringwoodite.

Sunnylvsfjord MS-Midnatsol
Sunnylvsfjord MS-Midnatsol

At even greater depths, around 660 km, ringwoodite decomposes into silicate perovskite and ferropericlase. These changes cause a jump in the density of the mantle. These transitions help move heat through the Earth, though the final reaction can actually slow down the flow of material.

Beyond our planet, olivine is a common sight in space. It has been discovered on both the Moon and Mars.

Météorite Esquel, exposition Météorites, Muséum national d'histoire naturelle de Paris 05
Météorite Esquel, exposition Météorites, Muséum national d'histoire naturelle de Paris 05

In our solar system, it is found in chondrites and pallasites, which are meteorites containing a mix of iron-nickel and olivine. Scientists have also detected the spectral signature of olivine in the dust disks surrounding young stars. In 2006, the Stardust spacecraft confirmed that olivine was present in a comet. Some rare A-type asteroids are also believed to have surfaces dominated by this mineral.

Humans use olivine in many industrial and decorative ways. When it is clear and high-quality, it is cut into a gemstone known as peridot. In the industrial sector, olivine sand is used in aluminum foundries to create molds. This sand is helpful because it requires less water than silica sand, which reduces the amount of steam gas produced during metal pouring.

Sunnylvsfjord MS-Midnatsol
Sunnylvsfjord MS-Midnatsol

In Norway, which produces about 50% of the world's industrial olivine, the mineral is also used in sauna stoves due to its density and heat resistance. Scientists are even studying ways to use crushed olivine on beaches to help capture carbon dioxide from the atmosphere.

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🖼️ Images & Media (5)
File:Olivine in polarizing light.jpg
Olivine in polarizing light.jpg
File:Météorite Esquel, exposition Météorites, Muséum national d'histoire naturelle de Paris 05.jpg
Météorite Esquel, exposition Météorites,...
File:Atomic structure of olivine 1.png
Atomic structure of olivine 1.png
File:Iddingsite.JPG
Iddingsite.JPG
File:Sunnylvsfjord MS-Midnatsol.jpg
Sunnylvsfjord MS-Midnatsol.jpg
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