Olivine is a ubiquitous, high-temperature, rock-forming silicate mineral named for its characteristic olive-green to yellowish-green color. It is one of the most common minerals on Earth by volume, playing a profound role in the planet’s interior structure and geological processes. As a primary subject of study in petrology, it contributes significantly to understanding magma crystallization and the composition of the deep Earth. Olivine’s presence in both terrestrial and extraterrestrial rocks highlights its importance to geology and planetary science.
Chemical Composition and Crystalline Structure
Olivine is classified chemically as a magnesium iron silicate, containing magnesium, iron, silicon, and oxygen. Its general chemical formula is (Mg,Fe)\(_2\)SiO\(_4\), where Mg and Fe substitute for each other within the crystal structure. This substitution creates a complete solid-solution series between the two end-members: forsterite (Mg\(_2\)SiO\(_4\)), the magnesium-rich variety, and fayalite (Fe\(_2\)SiO\(_4\)), the iron-rich variety. Composition is commonly expressed as a molar percentage of forsterite, such as Fo\(_{90}\).
Olivine belongs to the nesosilicate group, defined by its crystalline structure featuring isolated silicate tetrahedra. In this structure, each silicon atom bonds to four oxygen atoms, forming a tetrahedron separate from other units. These isolated tetrahedra are linked by divalent magnesium and iron cations, which occupy octahedral sites within the crystal lattice. Olivine crystallizes in the orthorhombic system.
Natural Distribution and Formation
Olivine is the most abundant mineral in the Earth’s upper mantle, often constituting over 50% of the rock volume. Its presence is inferred from geophysical data and confirmed by xenoliths—fragments of deep rock brought to the surface by volcanic eruptions. The high-pressure, high-temperature conditions of the mantle are the primary environment for olivine’s stability.
On the Earth’s surface, olivine is a signature component of mafic and ultramafic igneous rocks, which form from magmas with low silica content. Due to its high melting point, it is one of the first minerals to crystallize as magma cools. Rocks containing significant olivine include basalt and gabbro, common at divergent plate boundaries and oceanic hot spots. Ultramafic rocks like dunite and peridotite are composed of over 40% to 90% olivine.
Olivine is also found in extraterrestrial materials, providing clues about the formation of the solar system. Magnesium-rich olivine has been identified in meteorites, on the Moon, and on Mars. Pallasite meteorites are striking examples, consisting of large olivine crystals embedded within an iron-nickel metal matrix. These meteorites are thought to represent the boundary between the rocky mantle and metallic core of a differentiated asteroid.
Commercial Uses and Environmental Significance
The gem-quality variety of olivine is known as peridot, prized for its vibrant yellow-green to lime-green color. Peridot is one of the few gemstones that occurs in only one color, with shade variations determined by the amount of iron present. Historically, the ancient Egyptians referred to it as the “gem of the sun.” It continues to be a popular choice in modern jewelry.
In industrial applications, olivine is valued as a refractory material, meaning it is highly resistant to heat and chemical attack. Its high melting point makes it suitable for use as molding sand in metal casting and for manufacturing refractory bricks used in furnaces. The mineral withstands the extreme temperatures of these processes without breaking down.
Olivine has environmental significance due to its potential for geological sequestration of carbon dioxide. Its natural weathering involves a chemical reaction with water and dissolved CO\(_2\), converting the atmospheric gas into stable carbonate minerals. This process, known as enhanced weathering, can be artificially accelerated by grinding the rock into fine particles to increase surface area. This research aims to use olivine’s natural carbon-capturing ability on a large scale to mitigate climate change.