Olivine: The Common Rock-Forming Mineral

Olivine is a silicate mineral composed of magnesium iron silicate and is a primary component of the Earth's mantle. Known for its olive-green color, olivine is not just significant in geological processes but is also valued as a gemstone, most commonly referred to as peridot when of gem quality.

Olivine is a magnesium iron silicate mineral with the chemical formula (Mg, Fe)₂SiO₄. The magnesium-iron ratio within its structure defines a solid solution series, with forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄) representing the end members. Olivine's green color results from the partial substitution of ferrous iron (Fe²⁺) for magnesium (Mg²⁺) within its crystal lattice.

Olivine

Olivine is a crucial rock-forming mineral in mafic and ultramafic igneous rocks. Olivine is also a common component of the Earth's mantle, and is considered to be the most abundant mineral on Earth, though not very common on the surface as it weathers quickly when exposed to air and water. Because of this property, scientists are exploring the possibility of using olivine to capture carbon dioxide from the atmosphere to mitigate climate change.

Olivine forms during the cooling and crystallization of magma or lava. Due to its high melting point, it's one of the first minerals to solidify from a cooling magma chamber. This is why we find olivine in igneous rocks like basalt and peridotite. Olivine can also be formed in meteorites, when dust and gas condense in the early solar system.

The gem-quality variety of olivine, called peridot, is prized for its vibrant, lime-tinged green color. The ancient Egyptians called peridot the “gem of the sun” and it was used in many of their artifacts. This relatively affordable gemstone is a popular choice for rings, earrings, and other jewelry, adding a touch of nature's brilliance.

Olivine Properties

Olivine Composition

Olivine is classified as a nesosilicate, or orthosilicate. In this type of silicate mineral, each silicon (Si) atom is surrounded by four oxygen (O) atoms, forming separate SiO₄ tetrahedral units. These tetrahedra are not linked to each other, but rather surrounded by metallic cations like magnesium and iron.

Members of Olivine Mineral Group and Olivine structure

Olivine is a group of silicate minerals with the general chemical formula A₂SiO₄. In this formula, position A is primarily occupied by magnesium (Mg) or iron (Fe). However, in less common occurrences, calcium (Ca), manganese (Mn), or nickel (Ni) can also be present at position A.

The majority of naturally occurring olivine exhibit a solid solution between the two end members: forsterite (Mg₂(SiO₄)) and fayalite (Fe₂(SiO₄)). This solid solution signifies that magnesium and iron can substitute for each other within the crystal structure of olivine in any ratio. This continuous variation in composition is reflected by the olivine chemical formula (Mg, Fe)₂SiO₄. The composition of olivine serves as a valuable indicator of the pressure and temperature conditions during rock formation.

Olivine Varieties: Forsterite and fayalite,
two end members of the olivine mineral series.

Olivine Physical Properties

Color: Olivine is usually green in color, though it can also be yellow, greenish-yellow, brownish-green or brown, depending on its magnesium-iron ratio. Mg-rich olivine (forsterite) tends to be green, while Fe-rich olivine (fayalite) displays a brownish or yellowish hue.
Crystal System: Orthorhombic.
Streak: Olivine has a white or colorless streak regardless of its overall color.
Hardness: Olivine has a hardness of 6.5–7 on the Mohs scale, making it relatively hard.
Transparency: It ranges from transparent to translucent.
Luster: Olivine exhibits a vitreous (glassy) luster when polished.
Cleavage: Poor cleavage in two directions at 90 degrees.
Fracture: Olivine has a conchoidal fracture, which means it breaks in smooth, curved surfaces that resemble the inside of a seashell.
Density: Density: The density of olivine varies depending on its composition, ranging from 3.22 to 4.39 g/cm³. Forsterite typically has a density around 3.2 g/cm³, while fayalite's density can reach up to 4.4 g/cm³.
Habit: Olivine can occur in various forms, including granular (composed of tiny grains), massive (large, formless chunks), or disseminated (scattered grains throughout a rock). It can also occur as short prismatic or tabular crystals.
Tenacity: Olivine is brittle, meaning it breaks easily when struck sharply.
Solubility: Olivine is slightly soluble in weak acids like hydrochloric acid.
Magnetism: Olivine is not magnetic itself. However, some olivine-rich rocks may be slightly magnetic due to the presence of minor magnetic minerals like magnetite as impurities.
Melting Point: The melting point of olivine also varies depending on its composition. Forsterite (Mg-rich) has a higher melting point (around 1890°C) compared to fayalite (Fe-rich) which melts at a lower temperature (around 1200°C).
Fluorescence: Olivine typically does not fluoresce under ultraviolet light.
Refractive Index: The refractive index of olivine ranges from 1.65 to 1.70.

Peridot (gemstone variety of olivine)
From: Island (Zebirget), Red Sea Egypt

Olivine Properties in Thin Sections

Color: In thin section, olivine is typically colorless to pale green. This is a lighter shade compared to the olive green or yellow-green seen in hand sample. The exact color can vary slightly depending on the iron content, with forsterite (Mg-rich) being lighter and fayalite (Fe-rich) being slightly darker.

Crystal habit: Olivine crystals can be euhedral (well-formed with distinct crystal faces) or anhedral (irregular and lacking distinct crystal faces).

Relief: Olivine exhibits high relief. This means it stands out prominently from the surrounding minerals due to its higher refractive index. In thin section, olivine grains will appear bright with a distinct outline compared to lower relief minerals.

Fractures: Olivine commonly displays fractures throughout the grain. These fractures can be irregular or appear as a network of intersecting lines.

Pleochroism: Olivine typically exhibits weak pleochroism, particularly in fayalite-rich varieties. This means the color may appear slightly different depending on the direction of light passing through the crystal, ranging from colorless to pale yellow or green.

Birefringence: The birefringence of olivine is moderate, with a birefringence value around 0.036 to 0.040.

Interference Colors: Due to its moderate birefringence, olivine exhibits second-order interference colors under crossed nicols. These colors can range from yellow to orange to blue, depending on the thickness of the thin section.

Isotropy/Anisotropy: Olivine is an anisotropic mineral, meaning its optical properties vary depending on the direction of light transmission. This is in contrast to isotropic minerals, which have the same optical properties in all directions.

Extinction angle: Olivine typically shows straight or parallel extinction of 0°. This means the crystal goes completely dark (extinct) under crossed nicols when the light vibrates parallel to one of its crystallographic axes.

Optic sign: The optic sign of olivine refers to its birefringence behavior. Olivine can be either biaxial positive or negative, depending on its composition. Forsterite is typically biaxial positive, while fayalite can be either positive or negative.

Twinning: Olivine is not typically twinned.

Olivine crystals in thin section, XPL.
Photo: Bob Gooday

Olivine Gabbro thin section.
Photo:Crystal Luna

Additional Properties

Inclusions: Olivine commonly contains inclusions such as gas bubbles, mineral inclusions (e.g., chromite, magnetite), and sometimes liquid inclusions.
Associated Minerals: Pyroxene, amphibole, plagioclase feldspar, chromite, and magnetite are frequently associated with olivine in igneous environments.

Olivine in basalt

Where is Olivine Found?

Olivine is found in a variety of geological environments, both on Earth and in extraterrestrial materials. Its presence is significant in understanding the formation and composition of these environments.

Earth's Upper Mantle

Olivine is the most abundant mineral in the Earth's upper mantle, located beneath the Earth's crust, where it makes up about 60% of the rock. Rocks derived from the mantle, such as peridotite, are rich in olivine. Additionally, mantle xenoliths, fragments of mantle rock brought to the surface by volcanic eruptions, often contain olivine.

Mafic and Ultramafic Igneous Rocks

Basalt: A common mafic volcanic rock that often contains olivine phenocrysts (large, distinct crystals).
Gabbro: A coarse-grained mafic intrusive equivalent of basalt that can also contain olivine.
Peridotite: An ultramafic rock with very low silica content (less than 45%). Peridotite is coarse-grained and greenish-brown in color. It's composed mainly of olivine and pyroxene minerals, with minor amounts of other minerals.
Dunite: An ultramafic coarse-grained and dark green in color. It's almost entirely (>90%) composed of olivine, with very little pyroxene or other minerals.

Metamorphic Rocks

Olivine can be present in metamorphic rocks that were originally igneous rocks containing olivine, such as basalts or peridotites, and have undergone metamorphism. One common metamorphic rock associated with olivine is serpentinite. Serpentinite forms when olivine-rich rocks, such as peridotite, are metamorphosed in the presence of water. During this process, olivine reacts with water to form serpentine minerals, which give the resulting serpentinite its distinctive green color and greasy or soapy feel.

Metamorphic rock serpentinite containing remnants of olivine.

Meteorites

Pallasites: A type of stony-iron meteorite that contains large olivine crystals embedded in a metallic matrix.
Chondrites: Common stony meteorites that often include olivine among their constituent minerals.

Pallasite meteorite with green olivine crystals embedded in a metal matrix.

Extraterrestrial Locations

Lunar Rocks: Olivine has been identified in rocks from the Moon brought back by the Apollo missions.
Martian Surface: Olivine has been detected on the surface of Mars by various space missions, indicating volcanic activity on the planet.

Green Sands Beach, Hawaii

Olivine Sand: Green Sand Beaches

Olivine sand is a type of sand composed primarily of olivine. Green sand beaches are a rare sight, with only a handful existing worldwide. What makes these beaches so special is their unique green sand, caused by olivine eroding from the enclosing volcanic cone (tuff ring). Olivine isn't very stable at the Earth's surface, weathering quickly when exposed to air and water. That's why olivine sand beaches are relatively rare. Additionally, olivine is denser than most other volcanic components found in the sand. This allows it to accumulate on the beach while lighter elements are washed away by the waves.

Close-up of olivine sand grains, from the Green sand beach, Papakolea Beach, Hawaii

Green sand beaches are rare, with only four found around the world:

Hawaii (USA): Papakōlea Beach - Most famous, vibrant green sand.
Guam (USA): Talofofo Beach - Subtle green sand, known for surfing.
Galapagos Islands (Ecuador): Punta Cormorant - Green sand and flamingo sightings!
Norway: Hornindalsvatnet - Europe's only, green-hued lake shore.

Dunite (also called Olivinite) is intrusive igneous ultramafic rock
that is composed almost entirely of olivine.

Olivine Uses

Olivine has several uses:

Gemstones: Gem-quality olivine, known as peridot, is valued for its vibrant green color and used in jewelry. Significant deposits are found in Egypt, Myanmar, and the United States.
Refractory Material: Due to its high melting point, olivine is used as a refractory material in furnace linings for steel production and other high-temperature processes.
Foundry Sand: Olivine sand is used in foundries for casting molds. Its high thermal stability and low thermal expansion make it suitable for this purpose.
Abrasives: The hardness and angular fracture of olivine make it an effective abrasive material for sandblasting and surface preparation processes.
Carbon Sequestration: Olivine can be used to capture and store CO₂ through mineral carbonation, helping to reduce greenhouse gas concentrations in the atmosphere.

Olivine grains

Facts about Olivine

Historically, olivine was often referred to as chrysolite, derived from the Greek words for "gold" and "stone," reflecting its yellow-green to golden appearance.
Olivine is named after its olive-green color. In 1789, a mineralogist named Abraham Gottlob Werner decided the existing name "chrysolite" wasn't specific enough and opted for a more descriptive term based on the mineral's usual olive hue.
The largest known olivine deposit is in Norway, at Åheim, where the mineral is mined for industrial applications.
In Hawaiian culture, olivine is believed to be the tears of the goddess Pele, symbolizing peace and renewal.
Scientists can create synthetic olivine in laboratories, useful for studying its properties and potential industrial applications.
Peridot, the gemstone variety of olivine, is one of the few gemstones that occur in only one color.
Olivine's double refraction property means light passing through it splits into two beams, causing a distinctive sparkle in the gem variety peridot.
Olivine has been found in meteorites and on Mars, suggesting its presence in the early formation of the solar system.
Olivine weathers quickly on the surface, so it is not common in sedimentary rocks.

Large Olivine crystals, some with peridot quality

Olivine FAQs

What is the significance of olivine in geology?

Olivine is significant for geologists because:

Petrology: As a primary mineral in mafic and ultramafic rocks, olivine is essential for understanding igneous processes.
Mantle Composition: Olivine is a major component of the Earth's upper mantle. It helps geologists understand the composition and behavior of the mantle, which is crucial for studying plate tectonics and mantle convection processes. Also, Olivine's presence in subduction zones and mid-ocean ridges provides valuable data on plate tectonics and mantle convection processes.
Seismic Activity: The transformation of olivine into high-pressure minerals such as wadsleyite and ringwoodite affects seismic wave behavior, offering insights into the Earth's interior structure and dynamics.
It alters to form minerals like serpentine during hydrothermal metamorphism.
Planetary Geology: Olivine is found on other planetary bodies, such as the Moon, Mars, and asteroids, offering clues about their geological history and the processes that shaped them.

Why is the name "olivine" used instead of "forsterite" or "fayalite"?

The name "olivine" is used because determining whether forsterite or fayalite is dominant requires detailed chemical analysis. "Olivine" provides a quick and convenient way to identify the mineral without needing such testing.

How do you identify olivine?

Olivine can be easily identified by its olive-green color, glassy luster, absence of cleavage, and its presence in dark-colored rocks such as basalt and peridotite. In thin sections, olivine appears colorless to olive green with high retardation and fractures displaying irregular patterns, alongside second-order interference colors. It exhibits high relief and may show signs of alteration.

Is olivine rare?

Olivine is not considered rare, but gem-quality olivine suitable for use in jewelry is less common. Large deposits of olivine are found in certain geological formations, making it readily available for industrial use.