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Olivine is a nesosilicate mineral with the general chemical formula (Mg,Fe)2SiO4. It is a solid solution series between the magnesium-rich endmember forsterite (Mg2SiO4) and the iron-rich endmember fayalite (Fe2SiO4). The name 'olivine' refers to its characteristic olive-green color. It is a major component of the Earth's upper mantle and is one of the first minerals to crystallize from mafic and ultramafic magmas. Olivine is relatively unstable at the Earth's surface and readily alters to serpentine, iddingsite, or other secondary minerals through hydration and oxidation processes.
How to Identify
- Color
- Typically olive-green, yellowish-green, or brownish-green. Can also be yellow, brown, or rarely colorless.
- Luster
- Vitreous (glassy) to greasy.
- Texture
- Granular aggregates are common. Individual crystals are typically stubby prismatic.
- Crystal Form
- Orthorhombic system; typically forms stubby, equant, or tabular crystals. Often found as disseminated grains or granular masses in igneous rocks.
- Cleavage
- Poor to indistinct in two directions at 90 degrees, often appearing as conchoidal fracture rather than true cleavage.
- Geological Environment
- Mafic and ultramafic igneous rocks (basalt, gabbro, peridotite, dunite), mantle xenoliths, and some metamorphic rocks (e.g., serpentinites, marbles).
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 3.2-4.3 (increases with Fe content; forsterite ~3.2, fayalite ~4.3)
- Crystal System: Orthorhombic
- Color: Olive-green, yellowish-green, brownish-green, yellow, brown, rarely colorless
- Luster: Vitreous to greasy
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Poor to indistinct {010} and {110}
- Composition: Magnesium iron silicate
Quick Check
- Color: Olive-green to yellowish-green
- Luster: Vitreous to greasy
- Streak: White
Physical Characteristics
- Crystal Habit: Typically granular, massive, or as disseminated anhedral to subhedral grains. Individual crystals are usually stubby prismatic or tabular.
- Cleavage Type: Poor to indistinct, often masked by fracture. Two directions at approximately 90 degrees.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle
- Luster Type: Vitreous to greasy
Formation
Olivine forms at high temperatures and pressures, typically crystallizing from magnesium-rich, silica-poor magmas. It is a primary mineral in the Earth's upper mantle and is brought to the surface in basaltic lavas and ultramafic intrusions. It can also form during metamorphism of impure dolomitic limestones or silica-poor igneous rocks.
Usage
Industrial uses include refractory materials (due to high melting point), foundry sands, and as a source of magnesium. The gem-quality variety, peridot, is used in jewelry. It is also studied for its role in carbon sequestration and as a potential CO2 capture material.
Age Distribution
Found in rocks of all ages, particularly abundant in mafic and ultramafic igneous rocks which can be billions of years old (e.g., in mantle xenoliths) to relatively young volcanic rocks.
Where to Find
San Carlos Apache Reservation, Arizona, USA
World-renowned locality for gem-quality peridot (olivine) found in basaltic lava flows and volcanic bombs.
Zabargad Island (St. John's Island), Red Sea, Egypt
Historical source of large, fine-quality peridot crystals, often found in peridotite xenoliths within basalt.
Kilauea Volcano, Hawaii, USA
Olivine is a common phenocryst in Hawaiian basalts, and small crystals can be found in volcanic sands (e.g., Papakolea Green Sand Beach).
Norway
Significant industrial deposits of olivine, particularly for refractory uses, found in large dunite bodies.
Pakistan
Produces high-quality peridot, often from peridotite xenoliths in volcanic rocks.
Finding Tips
Look in Volcanic Rocks
Search for olivine in basaltic lava flows, volcanic bombs, and tuffs, especially in areas with recent volcanic activity. It often appears as small, green, glassy grains.
Examine Ultramafic Intrusions
Explore outcrops of peridotite, dunite, and gabbro. Olivine is a primary constituent of these rocks and can form large, granular masses.
Check for Mantle Xenoliths
In some volcanic areas, look for xenoliths (rock fragments) of mantle material brought to the surface by magma. These often contain abundant olivine.
Observe Color and Luster
The characteristic olive-green color and vitreous to greasy luster are key identifiers. Fresh surfaces are best for observation.
Consider Associated Minerals
Olivine is commonly found with pyroxenes (augite), plagioclase feldspar, and magnetite in mafic rocks. In ultramafic rocks, it may be associated with chromite and serpentine.
Similar Rocks
Serpentine
Mg3Si2O5(OH)4
Also known as: Antigorite, Lizardite, Chrysotile
Epidote
Ca2(Al,Fe)3(SiO4)3(OH)
Also known as: Pistacite
Garnet
X3Y2(SiO4)3
Also known as: Almandine, Pyrope, Spessartine, Grossular, Andradite, Uvarovite
Scientific Classification
- Mineral Class
- Nesosilicate
- Group
- Olivine Group
- Crystal System
- Orthorhombic
- Chemical Formula
- (Mg,Fe)2SiO4
- Composition
- A solid solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Minor amounts of Mn, Ni, and Ca can substitute for Mg or Fe.
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