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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). Most natural olivine falls between these two extremes, with the Mg:Fe ratio determining its specific properties. It is a characteristic mineral of mafic and ultramafic igneous rocks and is a major component of the Earth's upper mantle.
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 often stubby prismatic.
- Crystal Form
- Orthorhombic system; typically forms stubby, tabular, or prismatic crystals, often with rounded edges. Granular masses are very common.
- Cleavage
- Poor to indistinct in two directions at 90 degrees. Often appears to have no cleavage, fracturing conchoidally instead.
- Geological Environment
- Mafic and ultramafic igneous rocks (basalt, gabbro, peridotite, dunite), mantle xenoliths, some metamorphic rocks (e.g., serpentinites, dolomitic marbles), and meteorites.
Key Facts
- Hardness: 6.5-7 on Mohs scale
- Specific Gravity: 3.2-4.3 (increases with Fe content)
- 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 in two directions at 90 degrees
- Composition: Magnesium iron silicate (Mg,Fe)2SiO4
Quick Check
- Color: Olive-green to yellowish-green
- Luster: Vitreous to greasy
- Streak: Colorless (white)
Physical Characteristics
- Crystal Habit: Stubby prismatic, tabular, granular aggregates, massive
- Cleavage Type: Poor to indistinct {010} and {100}
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to greasy
Formation
Olivine forms at high temperatures and pressures, typically crystallizing from mafic and ultramafic magmas. It is a primary mineral in the Earth's upper mantle and oceanic crust. It can also form during metamorphism of impure dolomitic limestones.
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 ranging from Archean to Cenozoic, and in meteorites.
Where to Find
Peridot Mesa, San Carlos Apache Reservation, Arizona, USA
One of the world's most significant sources of gem-quality peridot (olivine).
Kilauea Volcano, Hawaii, USA
Olivine is a common phenocryst in Hawaiian basalts, leading to 'green sand' beaches (e.g., Papakolea Beach).
St. John's Island (Zabargad), Egypt
Historically important source of peridot, known since ancient times.
Pakistan (Kaghan Valley)
Produces fine gem-quality peridot.
Norway (Åheim, Nordfjord)
Significant industrial deposits of olivine, particularly for refractory uses.
Various Ophiolite Complexes Worldwide
Olivine is a major component of the mantle section of ophiolites (e.g., Oman, Cyprus).
Finding Tips
Look in Mafic and Ultramafic Rocks
Search for olivine in basalts, gabbros, peridotites, and dunites. It often appears as small, green, glassy grains.
Check Volcanic Areas
In volcanic regions, especially those with basaltic lavas, olivine crystals can be found as phenocrysts or in volcanic bombs.
Examine Weathered Surfaces
Olivine weathers relatively quickly, often altering to serpentine, iddingsite, or iron oxides. Fresh surfaces are best for identification.
Magnet Test (for fayalite-rich varieties)
Fayalite-rich olivine can be weakly magnetic due to its higher iron content. This is not a definitive test for all olivine.
Specific Gravity Test
Olivine has a relatively high specific gravity (3.2-4.3), which can be a useful distinguishing feature when compared to lighter minerals.
Similar Rocks
Epidote
Ca2(Al,Fe)3(SiO4)3(OH)
Also known as: Pistacite
Serpentine
Mg3Si2O5(OH)4
Also known as: Antigorite, Chrysotile, Lizardite
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
- Magnesium iron silicate, forming a solid solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4).
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