Olivine
A common mantle mineral that drives plate tectonics and weathers rapidly.
Olivine is a magnesium iron silicate mineral with the chemical formula (Mg,Fe)₂SiO₄, belonging to the nesosilicate or orthosilicate group. It is the primary component of the Earth's upper mantle and one of the most abundant minerals by volume on the planet, though it weathers rapidly when exposed at the surface. The mineral forms a solid solution series between two endmembers: forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). Composition is expressed as molar percentages of forsterite (Fo) or fayalite (Fa). Forsterite has an unusually high melting temperature near 1,890 °C at atmospheric pressure, while fayalite melts at a much lower temperature of about 1,205 °C, with properties varying smoothly between the two. Only minor amounts of elements other than oxygen, silicon, magnesium, and iron are incorporated, with manganese and nickel being the most common trace elements. Olivine typically appears olive-green, a color attributed to traces of nickel, but can turn reddish due to iron oxidation. Translucent olivine is used as the gemstone peridot, also historically called chrysolite; fine gem-quality specimens have come from mantle rocks on Zabargad Island in the Red Sea. Mg-rich olivine crystallizes from magnesium-rich, silica-poor magma, forming mafic rocks like gabbro and basalt. Ultramafic rocks such as peridotite contain over 40% olivine, while dunite exceeds 90% and likely forms from crystal settling in magma. Olivine and its high-pressure polymorphs constitute over half of the Earth's upper mantle, and its properties dominate mantle rheology, influencing plate tectonics. At high pressures, olivine can dissolve significant water—up to about 8,900 parts per million by weight—which drastically reduces its resistance to solid flow. Mg-rich olivine is also found in meteorites, on the Moon and Mars, on asteroid 25143 Itokawa, and in the dust disks around young stars and comets. The olivine group includes related minerals such as tephroite, monticellite, larnite, and kirschsteinite. Its crystal structure is orthorhombic (space group Pbnm) with isolated silicate tetrahedra, described as a hexagonal close-packed array of oxygen ions.
- chemical_formula
- (Mg,Fe)2SiO4
- crystal_system
- Orthorhombic
- endmembers
- Forsterite (Mg2SiO4) and fayalite (Fe2SiO4)
- common_uses
- Gemstone peridot (chrysolite), industrial metalworking
- color
- Typically olive-green, may alter to reddish
- major_elements
- O, Si, Mg, Fe; minor Mn and Ni
Lore & Background
Olivine is named for its typically olive-green color, thought to result from traces of nickel, though it may alter to reddish from iron oxidation. Translucent olivine is used as the gemstone peridot, also called chrysolite. Some of the finest gem-quality olivine has come from mantle rocks on Zabargad Island in the Red Sea. The mineral occurs in mafic and ultramafic igneous rocks and certain metamorphic rocks. Mg-rich olivine crystallizes from magma low in silica, forming rocks such as gabbro, basalt, peridotite, and dunite. Fe-rich fayalite is rarer, found in some granites and rhyolites, and can coexist with quartz.
Reader's Guide
Olivine's significance lies in its dominance of Earth's upper mantle, where it constitutes over 50% of the volume and influences the solid flow that drives plate tectonics. Experiments show that at high pressures, olivine can dissolve substantial water, potentially holding more water than Earth's oceans, and that water content drastically reduces its resistance to flow. Its phase transitions at depth—to wadsleyite, ringwoodite, and then perovskite—create seismic discontinuities and affect mantle convection. Olivine weathers rapidly on the surface, altering to iddingsite, and its presence on Mars suggests past liquid water. Artificially increasing olivine weathering has been proposed for CO2 sequestration. Norway produces about 50% of the world's industrial olivine. Extraterrestrially, Mg-rich olivine has been found in meteorites, on the Moon, Mars, asteroid Itokawa, and in comet dust.
Did You Know?
- Olivine is named for its typically olive-green color, thought to result from traces of nickel.
- Olivine and its high-pressure variants constitute over 50% of Earth's upper mantle.
- The spectral signature of olivine has been detected in comet tails and dust disks around young stars.
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