Olivine is a group of minerals, one of the most common mineral and a major rock forming mineral.
Good specimens and large crystals are uncommon.
Olivine is a mineral group with Forsterite and Fayalite members, they crate a solid solution series and most common specimens identified as olivine.
Olivine is not scientifically recognized as a separate mineral.
Olivine may contain some PO4 replacing SiO4
Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1900 Celsius and Fayalite much lower at 1200 Celsius.
Olivine rich in magnesium named olivinoid has been discovered in meteorites, on the Moon and Mars, as well on asteroid 25143 Itokawa.
At high temperatures and pressure in depth earth, olivine structure is no longer stable.
Below 410 Km, olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and at 520 Km depth, wadsleyite transforms exothermically into ringwoodite which has the spinel structure.
Olivine is one of the weaker common minerals being solubile in the presence of water, proposed to sequester CO2 by dispersing fine grained olivine on beaches.
It alters into iddingsite (combination of clay minerals, iron oxides and ferrihydriteS) in the presence of water.
The presence of iddigsite on Mars suggest that liquid water once existed there and might enable scientists to determine when there was on the planet.
Forsterite has a greater content of megnesium; pure form is uncommon.
Pure Fayalite is very rare.
Tephroite is a mineral which many consider a member of Olivine group, forms a series with Forsterite.
Olivine is a group composed of the primary members:
Forsterite: Mg2SiO4
Olivine ( Chrysolite ): (Mg,Fe)2SiO4 ; 25,37% magnesium; 14,57% iron; 18,32 Silicon; 41,74% oxygen; 42,06% MgO; 18,75% FeO; 39,19% SiO2
Fayalite: Fe2SiO4
Tephroite: Mn2SiO4
Color:
Forsterite and Olivine can be olive-green, light green, dark green, yellow-green, yellow brown and brown.
Forsterite is colorless but this is very rare
Fayalite is usually yellow-brown to brown.
Characteristics:
Crystal system orthorhombic
Streak colorless
Hardness 6,5-7 Mohs
Transparent to translucent
Specific gravity: 3,2-3,4
Density 3,27-3,37
Luster vitreous
Cleavage 2,1 ; 3,1 forming 90 degree angle
Fracture conchoidal
Tenacity brittle
Habit: often rounded grains, in dense aggregates of grainy crystals, as fractured masses and as rounded waterworn pebbles and grains
Large crystals which are prismatic and stubby, are uncommon except a few localities
Crystals are often rounded faces
Rock type: igneous, metamorphic, meteoric
Optical properties Biaxial (+)
Composition: magnesium iron silicate, group of silicates, nesosilicates
Olivine occurs in mafic and ultramafic igneous rocks and is also found in metamorphic rocks and serpentine deposits as primary mineral
Varieties:
Chrysolite or Olivine - yellowish, green form of olivine; often as synonym for Olivine; old name to describe
yellow, transparent chrysoberyl; can be olive-green, light green, dark green, yellow-green, yellow brown and brown; (Mg,Fe)2SiO4
Forsterite is colorless but this is very rare; Mg2SiO4
Dunite - solid, grainy masses of olivine usually classified as a rock
Olivinoid - extraterrestrial form of olivine found in meteorites
Peridot - transparent green variety of olivine often used as gemstone
Fayalite is usually yellow-brown to brown; Fe2SiO4
Tephroite: Mn2SiO4
Liebenbergite (Ni,Mg)2SiO4
Laihunite FeFe2(SiO4)2
Glaucochroite CaMnSiO4
Kirschsteinite CaFeSiO4
Uses:in industrial for flux for steel production and important ore of magnesium
Peridot, the transparent olive-green to yellow-green is used as gemstone
Used to process CO2 called enhanced weathering.
When olivine is crushed, it weathers completely within a few years, depending on the grain size, all the CO2 is produced by burning one liter of oil can be sequestered by less than one liter of olivine.
The reaction is exothermic but slow.
In Finland is marketed as ideal rock for sauna stoves because of its comparatively high density and resistance to weathering under repeated heating and cooling.
Common mineral associations: feldspars, serpentine, hornblende, augite, spinel, diopside, chromite, magnetite, spinel, iron-nickel
Distinguishing similar minerals:
Tourmaline - different environment and crystal structure, is harder 7-7,5 Mohs
Apatite - lower hardness, different crystal habits
Garnet - occurs in different crystals, lacks cleavage
other names: olivin olivine olivina chrysolite forsterite dunite peridot fayalite tephroite olivinoid meteorite meteorit meteor
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