Olivine is the mineral group crystallizes first during the magma cooling as per Bowen reaction series.
It has the general formula X2SiO4 , where X = Fe, Mg, Mn and Ca. with charge of +2.
- All olivine minerals crystallizes in the orthorhombic system.
- Olivine found primarily in igneous but can also found in metamorphic rocks.
- Olivines are olive green and brown in color.
- They are silicates from ortho/nesosilicate group.
Structure –
Olivines are nesosilicates which consists of isolated tetrahedra of SiO4 linked together by means of metal cations.
Metal cations has 6 nearest oxygen neighbours.
Si : O ratio is 1 : 4
Imp minerals –
- Forsterite – Mg2SiO4
- Fayalite – Fe2SiO4
- Olivine – [Mg, Fe]2SiO4
- Monticellite – CaMgSiO4
- Tephroite – Mn2SiO4
Olivine solid solution –
Because there is similar ionic radius and ionic charge of cations. olivines make a complete solid solution series between Forsterite and Fayalite, Fayalite and Tephroite.
The intermediate members between Fayalite and Forsterite are – Forsterite – Crysolite – Hyalosiderite – Hortonolite – Ferrohortonolite – Fayalite.
Reactions –
- Olivine reacts with silica to form pyroxene, thats olivine and silica can not found together.
- Olivine alters into serpentine [antigorite].
- In basaltic rocks olivine alter into outer iron-rich layer of brownish red mineral called Iddingsite.
- Metamorphism of dolomite can form Olivine.
Mg2SiO4 + SiO2 = 2MgSiO3 [olivine to pyroxene]
Fe2SiO4 + 5Mg2SiO4 + 9H2O = 3Mg3Si2O5[OH]4 + Mg[OH]2 + 2Fe[OH]2 [Olivine to serpentine]
2CaMg(CO3 )2 +SiO2 = Mg2SiO4 + 2CaCO3 +2CO2 [metamorphism of dolomite to forsterite]
Physical and Optical Properties
Physical properties
- color – olive green, pale green, brown
- form – massive granular and prismatic
- streak – colorless
- lusture – vitreous
- hardness – 6-7
- fracture – conchoidal
- cleavage – absent
- crystal system – orthorhombic
Optical properties
- Color – colorless
- Paleochroism – non-paleochroic
- RI – high
- Relief – High
- Bireferengence – strong 3rd order
- Extinction – straight
- Alterations – generally present
Variety – Peridot is the gem variety of olivine. used in ornamentation.
Mode of occurance –
- In ultramafic rocks like dunite, peridotite.
- common association of minerlas are chromite, spinel, pyropemetc.
- metamorphic rocks as dedolomitisation.
REFERENCES –
- https://hotcore.info/babki/olivine-mineral-structure.htm
- Research gate -for phase diagram image
- GB.Mahapatra ‘ A Textbook of Geology’
- John D Winter – Principles of Igneous and Metamorphic Petrology
- Dexter Perkins – Minerology
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