Olivine - (Fe,Mg)2SiO4

Olivine minerals constitute an isomorphous group of orthosilicates; independent silicate tetrahedra, in layers parallels to (100), point alternately up and down along a, linking zigzag bands of edge-sharing MO6 (M = metal atom), that extend along c. Extensive solid solution exist between end-members composition of the Olivine group when large cation are similar size ( Mg---Fe; Fe---Mn).

Members of the Olivine group are:

• Forsterite Mg2SiO4
• Fayalite Fe2SiO4
• Tephroite Mn2SiO4
• Monticellite CaMgSiO4
• Kirschteinite CaFeSiO4
• Glaucochroite CaMn SiO4
• Picrotephroite (Mg,Mn)2SiO4

Alteration:

Olivine is highly subject to weathering, hydrothermal alteration, and metasomatic alteration. Alteretion product are often complex mixture of fine-grained minerals that are not distinguished by optical means. Some characteristic form of olivine alteration are:

Serpentine: Serpentine alteration is common in plutonic or metamorphic rocks, it being along fracture, as the result of hydrothermal or deuteric processes and may completely convert olivine rocks in a serpentine rock, with a structure characteristic of the olivine fracture pattern ("mesh" structure).

Serpentine alteration can be described by the reaction:

2Mg2SiO4 + 3H2O = Mg3Si2O5(OH)2 + Mg(OH)2

If CO2 is present in the sistem:

2Mg3Si2O5(OH)4 + 3CO2 = Mg3Si4O10(OH)2 + 3MgCO3+ 3H2O (serpentine + CO2 = Magnesite + talc)

Iddingsite and bowlingite: Iddingsite and bowlingite alteration are most characteristic of oxidation, deuterici alteration or weathering. Iddingsite appears as reddish-brown replacement of olivine, along fracture, rims or complete replacement. Iddingsite may appears optically homogeneous , with high relief, and high interference colors. Bowlingite is a fine-grained aggregate containing smectite, goethite, chlorite, calcite, silica, talc.

Chlorophaeite: Chlorophaeite alteration is also characteristic of volcanic or shallow intrusion, this fine-grained aggregate tend to be isotropic. Probably contain limonite, chlorite and serpentine.

Olivine crystallize very early in the crystallization sequence of a magma and it is characteristic of mafic/ultramafic igneous rocks; early crystals may form layered intrusion by gravity settling (dunite cumulate). Fayalite may appear with either quarz or nepheline and it is occasionally found in salic igneous rocks (alkali granite or rhyolite).

Optical Properties:

Relief: High.
Color: Colorless or pale green-yellow with increasing Fe.
Interference colors: Strong, III order colors.
Cleavage: Rather poor on (010) and (110).
Form: Six sides circular section on (001), rectangular section on (010), six or eigth sides section elongated on c in (100) section.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. PPL image , 2x (Field of view = 7mm)
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. PPL image , 2x (Field of view = 7mm)
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. XPL image , 2x (Field of view = 7mm)
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. XPL image , 2x (Field of view = 7mm)
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. XPL image , 2x (Field of view = 7mm)
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Olivine in a Troctolite. Note the fractures due to the increase in volume due to the serpentinization. XPL image , 2x (Field of view = 7mm)
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Olivine crystals in a dunite. XPL image , 2x (Field of view = 7mm)
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Olivine crystals in a dunite. XPL image , 2x (Field of view = 7mm)
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Olivine and chromite crystals in a dunite. PPL image , 2x (Field of view = 7mm)
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Olivine crystals in a dunite. XPL image , 2x (Field of view = 7mm)
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Olivine crystals in a dunite. XPL image , 2x (Field of view = 7mm)
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Olivine crystals in a dunite. XPL image , 2x (Field of view = 7mm)
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Euhedral Olivine crystals with interstitial plagioclase in a Orthocumulates from New Zeland. XPL image , 10x (Field of view = 2mm)
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Euhedral Olivine crystals with interstitial plagioclase in a Orthocumulates from New Zeland. XPL image , 10x (Field of view = 2mm)
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Euhedral Olivine crystals with interstitial plagioclase in a Orthocumulates from New Zeland. XPL image , 10x (Field of view = 2mm)
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Euhedral Olivine crystals with interstitial plagioclase in a Orthocumulates from New Zeland. XPL image , 10x (Field of view = 2mm)
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Euhedral Olivine crystals with interstitial plagioclase in a Orthocumulates from New Zeland. XPL image , 10x (Field of view = 2mm)