6-OLIVINE

• �Structural Formula-

(X²⁺)₂ZO4

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• Site X Z

C. N. 6 4

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Olivine Structure

• Crystallizes with orthorhombic symmetry
• Individual SiO4 tetrahedra
• Si in 4-fold coordination, occupy tetrahedral site
• Fe, Mg other divalent cations in 6-fold coordination, occupy octahedral site
• Complete solid solution between Fe2SiO4 (Fayalite)-Mg2SiO4 (Forsterite)
• Other important end members-Tephroite (Mn2SiO4), Monticellite (CaMgSiO4)
• Fo contain Ni, Cr

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Each tetrahedra point alternately either way

Olivine Structure

• Mg rich Ol has higher melting temp compared to Fe rich Ol
• Cation-O bonding are weaker for larger cations with same charge.
• As more larger cations enter the structure the melting temp. Reduces
• First Ol to sepearte from liquid is Mg rich and larger Fe+2 in consequence concentrates in melt
• Very susceptible to hydrothermal alteration, weathering and low grade metamorphism and converts to serpentine ((Mg, Fe)₃Si₂O₅(OH)₄)

• (Mg,Fe)₂SiO₄ at low pressure it occurs as olivine (α-Mg₂SiO₄).
• With increases in pressure olivine structure become unstable converts to Wadsleyite (β-Mg2SiO4, distorted spinel structure) and Ringwoodite (γ-Mg₂SiO₄, spinel structure) which are denser. The transitions in the earth occur at ~410 km and ~520 km respectively.
• Wadsleyite (β-Mg2SiO4, distorted spinel) and Ringwoodite (γ-Mg₂SiO₄) are polymorph of Olivine
• At ~670 km depth, Ringwoodite disproportionates into a perovskite structured phase and magnesiowustite structured phase

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• α-β transition occurs at high Mg side otherwise directly transforms to γ

gamma from α

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Change in Velocity and �density with depth

• Deeper in the earth greater mineral density.
• At relatively shallow depths of upper mantle Si in tetrahedral sites and Mg/Fe in the octahedral sites.
• As pressure increases atoms pack more tightly so atoms undergo structural transformations to phases with intrinsically denser packing.
• One of the ways to do this is to put Si into octahedral coordination. The transition zone is made up of minerals with a mixture of 4 and 6 coordinated Si.
• Below the transition zone, in the lower mantle, all Si is 6 coordinated. Above the transition zone, in the upper mantle, all Si is 4 coordinated.
• In the Pauling sense, in order for Si to fit in a six-coordinated site, the radius of the O atoms must decrease with increasing pressure, at a rate greater than the Si atoms.

• calculate the weight % of the elements in the mineral, Fo (Mg₂SiO₄)-

Problem III

• calculate the weight % of the oxides in the mineral, Fo (Mg₂SiO₄)-

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Occurrence?

Rock types

• Igneous –
• dunite,
• peridotite (UB / UM)
• Metamorphic –
• Metamorphosed UM / UB (from Serpentine)
• Impure carbonate (from Tremolite)
• Sedimentary ??????
• Association?

Why Fo does not occur with Quartz?

Rock types

• Igneous –
• Basic rocks
• Acidic rocks
• Metamorphic –
• Metamorphosed BIF (iron-rich sedi)
• Sedimentary ??????
• Association?????

Gabbroic rocks

Metamorphosed impure CO3 rocks, metasomatised skarn

Properties in hand specimen

• Crystal System

orthorhombic

• Crystal Habit

granular masses or rounded grains

• Cleavage

no distinct cleavage

• Color

Olive or yellowish-green in hand samples. 

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Imp. Optical property

• Pleochroism

Colorless to pale green in thin section.  Weak, pale green pleochroism in thin section.

• Distinguishing property:

Colorless to olive green in thin section. Second-order interference colors. High relief. Lack of cleavage.

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Olivine in PPL in Basalt

Olivine in CPL in Basalt

Olivine in Gabbro

Difference between associations or ......??????????