1) The document discusses olivine, a group of minerals that form a solid solution between magnesium-iron silicates. 2) Olivine has an orthorhombic crystal structure consisting of silicon-oxygen tetrahedra linked by magnesium and iron atoms. The distribution of magnesium and iron can vary within the structure. 3) Olivine is common in ultramafic rocks and mafic igneous rocks. Its composition ranges from pure magnesium silicate (forsterite) to iron-rich (fayalite) depending on the rock type. Olivine is susceptible to alteration by weathering and hydrothermal processes.

1. DEPARTMENT OF EARTH SCIENCES
UNIVERSITY OF SARGODHA, SARGODHA
ASSIGNMENT
OLIVINE
SUBMITTED BY:
SALAH UD DIN SHABAB
SUBMITTED TO:
SIR ABDUL HANNAN

2. Olivine Group of Minerals
Introduction:
 The members of Olivine group crystallize with orthorhombic symmetry.
 The structure consisting of independent SiO4 tetrahedra linked by divalent
cations.
 In the (Mg, Fe)-Olivine there is complete solid solution between Mg2 SiO4
(Forsterite) and Fe2SiO4 (Fayalite).
 Similarly the (Fe, Mn)-Olivine form a continuous series.
Olivine Crystal
Structure of Olivine:
 The structure consists of individual silicon-oxygen tetrahedra linked by (Mg,
Fe) atoms each of which six nearest oxygen neighbors.
 In accordance with full orthorhombic symmetry the Si-O tetrahedra point
alternately either way along each of the x and y direction.
 The distribution of Mg2+ and Fe2+ in M1 and M2 sites show varying degree of
ordering with the larger Fe2+ cation commonly and paradoxially showing a
small preference for smaller M1 site.
 At high pressure olivine adopt the denser structure of spinal.

3. General formula: А2TO4.
A – Divalent cations (Mg, Fe, Mn,
Ni, Zn, Co, Ca, Pb).
T – Three - and tetravalent cations (Si, Al, Ti)
.

4. The crystal lattice is constructed of individual blocks (А and Б).
Tetrahedrons of SiO4 are directed up and down. Small circles is magnesium ions.
Chemistry of Olivine:
 Olivine very in composition from Mg2 SO4 (Fortsterite) to Fe2 SiO4
(Fayalite), there being complete diadochy between Mg2+ and Fe2+in the
structure.
 In many natural crystals and particularly in more iron-rich olivine there is
a little replacement of (Mg, Fe) by Mn and Ca.
 Nickel and chromium are commonly present in Mg rich Olivine, but the
chromium occur in minute exsolved plates of chromite.
 Relatively small amounts of calcium are present in the majority of olivine.
 Phosphorous in trace amount (up to 400ppm) occurs in some olivines.
 The melting point of forsterite is 1890 -
＋
20C
ｏ
.
 The melting point of forsterite under anhydrous condition increases with
pressure, and under water-saturated condition decreases with pressure.
 Olivine is very susceptible to hydrothermal alteration, to the effects of
weathering and to low grade metamorphism.
 Serpentinization is the most widespread form of olivine alteration and the
most common process of metamorphism in olivine rich rocks.
Optical & physical properties of
Olivine:
 The refractive indices very linearly with composition, both α and γ indices
increasing by about 0.002 per mol percent Fe2SiO4.
 Zoned olivines with less iron-rich cores and more iron-rich margins are
common in some basic volcanic and hypabissal rocks.
 Magnesium-rich Olivine is distinguished from diopside by its poor
cleavage, large optic axial angle and higher birefringence.
Paragenesis:
 Olivine is a major constituent of Dunite and Peridotite.
 In the ultramafic nodules in basalts and kimberlites, olivine compositions
range between Fo91 Fo86.

5.  Olivine in the compositional range Fo80-Fo50 are common constituents in
gabbroic rocks.
 More iron-rich olivines do occur in basic rocks but in general are
restricted to ferrodiorites and mangerite, the olivine of which may extend
fayalitic composition.
 Iron-rich olivine occur in both alkaline and acid plutonic and hypabyssal
rocks,
 Fayalite is present in arfredsonite-fayalite-hedenbergite granite and in
small amounts in many acid and alkaline volcanic rocks, e.g. obsidian,
rhyolites, trachytes and phonolites.
 Olivine of metamorphic origin occur principally in rocks of ultramafic
composition, in impure carbonates and in iron-rich sediments.