Descriptive Mineralogy of Olivine

1. CHRONOLOGY OF OLIVINE TO REVEAL
COMMENCEMENT IN PETROLOGY
Madhusmriti Adhikary
Department of Earth Sciences
UG III
SEM 5
Roll no.- 181212003001

2. INTRODUCTION
 Silicate minerals are classified into different types on the basis of their
polyhedral linkage.
 The different types of silicate classifications are as mentioned below:
Silicates Si:O ratio Minerals
Nesosilicate 1:4 Olivine, Garnet,
Aluminosilicates
Sorosilicate 2:7 Melilite, Hemimorphite,
Lawsonite, Pumpellyite
Cyclosilicate 1:3 Beryl, Tourmaline,
Cordierite
Single Chain 1:3 Pyroxene group
Double Chain 4:11 Amphibole group
Phyllosilicate 2:5 Mica group, Talc,
Serpentine
Tectosilicate 1:2 Quartz, Feldspar
Chain
silicates

3. INTRODUCTION CONT.
I
VI
V
IV
III
II
(b)
(a)
I. Nesosilicate structure II. Sorosilicate structure III. Cyclosilicate structure IV. (a) Single chain
silicate (b) Double chain silicate V. Phyllosilicate VI. Tectosilicate
Diagram reference- Tulane University notes

4. BRIEF ON NESOSILICATES
 Isolated SiO4 tetrahedral [SiO4]
−4
interspersed with other cations for charge
balance
 Important rock forming nesosilicates:
Olivine
Garnet
Aluminosilicates

5. OLIVINE
 A group of mineral crystallizing in Orthorhombic system.
 Crystal class- 2/m 2/m 2/m
 General Formula [M1M2SiO4]/ [M2SiO4]
M1= 𝐶𝑎2+, 𝑀𝑔2+, 𝐹𝑒2+ 3+, 𝑀𝑛2+
M2= 𝑀𝑔2+, 𝐹𝑒2+ 3+, 𝑀𝑛2+
 Radii: M2>M1

6. ISOLATED TETRAHEDRAL STRUCTURE
SiO4
SiO4
M1 and M2 sites are bonded to Si tetrahedral by
edge sharing.
• M1 cations are present at the Centre of
Symmetry
• M2 cations are present on the Mirror Planes
Image source –cnx.org
Image
source
-Mdpi.com
• M1: 6 Co-ordination
• M2: 6 Co-ordination
• Si: 4 Co-ordination

7. ISOLATED TETRAHEDRAL STRUCTURE (Cont.)
 Pauling’s 3rd rule: Sharing of Polyhedral Elements I:
The existence of edges, and particularly faces, common
to two anion polyhedra in a co-ordinated structure
decreases the stability.
 But here the sharing of edges are between cationic
polyhedrons.
Hence,
 The bonding of Polyhedra is very strong;
Hardness=6.5- 7; High RI
 No weak planes defining cleavage
Image
source
-Mdpi.com

8. OLIVINE: CHEMISTRY
Image source- Mineral Science by Klein & Dutrow
Larnite
Image source-www.chegg.com

9. Two Minerals form solid solution because:
 Ionic Size Difference should be less than 15%.
For Forsterite and fayalite:
Ionic Radius: Mg2+
= 0.72Å; Fe2+
= 0.78Å
Size difference= 0.06≈4%
 The mineral which form solid-solution should have similar crystal structure.
 Valency of the compounds should not be widely difference.
Hence, Forsterite and Fayalite forms complete solid solution

10. PHYSICAL PROPERTIES
Forsterite:-
 Crystal Form: Prismatic to Tabular
 Colour : Green
 Streak : white
 Lustre : Vitreous
 Fracture: Conchoidal
 Cleavage: Poor
 Hardness: 6.5
 Specific gravity:3.2
 Diaphaneity: Translucent
Forsterite handspecimen
Image source: webmineral.com

11. PHYSICAL PROPERTIES
Fayalite:-
 Crystal Form: Granular to massive
 Colour : Yellow to brown
 Streak : white
 Lustre : Vitreous to resinous
 Fracture: Conchoidal
 Cleavage: Poor
 Hardness: 6.5
 Specific gravity:4.4
 Diaphaneity: Transparent to translucent
Fayalite handspecimen
Image source: Wikipedia.org

12. OPTICAL PROPERTIES
Forsterite:-
Under PPL-
 Colour: Colourless
 Form: Prismatic
 Refractive Index: High
 Pleochroism: Non Pleochroic
Under XPL-
 Optical Character- Anisotropic
 Interference Colour: 2nd Order
 Extinction: Parallel
 Optic Sign: Biaxial(+)
Fayalite:-
Under PPL-
 Colour: Yellow
 Refractive Index: High
 Pleochroism: weakly pleochroic
Under XPL-
 Optical Character- Anisotropic
 Interference Colour: 2nd Order
 Extinction: Parallel
 Alteration: Alter to Serpentine
 Optic Sign: Biaxial(-)

13. OPTICAL PROPERTIES
Basalt (from Melbourne, Victoria) seen in thin section through a polarizing microscope
showing a euhedral phenocryst of olivine in plane polarized light (A), and with crossed
polars (B)
Image source- www.Sciencedirect.com
A B

14. OPTICAL PROPERTIES CONT.
Optic Sign and Extinction:
Obtuse Bisectrix Figure
Acute Bisectrix Figure

15. HIGH PRESSURE POLYMORPHS
Polymorphs are crystalline materials that have
the same chemical composition but different
structure.
MOHO
410KM
520KM
660KM
2900KM
Crust
Olivine
Wadsleyite
Ringwoodite
Perovskite+
Magneso-Wusite

16. OLIVINE: SIGNIFICANCE IN PETROLOGY
Igneous Petrology:-
 Olivine comprises 50% of the mantle composition, hence it can help in
petrographic analysis by determining the melting parameters.
Image source-www.chegg.com
Image source-Tulane university notes

17. OLIVINE: SIGNIFICANCE IN PETROLOGY
Goldich stability series
• Provenance indicator
• Compositional Maturity
Sedimentary Petrology:-

18. OLIVINE: SIGNIFICANCE IN PETROLOGY
Metamorphic Petrology:-
Serpentinization- processes whereby rock (usually ultramafic) is altered, with
the addition of water into the crystal structure of the minerals found within the
rock.
Image
source-
www.sandatlas.com
Serpentinite

19. PARAGENESIS
 Olivine is a major constituent of Dunite and Peridotite.
 In the ultramafic nodules in basalts and kimberlites, olivine compositions
range between Fo91 Fo86.
 Olivine of metamorphic origin occur principally in rocks of ultramafic
composition, in impure carbonates and in iron-rich sediments.

20. ECONOMIC SIGNIFICANCE
 Gemstone variety of olivine
‘Peridot’.
 Also used in several Metallurgical
processes.

21. REFERENCES
 Mineralogy- Dexter Perkins
 Mineral Science- Klein and Dutrow
 Optical Mineralogy- W.D.Nesse
 www.Wikipedia.org
 www.britanica.com
 Tulane University Notes