Chronology of Olivine to reveal commencement in petrology, Assignments of Geology

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CHRONOLOGY OF OLIVINE TO REVEAL

COMMENCEMENT IN PETROLOGY

Madhusmriti Adhikary Department of Earth Sciences UG III SEM 5 Roll no.- 181212003001

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 silicate s

BRIEF ON NESOSILICATES

 (^) Isolated SiO 4 tetrahedral [SiO interspersed with other cations for charge balance  (^) Important rock forming nesosilicates: Olivine Garnet Aluminosilicates 

OLIVINE

 (^) A group of mineral crystallizing in Orthorhombic system.  (^) Crystal class- 2/m 2/m 2/m  (^) General Formula [M1M2SiO 4 ]/ [M 2 SiO 4 ] M1= M2= ,  (^) Radii: M2>M 

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

OLIVINE: CHEMISTRY

Image source- Mineral Science by Klein & Dutrow Larnite Image source-www.chegg.com

PHYSICAL PROPERTIES

Forsterite:-

 (^) Crystal Form: Prismatic to Tabular  (^) Colour : Green  (^) Streak : white  (^) Lustre : Vitreous  (^) Fracture: Conchoidal  (^) Cleavage: Poor  (^) Hardness: 6.  (^) Specific gravity: 3.  (^) Diaphaneity: Translucent Forsterite handspecimen Image source: webmineral.com

PHYSICAL PROPERTIES

Fayalite:-

 (^) Crystal Form: Granular to massive  (^) Colour : Yellow to brown  (^) Streak : white  (^) Lustre : Vitreous to resinous  (^) Fracture: Conchoidal  (^) Cleavage: Poor  (^) Hardness: 6.  (^) Specific gravity: 4.  (^) Diaphaneity: Transparent to translucent Fayalite handspecimen Image source: Wikipedia.org

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

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

OLIVINE: SIGNIFICANCE IN

Igneous Petrology:-^ PETROLOGY

 (^) Olivine comprises 50% of the mantle composition, hence it can help in petrographic analysis by determining the melting parameters. Image source-Tulane university^ Image source-www.chegg.com notes

OLIVINE: SIGNIFICANCE IN

PETROLOGY

Goldich stability series

• (^) Provenance indicator
• (^) Compositional Maturity Sedimentary Petrology:-

PARAGENESIS

 (^) Olivine is a major constituent of Dunite and Peridotite.  (^) In the ultramafic nodules in basalts and kimberlites, olivine compositions range between Fo 91 Fo 86.  (^) Olivine of metamorphic origin occur principally in rocks of ultramafic composition, in impure carbonates and in iron-rich sediments.

ECONOMIC SIGNIFICANCE

 (^) Gemstone variety of olivine ‘Peridot’.  (^) Also used in several Metallurgical processes.