Enstatite- Orthoferrosilite

Content•Introduction.•Structure.•Chemistry.•Optical and Physical Properties.•Paragenesis.•Conclusion.•Refernces.

Enstatite-Orthoferrosititebelongs to Pyroxene Group

Orthorhombic System General formula is X1-p Y1+p Z2 O6

where X=Ca or Na; Y=Mg,Fe2+,Ni,Li,Fe3+,Cr or Ti

Z=Si or Al. p≈1

In the orthopyroxene serice p≈1 and the content of trivalent ions is small, in the monoclinic pyroxenes the value of p varies from zero.

INTRODUCTION

Structure

The essential feature of the structure of all Pyroxenes is the linkage of SiO4

tetrahedra by Shareing two of the four corner to form continuous chains of compostion(SiO3)n.

•The along the length of chain is approximately 5.3 A

•C is a parameter of th e unit cell.

•The chains which are linked laterally by cations(Ca,Mg,Fe,Na,Al).

•The grossly different arrangements of chains that lead to the main sub division of pyroxenes into clino and the orthopyroxenes which are monoclinic and orthorhombic respectively.

•Pyroxenes having a compostion that can be expressed in terms of CaMgSi2 O6-CaFeSi2 O6-MgSiO3-FeSiO3can be divided into two Structural groups.

• CaMgSi2 O6-CaFeSi2 O6 with up to about 25 mol.per cent (Mg,Fe)SiO3has monoclinic symmetry.

• The other group lie near the Mg SiO3-FeSiO3 join and have a Maximum content of about 15 mol.per cent CaSiO3

ChemistryIons other than Mg andFe+2 are invariably present in the orthopyroxenes and these commonly include Ca,Mn,Ni,Fe+3,Cr, Al and Ti, in most specimens, however, the sum of these constituents does not exceed 10 Mol%.Chromium and nickel occur mainly in the magnesium rich orthopyroxenes of igneous rocks.

•High contents of manganese are found in Iron-rich Orthopyroxenes of igneous rocks as well as in wide range of compositions in the orthrohombic pyroxenes of metamorphic rocks.

* High contents of aluminium occur mainly in orthopyroxenes of high grade metamorphic rocks.

*

Optical and Physical Propertiesα. 1.650-1.768β. 1.653-1.770 γ. 1.658-1.788 δ. 0.007-0.020Dispersion :r > v Weak to Strong.Density: 3.21-3.96.Hardness: 5-6.Cleavage:{210}Colour: grey, green, yellow, brown.Pleochroism: Enstatite none, other composition variable weak to strong α pink, pale reddish brown, purple-violet smoky brown: β yellow, pale greenish brown :γ green pale green, somky green.

Occurrence Orthopyroxenes occur in basic and ultrabasic igneous rocks of all types, and are the dominant pyroxene type present in norites. Mg-rich opx occur in ultrabasic and ultramafic igneous rocks such as pyroxenites, harzolite, in association with forsterite, Mg-rich augite and Mg-rich spinel or garnet. Orthopyroxenes occur in rocks affected by high-temperature regional metamorphism, particulary charnokites and granulite, and also may occur in some high grade hornfels formed from pelitic rocks in the inner most zone of thermal aureoles. Orthopyroxenes commonly occur in some chondritic meteorites.

Conclusion• Enstatite-Orthoferrosillite belongs to Pyroxene group Crystallize in orthrhombic system.

• OPX is an important rock forming minerals in basic and ultrabasic igneous rocks

• High contents of aluminium occur mainly in orthopyroxenes of high grade metamorphic rocks.

• Orthopyroxenes occur in rocks affected by high-temperature regional metamorphism

• Orthoferrosilite are characteristics consistuents, associated with fayalite, hedenbergite, gerunerite and almandine –spessartine garnet of a regionally metamorphosed iron rich sediments .

Green.D.H, (1954) The Petrogenesis of the high temperature peridotite in the Lizard area, Cronwall, Journ. Petro. Vol.05, p.135.

Deer, W.A, Howie, R.A., Zussman, J, (1979), An Introduction to the Rock Forming Minerals, ELBS pub. Pp.99-117

Refences

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