THE PYROXENE GROUP OF MINERALS

The Pyroxene Group

Silicate Structure Of PyroxenesThe pyroxene group of minerals is in the INOSILICATE subdivision of the silicates group

Inosilicates - Group of silicate minerals that have their tetrahedrons form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains. Si: O =1:3.

INOSILICATE STRUCTUREDivided into two types:Single Chain StructureDouble Chain Structure

Silica tetrahedrons

Single Chain Inosilicate Structure

Double Chain Inosilicate Structure

This difference produces a difference in anglesThe cleavage of the two groups results

between chains and does not break the chains thus producing prismatic cleavage

In the single chained silicates the two directions of cleavage are at 87° and 93° (close to 90 degrees) forming nearly square cross sections

In the double chain silicates the cleavage angles are at 56° and 124° (close to 120 and 60 degrees) forming rhombic cross sections

This makes it a convenient way to distinguish between single chain and double chain silicates or more precisely between pyroxene minerals and amphibole minerals

The amphiboles contain the same elements that the pyroxenes do, except they have hydroxyl (OH) in their structure, which alters both its physical and chemical properties

For example:- Augite- (Ca,Mg,Fe)SiO3

Hornblende- Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2

Clino pyroxene groupClino pyroxene (Diopside series)Diopside Ca Mg Si2 O6Hedenbergite Ca Fe Si2 O6Pyroxene Solid solutionsPigeonite (Ca, Mg, Fe)2 Si2 O6Augite (Ca, Mg, Fe, Na) (Mg, Fe, Al) (Si, Al)2 O6Omphacite (Ca, Na )(Fe Mg Al) (Si, Al)2 O6Aegirine Na (Al, Fe3+) Si2 O6Na and Li pyroxenesJadeite Na Al Si2 O6Spodumene Li Al Si2 O6

General formula (w,x,y)2 Z2 O6In which w, x, y, z indicate elements having

similar ionic radii and capable of replacing each otherin the structure.

W= Ca, NaX= Mg, Fe2+, Mn, LiY= Al, Fe 3+, TiZ= Si, Al

Diopside- hedenbergiteComplete solid solution exist between diopside

and hedenbergite and johannssenite (Ca Mn Si2 O6)

Diopside is the most common pyroxene. Found in mafic and ultramafic igneous rocks.

Found in marbles associated with calcite, quartz or forsterite, tremolite and garnet.

Also found in medium and high grade metamorphosed mafic rocks.

Hedenbergite is often found associated with ore deposits formed at high temperatures

AUGITE

AEGIRINE

DIOPSIDE

Enstatite- FerrosiliteEnstatite Mg Si O3Ferrosilite FeSiO3Hypersthene& Bronzite (Mg, Fe)2 Si2 O6.Enstatite usually massive , blocky, fibrous or

lamellar. Individual crystals may be prismatic or acicular.

Complete solid solution exist between Enstatite- Ferrosilite

Except at high temperature, only limited solid solution exist between clino pyroxene end members diopside and heldenbergite.

Enstatite- Ferrosilite

Enstatite 0-10(% Fe)Bronzite 10-30Hypersthene 30-50Ferrohypersthene 50-70Eulite 70-90Orthoferrosilite 90-100

Occurrence and associationsEnstatite is more common in mafic igneous

rocks.Commonly associating with plagioclase and

clinopyroxeneAlso in high- grade metamorphic rocks and is

considered diagnostic for the granulite facies .Hypersthene is a characteristic mineral

associated with charnockite series of rocks.Eulite and Orthoferrosilite- associated with

fayalite, hedenbergite, grunerite and almandine – spessartine garnet, of eulysite, a regionally metamorphosed Iron rich sediment.

AugiteMost common pyroxene found in mafic to

intermediate igneous rocks, both plutonic and volcanic

Associated with hornblende and plagioclaseAugite is equivalent to diopside with may

impurities, especially Na.Omphacite is a bright green variety of augite

rich in Na and Al.

JadeiteThe term jade refers to either jadeite or to

the amphibole, nephrite.It is a high pressure pyroxene found in

metamorphic rocks of the blueschist facies.Associated with glaucophane, lawsonite or

aragonite, and with quartz and epidote.A bright green variety of jadeite, omphacite,

occurs in eclogites with pyrope rich garnets. Omphacite is also found in kimberlites.

SpodumeneSpodumene found in granitic pegmatites,

where it associated with k- feldspar, quartz, muscovite, tourmaline, beryl and lepidolite.

Hiddenite is a name given to emerald green spodumene.

Kunzite to lilac/pink colour spodumene.Triphane to colourless or yellow spodumene.

ENSTATITE

FERROSILITE

HYPERSTHENE

THE END