THE PYROXENE GROUP OF MINERALS

Silicate Structure Of Pyroxenes The pyroxene group of minerals is in the INOSILICATE subdivision of the silicates group  Inosilicates - Group of silicate minerals that have their tetr

THE PYROXENE GROUP OF MINERALS

The Pyroxene Group

Silicate Structure Of Pyroxenes

 The 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 STRUCTURE Divided into two types:

Silica tetrahedrons

Single Chain Inosilicate Structure

Double Chain Inosilicate Structure

 This difference produces a difference in angles

 The 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 group

Clino pyroxene (Diopside series)

Diopside Ca Mg Si2 O6

Hedenbergite Ca Fe Si2 O6

Pyroxene Solid solutions

Pigeonite (Ca, Mg, Fe)2 Si2 O6

Augite (Ca, Mg, Fe, Na) (Mg, Fe, Al) (Si, Al)2 O6 Omphacite (Ca, Na )(Fe Mg Al) (Si, Al)2 O6 Aegirine Na (Al, Fe3+) Si2 O6

Na and Li pyroxenes

Jadeite Na Al Si2 O6

Spodumene Li Al Si2 O6

General formula (w,x,y)2 Z2 O6

 In which w, x, y, z indicate elements having similar ionic radii and capable of replacing each otherin the structure

W= Ca, Na

X= Mg, Fe2+, Mn, Li

Y= Al, Fe 3+, Ti

Z= Si, Al

 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- Ferrosilite

Enstatite Mg Si O3

Ferrosilite FeSiO3

Hypersthene& 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 10-30

Hypersthene 30-50Ferrohypersthene 50-70

Eulite 70-90

Orthoferrosilite 90-100

Occurrence and associations

 Enstatite is more common in mafic igneous rocks

 Commonly associating with plagioclase and

clinopyroxene

 Also 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

 Most common pyroxene found in mafic to

intermediate igneous rocks, both plutonic and

volcanic

 Associated with hornblende and plagioclase

 Augite is equivalent to diopside with may impurities, especially Na

 Omphacite is a bright green variety of augite rich in

Na and Al

 Spodumene 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