THE PYROXENE GROUP OF MINERALS

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:

Single Chain Structure

Double Chain Structure
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)SiO
3


Hornblende- Ca
2
(Mg, Fe, Al)
5
(Al, Si)

8
O
22
(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
Diopside- hedenbergite

Complete 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- 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 30-50
Ferrohypersthene 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.
Augite

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.