Pyroxene Group of Minerals, MSc. Geology It contains Pyroxene Group of Minerals Clinopyroxene Orthopyroxene examples with physical and optical properties of minerals of orthopyroxene and clinopyroxene group. IUGS Classification References . It will help in MSc Pyroxene Group of Minerals entitled Presentations as well as notes for exams of MSc program under Geology Honors. I had presented it in my 1st Semester presentation of Pyroxene Group of Minerals in Jiwaji University, Gwalior, Madhya Pradesh, India

1. ACKNOWLEDGEMENT
It gives me immense pleasure to thank Dr. S.N. Mohapatra, HOD, S.O.S in Earth Science,
Jiwaji University for allotting me interesting seminar to pen entitled “The Pyroxene Group
of Minerals” for the fulfilment as the award of degree in Master in Science in Geology 1st
Semester and also for his help and guidance extension through various suggestions.
I want to convey my sincere regards to Dr. S.N. Mohapatra under whose supervision we
have got this valuable chance. I am also thankful to Dr. S.N. Mohapatra for providing their
continuous guidance and making this seminar.
We have our deep appreciation for all technical and non-technical staff members of the
S.O.S in Earth Science for their friendly and cooperative behaviours.
At last, but not the least I am also thankful to my Colleagues for their constant help and
cooperation during my seminar presentation.
Thank You
SP Swapnajit Das
M.Sc. 1st Semester

2. Contents :
 Introduction
 Classification
 Silicate structure of Pyroxene
✓ Ionosilicate structure
 Comparison between Single Chain and Double Chain Silica tetrahedrons.
 Clinopyroxene Group
✓ Physical and Optical properties of Clinopyroxene minerals
 Ortho-pyroxene Group
✓ Physical and Optical properties of Orthopyroxene minerals
 Comparison between Orthopyroxene and Clinopyroxene
 IUGS Classification of Olivine, Orthopyroxene and Clinopyroxene
 Conclusion
 Reference

3. Introduction :
• Pyroxenes are the most significant and abundant group of rock-forming ferromagnesian silicates.
• Thecompositions in almost every variety of igneous rock and also occur in rocks of widely
different compositions.
Classification of the pyroxene group :
• The pyroxene group includes minerals that form in both the orthorhombic and monoclinic crystal
systems.
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 from single or multiple
chains, with two oxygen atoms of each tetrahedron forming long, thin, chains.
Si : O = 1 : 3
Inosilicate structure :
It is divided into two types
1. Single Chain Structure
2. Double Chain Structure
ORTHO
PYROXENE
CLINO
PYROXENE
Name derived as minerals belongs to
Orthorhombic system
Name derived as minerals belongs to Monoclinic
system

4. Comparison between
Single Chain and Double Chain structure :
SINGLE CHAIN DOUBLE CHAIN
➢ The tetrahedrons share two oxygens with
two other tetrahedrons and form a
seemingly endless chain.
➢ Two single chains lie side by side so that
all the right sided tetrahedrons of the left
chain are linked by an oxygen to the left
sided tetrahedrons of the right chain.
➢ The ratio of silicon to oxygen is 1 : 3 ➢ The extra shared oxygen for every four
silicon reduces the ratio of silicon to oxygen
to 4 : 11
➢ The tetrahedrons alternate to the left and
then to the right along the line formed by
the linked oxygens although more
complex chains seems to spiral
➢ The double chain looks like a chain of six
sided rings
Single Chain Structure :

5. Double Chain Structure :
The Clino Pyroxene Group :
• It is a member of the pyroxene group of minerals having a monoclinic crystal structure.
• General formula of Clino pyroxene group is (w,x,y)2 z2 o6
• Here w - Ca, Na
x – Mg, Fe2, Mn, Li
y – Al, Fe3, Ti
z – Si, Al
Examples of Clino pyroxene group minerals
Augite
Diopside
Argirine
Hedenbergite

6. AUGITE :
• Augite is a common rock-forming pyroxene
mineral.
• The crystals are monoclinic and prismatic.
• Augite has two prominent cleavages, meeting at
angles near 90 degrees.
Physical Properties :
Formula: (Ca,Mg,Fe)2Si2O6
System: Monoclinic
Color: Brown-green, black, etc.
Lustre: Vitreous, Resinous, Dull
Hardness: 5½–6
Density: 3.19–3.56
Optical Properties :
PPL properties :
Relief: High positive
Habit/Form: Crystals are usually stubby prisms
elongate along the c axis. Basal cross sections are
four or eight sided and show the two cleavages at
~87°. Longitudinal sections are roughly rectangular
and show only one cleavage direction. Augite also forms anhedral grains, or irregular masses
that may enclose associated minerals. Overgrowths of hornblende are relatively common.
Lamellae of exsolved orthopyroxene are commonly found parallel to {100} , whereas pigeonite
lamellae are inclined somewhat to either P {100} or {001}. The lamellae may be inclined up to 22
and 17° from (100) and (001) respectively for Mg-rich augite. Smaller inclinations are foundin
Fe-rich augite.
Color: Colorless, pale green, pale brown, green, greenish-yellow, pale yellow green, grayish,
green, violet. Zonal structure is sometimes present.
Pleochroism: Absent or weak; it is best shown in {100} sections.
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage
angle in two directions are at nearly right angles 87° and 93°. Prismatic sections parallel to(100)
or (010) to which both cleavages are at too acute an angle to be always visible, especially in
slightly thick sections.

7. XPL Properties :
Isotropy/Anisotropy: Anisotropic
Interference color: Bright
Extinction angle: The maximum extinction
angle of longitudinal sections varies from 35°
to 45°. These sections have the maximum
interference colors for the slide. Some
varieties have a peculiar concentric wavy
extinction known as the hourglass structure.
Cross sections have parallel or symmetrical
extinction depending upon whether {100} and
{010} or {110} predominates.
Twins: Twins with {100} as twin-plane are common; these often appear as twin seams.
Polysynthetic twins with {001} as twin-plane are occasionally found. Combined {100} twins with
{001} polysynthetic twins give what is known as herringbone structure.
Uniaxial/Biaxial: Biaxial (+)
Optic axial angle (2V): 2V measured: 40 – 52°, 48 – 68°
AEGIRINE :
• Aegirine is a member of the clinopyroxene
group of inosilicate minerals.
• Aegirine is the sodium endmember of the
aegirine-augite series.
• Aegirine has the chemical formula NaFeSi₂O₆
in which the iron is present as Fe³⁺.
Physical Properties :
Formula: NaFe3+Si2O6
System: Monoclinic
Color: Dark green to greenish black, reddish
brown, black
Lustre: Vitreous
Hardness: 6
Density: 3.5–3.6

8. Optical Properties :
PPL Properties :
Relief: Very High positive
Habit/Form: Aegirine form stubby to quite
elongate prisms. Eight-sided cross sections
show both cleavages at about 87°.
Longitudinal sections are elongate to roughly
rectangular and show one direction of
cleavage. The brown varieties may have
acute or sharply pointed
terminations. In the process of fenitization,
fibrous aegirine may form at the expense of
biotite or hornblende. Spongy intergrowths of riebeckite and aegirine may be found.
Color: Generally brown to green, but also black, emerald-green, grass-green, deep green, yellow,
brownish-green, yellowish brown
Pleochroism: Intense pleochrosim from bright green toyellow-green and brown-green. The
acmite variety of aegirine is weakly pleochroic in pale brown and yellow.
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage
angle in two directions are at nearly right angles 87° and 93°. Prismatic sections parallel to
(100) or (010) to which both cleavages are at too acute an angle to be always visible, especially
in slightly thick sections.
XPL Properties :
Isotropy/Anisotropy: Anisotropic
Interference color: Brilliant; or anomalous
blue/brown
Extinction angle: Very slightly inclined; the
maximum extinction angle in longitudinal
sections is very small: from 2° to 10° (in
some crystals you can have 15-20°
extinction angle). {100} sections show
parallel extinction.
Twins: Simple and lamellar twinning on
{100} and {001}
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 60 – 90°, calculated: 68 – 84°

9. DIOPSIDE :
• Diopside is a monoclinic pyroxene mineral with
composition MgCaSi ₂O ₆.
• It forms complete solid solution series with hedenbergite
and augite, and partial solid solutions with orthopyroxene
and pigeonite.
• It forms variably colored, but typically dull green crystals
in the monoclinic prismatic class
Physical Properties :
Formula: CaMgSi2O6
System: Monoclinic
Color: Light to dark green, blue, brown, colourless, snow
white, grey
Lustre: Vitreous, Dull
Hardness: 5½–6½
Density: 3.22–3.38
Optical Properties :
PPL Properties :
Relief: High positive
Habit/Form: Crystals are usually stubby
prisms elongate along the c axis. Basal
cross sections are four or eight sided and
show the two cleavages at ~87°.
Longitudinal sections are roughly
rectangular and show only one cleavage
direction. Augite also forms anhedral
grains, or irregular masses that may
enclose associated minerals.
Overgrowths of hornblende are relatively
common. Lamellae of exsolved orthopyroxene are commonly found parallel to {100} , whereas
pigeonite lamellae are inclined somewhat to either P100} or {001}. The lamellae may be inclined
up to 22 and 17° from (100) and (001) respectively for Mg-rich augite. Smaller inclinations are
found in Fe-rich augite.
Color: Diopside (Mg-member) is colorless; Hedenbergite (Fe-member) is pale green to bright-
green/yellow-green
Pleochroism: For Diopside is absent; with increasing Fe2+, pleochroism also appears.
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage
angle in two directions are at nearly right angles 87° and 93°.

10. XPL Properties :
Isotropy/Anisotropy: Anisotropic
Interference color: Bright
Extinction angle: The maximum extinction
angle in sections cut parallel to the c axis
varies from 38° to 45°. In cross sections the
extinction is symmetrical to the cleavage
traces. In {100} sections the extinction is
parallel.
Twins: Simple or polysynthetic twinning is
relatively common on {100} or {001}.
Uniaxial/Biaxial: Biaxial (+)
Optic axial angle (2V): 2V measured: 58 – 63°, calculated: 56 – 64°
Ortho-Pyroxene Group :
• It is a member of the pyroxene group of minerals having an orthorhombic crystal structure, such as
enstatite and hypersthene.
• Orthopyroxenes have general formula (Mg,Fe,Ca)(Mg,Fe,Al)(Si,Al)2O6
Examples of minerals of Ortho-pytoxene group
1. Enstatite
2. Hypersthene
3. Ferrosilite
4. Bronzite
ENSTATITE :
• Enstatite is a mineral; the magnesium endmember of the
pyroxene silicate mineral series enstatite – ferrosilite.
• The magnesium rich members of the solid solution
series are common rock-forming minerals found in
igneous and metamorphic rocks.

11. Physical Properties :
Formula: Mg2Si2O6
System: Orthorhombic
Color: White, yellowish green, dark brown to black, greenish white or grey, olive-green
Lustre: Vitreous, Pearly
Hardness: 5–6
Density: 3.2–4.0
Optical Properties :
PPL Properties :
Relief: Moderate positive
Habit/Form: Euhedral crystals are usually stubby
prisms. Basal sections are four or eight-sided and
show the prismatic cleavages intersecting at
nearly 90°. Longitudinal sections are usually
roughly rectangular and
show only one direction of cleavage.
Orthopyroxene also forms
anhedral, irregular grains occupying the space
between other
minerals, and poikiloblastic with numerous inclusions of associated minerals. Fibrous
orthopyroxene may form reaction rims around other minerals such as olivine or garnet.
Orthopyroxene commonly contains exsolution lamellae of augite. The lamellae may be uniform
and tabular, or they may pinch and swell and form rows of blebs. Lamellae of plagioclase in
orthopyroxene are sometimes found in anorthositic rocks and apparently formed by exsolution.
Color: Colorless, slight greenish or grayish tinge. Color increase with Fe.
Pleochroism: Absent or weak at the Fe-member (see Bronzite)
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the cleavage
angle in two directions are at nearly right angles 87° and 93°.
XPL Properties :
Isotropy/Anisotropy: Anisotropic
Interference color: Order I gray or white to high
order I; the maximum interference color is pale
yellow of the
first order.
Extinction angle: Parallel / 0° / straight in
longitudinal sections and symmetrical in
basal/cross sections.
Twins: Simple and lamellar twinning on {100} and
{001}
Uniaxial/Biaxial: Biaxial (+/-)
Optic axial angle (2V): 2V measured: 54 – 90°, calculated: 58 – 86°

12. HYPERSTHENE :
• Hypersthene is a common rock-forming
inosilicate mineral belonging to the group
of orthorhombic pyroxenes.
• Its chemical formula is SiO₃.
Physical Properties :
Formula : (Mg,Fe)SiO3
System : Orthorohmbic
Colour: Greyish white
Lustre: Vitreous, Silky
Hardness: 5½ - 6
Density : 3.2 – 3.9
Optical Properties :
PPL IMAGE OF HYPERSTHENE THIN
SECTION
XPL IMAGE OF HYPERSTHENE THIN
SECTION

13. FERROSILITE :
Ferrosilite, silicate mineral (Fe2Si2O6) and
iron-rich end member of the orthopyroxene
solid solution series
Physical Properties :
Formula : Fe2Si2O6
System : Orthorohmbic
Colour : Dark brown to Black
Lustre : Vitreous, Sub-vitreous, Greasy
Hardness : 5-6
Density : 3.88
Optical Properties :
PPL IMAGE OF FERROSILITE THIN
SECTION
XPL IMAGE OF FERROSILITE THIN
SECTION

14. BRONZITE :
• Bronzite belongs to the mineral group
known as pyroxene, and so is a silicate
compound of magnesium.
• An iron-bearing variety of enstatite,
bronzite has a metallic luster when
polished.
Physical Properties :
Formula : (Mg, Fe2+)2Si2O6
System : Orthorhombic
Color : White, yellowish green, dark brown
to black, greenish white or grey, olive-green
Lustre : Vitreous, Pearly
Hardness : 5–6
Density : 3.2–4.0
Optical Properties :
PPL Properties :
Relief: Moderate positive
Habit/Form: Euhedral crystals are usually stubby
prisms. Basal sections are four or eight sided and
show the prismatic cleavages intersecting at
nearly 90°. Longitudinal sections are usually
roughly rectangular and show only one direction
of cleavage. Orthopyroxene also
forms anhedra, irregular grains occupying the
space between other minerals, and pokiloblastic with numerous inclusions of associated
minerals. Fibrous orthopyroxene may form reaction
rims around other minerals such as olivine or garnet. Orthopyroxene commonly contains
exsolution lamellae of augite. The lamellae may be uniform and tabular, or they may pinch and
swell and form rows of blebs. Lamellae of plagioclase in orthopyroxene are sometimes found in
anorthositic rocks and apparently formed by exsolution.
Color: Colorless, slight brownish or grayish tinge
Pleochroism: Absent or weak at the Fe-member (from greenish to pale reddish)
Cleavage: Good in two directions {110} (at right angles to the c-axis – on (001)) – the clivage
angle in two directions are at nearly right angles 87° and 93°. Prismatic sections parallel to
(100) or (010) to which both cleavages are at too acute an angle to be always visible, especially
in slightly thick sections.

15. XPL Properties :
Isotropy/Anisotropy: Anisotropic
Interference color: Order I – II
Extinction angle: Parallel / 0° / straight in
longitudinal sections and symmetrical in basal
sections
Twins: Simple and lamellar twinning on {100}
and {001}
Uniaxial/Biaxial: Biaxial (-)
Optic axial angle (2V): 2V measured: 2V 125 –
53 -125°
Comparison between Orthopyroxene
and Clinopyroxene group of Minerals :
ORTHOPYROXENE CLINOPYROXENE
• In Orthopyroxene the M2 site is usually
octahedral and commonly contains Fe &
Mg.
• In Clinopyroxene, larger cations are in
eightfold coordination in the M2 site.
• Due to poor content of Cr in
Orthopyroxene so it is colourless.
• The significantly higher Cr2O3 content of
the Clinopyroxene in this sample gives it
a pale green colour in plane polarized
light.
• Orthopyroxene has maximum
interference colours of upper first order.
• Clinopyroxene ranges to upper second
order.
• Orthopyroxene has parallel extension. • Clinopyroxene has inclined extinction.
• Orthopyroxene occurs commonly in
tholeiitic basalts and in plutonic mafic
and ultramafic igneous rocks, and in
granulite facies meta pelites & meta
basic rocks.
• Clinopyroxene is common in plutonic
gabros, pyroxenites and peridotites, and
a phenocryst in mafic to intermediate
volcanic rocks, and in various medium to
high grade metamorphic rocks.

16. IUGS Classification of Ultramafic rocks with Olivine
Orthopyroxene and Clinopyroxene :
• Ultramafic rocks are a vast family of rocks consisting predominantly of mafic minerals
(olivine, pyroxene, and amphibole) with very limited contents of felsic minerals (generally
calcium plagioclase) and other accessories (garnet, spinel, phlogopite, etc.).
• These rocks comprise peridotites, constituting the Earth’s mantle, and various ultramafic
cumulates that form by segregation of mafic minerals within peridotites or by settling at the
base of magma chambers, generally of gabbroid rocks.
The QAPF classification diagram considers ultramafic any rock with a modal content of
mafic minerals (color index M) > 90%.
• Therefore, their classification requires specific ternary diagrams based on the proportion of
olivine – orthopyroxene – clinopyroxene (Ol – Opx – Cpx), and olivine – pyroxene –
hornblende (Ol – Px – Hbl) [see also: how to use a ternary plot]. Let’s consider just the Ol –
Opx – Cpx diagram for the moment.
Conclusion :
• Pyroxene minerals are minor to secondary components of some rocks that are used as
crushed stone and dimension stone.
• A few pyroxene minerals are used as gem materials, but only in rare instances when they
have attractive color and clarity.
• Jadeite, diopside, and spodumene are the most important pyroxene gem materials.

17. Reference :
• Dana, E.S. (2006) Text book of Mineralogy, 4th
ed. CBS Publishers
• Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming
minerals (pp. 498). Mineralogical Society of Great Britain and Ireland, London.
• Washington, H.S. (1932) Min. Petr. Mitt. (Tschermak), vol. 43, 63 [M.A. 5-222].
• Cox, K. G. (Ed.). (2013). The interpretation of igneous rocks. Springer Science &
Business Media.