PyroxenesPyroxenes http://www.es.mq.edu.au/ndaczko/Web/http://www.es.mq.edu.au/ndaczko/Web/

GEOS254/GEOS254.htmlGEOS254/GEOS254.html Pyroxenes are a major component of the mantle Pyroxenes are a major component of the mantle

(peridotite is olivine plus pyroxene) and the oceanic (peridotite is olivine plus pyroxene) and the oceanic crust (basalt, dolerite, gabbro are mainly calcium-crust (basalt, dolerite, gabbro are mainly calcium-rich plagioclase plus pyroxene).rich plagioclase plus pyroxene).

Pyroxenes are also present in many high grade Pyroxenes are also present in many high grade (high temperature) metamorphic rocks. The calcium (high temperature) metamorphic rocks. The calcium magnesium pyroxene diopside forms in meta-magnesium pyroxene diopside forms in meta-limestone even at low temperature.limestone even at low temperature.

Fragments of pyroxene are present in many Fragments of pyroxene are present in many volcaniclastic sandstones.volcaniclastic sandstones.

PYROXENES PYROXENES X Y ZX Y Z22OO66 (X=Ca,Mg,Fe,Na); Y= Mg,Fe,Al,Ti0; Z=Si)(X=Ca,Mg,Fe,Na); Y= Mg,Fe,Al,Ti0; Z=Si)

Most common pyroxenes are solid solutions ofMost common pyroxenes are solid solutions of DiopsideDiopside CaMgSiCaMgSi22OO66 Hedenbergite CaFeSiHedenbergite CaFeSi22OO66

EnstatiteEnstatite MgMgSiMgMgSi22OO66 Ferrosilite FeFeSiFerrosilite FeFeSi22OO66

Two other important pyroxene end-members areJadeite NaAlSi2O6 Aegirine NaFe3+Si2O6

3 other well recognised pyroxenes are: Omphacite (about half augite/half jadeite) in eclogites.Aegirine-augite (about equal amounts of both) in trachytes (green)Titan-augite (Ti replaces Fe) in intra-plate basalts (pink/mauve)

The pyroxene The pyroxene quadrilateralquadrilateral

Most monoclinic Most monoclinic or “clino”-or “clino”-pyroxenes are pyroxenes are close to the Di –close to the Di –He join. He join.

Most Most orthorhombic or orthorhombic or “ortho”-pyroxenes “ortho”-pyroxenes are close to the are close to the En – Fs join.En – Fs join.

Pigeonite is a Pigeonite is a calcium-poor calcium-poor clinopyroxene that clinopyroxene that occurs in some occurs in some dolerites.dolerites.

Augite is a Augite is a common igneous common igneous clinopyroxene. clinopyroxene.

PYROXENE STRUCTUREPYROXENE STRUCTURE

A: Single chain parallel to c-axis.A: Single chain parallel to c-axis. Looking along the b-axis of diopside (clinopyroxene). Looking along the b-axis of diopside (clinopyroxene).

Red atoms are Mg, grey are Ca.Red atoms are Mg, grey are Ca.

A B

Pyroxene cleavagePyroxene cleavage Pyroxene has two Pyroxene has two

cleavages at 87cleavages at 87oo. True . True angle seen in sections angle seen in sections cut normal to the c-cut normal to the c-axis.axis.

Sections cut parallel to Sections cut parallel to the C-axis will show the C-axis will show one direction of one direction of cleavage.cleavage.

Ophitic microstructure (dolerite) Ophitic microstructure (dolerite) plagioclase is included in large plagioclase is included in large

pyroxene crystals.pyroxene crystals.

Close up of plagioclase Close up of plagioclase partly included in partly included in

pyroxene in sub-ophitic pyroxene in sub-ophitic dolerite.dolerite.

Top PPL,bottom is XPTop PPL,bottom is XPThis pyroxene is This pyroxene is

colourless with high colourless with high relief (cleavage shows relief (cleavage shows

clearly).clearly).The two grains on the The two grains on the left have interference left have interference colours towards the colours towards the

lower end of the range.lower end of the range.

Eclogite (ultra high grade meta-Eclogite (ultra high grade meta-basalt) with pyroxene and basalt) with pyroxene and

garnet (black)garnet (black)

The pyroxene in eclogite isA solid solution betweenAugite and jadeite

Radiating aggregate of Radiating aggregate of pyroxene in quartz monzonite pyroxene in quartz monzonite with hornblende forming a rim with hornblende forming a rim

around the pyroxene. around the pyroxene. Typical bright interference Typical bright interference colours of clinopyroxene.colours of clinopyroxene.

Clinopyroxene exhibits Clinopyroxene exhibits inclined extinction for most inclined extinction for most grains with one cleavage. grains with one cleavage.

Most orthopyroxene grains Most orthopyroxene grains show straight or parallel show straight or parallel

extinction (black under X-extinction (black under X-polars) when cleavage (one) polars) when cleavage (one)

is N-S or E-Wis N-S or E-W

Pyroxene in Pyroxene in Meta-Meta-

basalts.basalts. At Broken Hill some At Broken Hill some

metabasalts in zone 2 metabasalts in zone 2 have clinopyroxene have clinopyroxene but it is only at the but it is only at the highest grade (Zone highest grade (Zone 3) that hornblende 3) that hornblende starts to react out to starts to react out to produce produce orthopyroxene and orthopyroxene and clinopyroxene.clinopyroxene.

Olivine Olivine (Mg,Fe)(Mg,Fe)22SiOSiO

44

MgMg22SiOSiO4 4 is forsterite (Fo); Feis forsterite (Fo); Fe22SiOSiO44 is is fayalite (Fa).Complete solid solution.fayalite (Fa).Complete solid solution.

Mantle olivine is about Fo90-93.Mantle olivine is about Fo90-93. Basalts, gabbros have olivine Fo80-40.Basalts, gabbros have olivine Fo80-40. Rare dolerites, ferrogabbros and Rare dolerites, ferrogabbros and

syenites have fayalite (pale yellow in syenites have fayalite (pale yellow in thin section).thin section).

Meta-dolomitic limestone has Fo100 Meta-dolomitic limestone has Fo100 (dolomite + quartz (dolomite + quartz forsterite + calcite + forsterite + calcite + COCO22))

Meta banded iron formations Fa100,Meta banded iron formations Fa100, Fo-rich olivine does not co-exist with Fo-rich olivine does not co-exist with

quartz. Fa-rich does. quartz. Fa-rich does. Olivine is not stable in a low temp. wet Olivine is not stable in a low temp. wet

environment and is replaced by environment and is replaced by serpentine. This replacement is serpentine. This replacement is commonly pseudomorphous (cracks).commonly pseudomorphous (cracks).