OrthopyroxeneOrthopyroxene

By Dominic PapineauBy Dominic Papineau

The varieties of orthopyroxeneThe varieties of orthopyroxene

Enstatite Enstatite

ClinoenstatiteClinoenstatite

BronziteBronzite

HypersteneHyperstene

FerrohypersteneFerrohyperstene

EuliteEulite

OrthoferrosiliteOrthoferrosilite

The enstatite series of isomorphsThe enstatite series of isomorphs

The physical properties of orthopyroxenesThe physical properties of orthopyroxenes

Luster: vitreous to pearly on cleavage facesLuster: vitreous to pearly on cleavage faces

Cleavage: goodCleavage: good

Hardness: 5.5 - 6Hardness: 5.5 - 6

Specific gravity: 3.2 – 3.6 increasing with Fe contentSpecific gravity: 3.2 – 3.6 increasing with Fe content

Crystal habit: massive, fibrous, or lamellarCrystal habit: massive, fibrous, or lamellar

Group: orthopyroxenes are inosilicates (single chain silicates) Group: orthopyroxenes are inosilicates (single chain silicates)

The chemical properties of orthopyroxenesThe chemical properties of orthopyroxenes

Orthopyroxenes are solid solutions of MgSiOOrthopyroxenes are solid solutions of MgSiO33 – (Mg,Fe)SiO – (Mg,Fe)SiO33

Composition:Composition: Pure enstatite contains 40.0% MgO and 60.0% SiOPure enstatite contains 40.0% MgO and 60.0% SiO22

Pure ferrosilite (very rare) contains 40.0% FeO and Pure ferrosilite (very rare) contains 40.0% FeO and 60.0% SiO60.0% SiO22

Pure ferrosilite is very rare because in most geologically observed temperature Pure ferrosilite is very rare because in most geologically observed temperature and pressure ranges, fayalite (Feand pressure ranges, fayalite (Fe22SiOSiO44) and quartz (SiO) and quartz (SiO22) are more stable.) are more stable.

The optical properties of orthopyroxeneThe optical properties of orthopyroxeneorthopyroxene is biaxial negative or positiveorthopyroxene is biaxial negative or positive

= 1.649 – 1.768= 1.649 – 1.768

= 1.653 – 1.770= 1.653 – 1.770

= 1.657 – 1.788= 1.657 – 1.788

Weak pleochroismWeak pleochroism

2V2VZZ = 50-132° = 50-132°

May be twinnedMay be twinned

= 0.007 – 0.020 (usually first order yellow or lower interference colors)= 0.007 – 0.020 (usually first order yellow or lower interference colors)

Relation between the physical, chemical and Relation between the physical, chemical and optical properties of orthopyroxenesoptical properties of orthopyroxenes

The crystallographic properties of orthopyroxeneThe crystallographic properties of orthopyroxene

Crystal system: orthorhombicCrystal system: orthorhombic

Point Group: 2/m 2/m 2/mPoint Group: 2/m 2/m 2/m

Space group: PbcaSpace group: Pbca

Unit cell parameters:Unit cell parameters: Z = 8Z = 8

Enstatite and ferrosilite: the end membersEnstatite and ferrosilite: the end members

ba

c

b

a

cEnstatiteEnstatite

FerrosiliteFerrosilite

Magnesium polyhedronMagnesium polyhedron

Silicon tetrahedronSilicon tetrahedron

Oxygen atomOxygen atom

Iron polyhedronIron polyhedron

Silicon tetrahedronSilicon tetrahedron

Oxygen atomOxygen atom

The crystallographic properties of enstatiteThe crystallographic properties of enstatite

Crystal system: orthorhombicCrystal system: orthorhombic

Point Group: 2/m 2/m 2/mPoint Group: 2/m 2/m 2/m

Space group: PbcaSpace group: Pbca

Unit cell parameters:Unit cell parameters: a = 18.22 a = 18.22 ÅÅ

c = 5.17c = 5.17ÅÅZ = 8Z = 8

Magnesium polyhedronMagnesium polyhedron

Silicon tetrahedronSilicon tetrahedron

Oxygen atomOxygen atom

b = 8.81 b = 8.81 ÅÅ

c

ab

The crystallographic properties of ferrosiliteThe crystallographic properties of ferrosilite

Crystal system: orthorhombicCrystal system: orthorhombic

Point Group: 2/m 2/m 2/mPoint Group: 2/m 2/m 2/m

Space group: PbcaSpace group: Pbca

Unit cell parameters:Unit cell parameters: a = 18.41 a = 18.41 ÅÅ

c = 5.24c = 5.24ÅÅZ = 8Z = 8

Iron polyhedronIron polyhedron

Silicon tetrahedronSilicon tetrahedron

Oxygen atomOxygen atom

b = 9.07 b = 9.07 ÅÅ

a

cb

Crystallography of enstatiteCrystallography of enstatite

This is a high resolution TEM image of enstatite along the c-axisThis is a high resolution TEM image of enstatite along the c-axis

The white regions is the area between the M2 sites in the structureThe white regions is the area between the M2 sites in the structure

Unit cellUnit cell

CleavageCleavage

Enstatite structure Enstatite structure (to scale)(to scale)

The thermodynamic properties of orthopyroxenesThe thermodynamic properties of orthopyroxenes

Thermal expansion of the three Thermal expansion of the three crystallographic axes of B1-9 orthopyroxenecrystallographic axes of B1-9 orthopyroxene

More thermodynamic properties of orthopyroxenesMore thermodynamic properties of orthopyroxenes

Percentage variation of mean interatomic Percentage variation of mean interatomic distance with temperaturedistance with temperature

M1-O distance (Mg)M1-O distance (Mg)

M2-O distance (Fe)M2-O distance (Fe)

Si-O distanceSi-O distance

Corrected for riding motionCorrected for riding motion

Corrected for noncorrelated thermal motionCorrected for noncorrelated thermal motion

Occurrences of OrthopyroxenesOccurrences of Orthopyroxenes

Igneous rocksIgneous rocks

Metamorphic rocksMetamorphic rocks

In intrusive rocks like gabbro, norite, peridotite, In intrusive rocks like gabbro, norite, peridotite, diorite, and syenite, but also in extrusive rocks diorite, and syenite, but also in extrusive rocks like basalts.like basalts.

In high temperature metamorphic rocks (granulite In high temperature metamorphic rocks (granulite facies)facies)

Associated minerals include clinopyroxenes, Associated minerals include clinopyroxenes, amphiboles, olivine, feldspars, garnets, etc. amphiboles, olivine, feldspars, garnets, etc.

Occurrences of orthopyroxenesOccurrences of orthopyroxenes

Iron and stony iron Iron and stony iron meteorite, and thus meteorite, and thus

presumably on asteroids presumably on asteroids (very common)(very common)

Locations on Earth:Locations on Earth:

Extraterrestrial locations:Extraterrestrial locations:

On the Moon On the Moon (very common)(very common)

On Mars On Mars (apparently very common)(apparently very common)

United States (NY, Co, NC, Pa, Md, Az), Canada (Labrador), United States (NY, Co, NC, Pa, Md, Az), Canada (Labrador), India, Sri Lanka, Finland, Tanzania, etc. India, Sri Lanka, Finland, Tanzania, etc.

From the Kapoeta meteoriteFrom the Kapoeta meteorite

From lunar highland From lunar highland Apollo samplesApollo samples From Pathfinder analysisFrom Pathfinder analysis

Enstatite chondritesEnstatite chondrites

Enstatite chondrite meteorites are characterized by the Enstatite chondrite meteorites are characterized by the presence of orthopyroxene, but without olivine, because:presence of orthopyroxene, but without olivine, because:

(Mg, Fe)SiO(Mg, Fe)SiO44 MgSiOMgSiO33 FeFeoo 1/2O1/2O22 ++ ++

Their Fe content occurs as a metallic alloy Their Fe content occurs as a metallic alloy

Chondrite meteorites represents the oldest rocks found: Chondrite meteorites represents the oldest rocks found: from 4.4 to 4.56 billion years old from 4.4 to 4.56 billion years old