Pyroxenes pdf

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Pyroxene group

Fundamental group of mineral

Composition of basalt ~ composition ofpyroxene

Abundantly found from mantle rocks torhyolites

Both metamorphic and igneous

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

Diopside: Diopside: CaMgCaMg [Si [Si22OO66]]

Where are the Si-O-Si-O chains??Where are the Si-O-Si-O chains??

Perspective viewPerspective view

SiOSiO44 as polygons as polygons

(and larger area)(and larger area)IV slabIV slab

IV slabIV slab

VI slabVI slab

M1 octahedronM1 octahedron

(+) type by convention(+) type by convention

This is a (-) typeThis is a (-) type

Creates an Creates an ““I-beamI-beam””

like unit in thelike unit in the

structure.structure.

Creates an Creates an ““I-beamI-beam””

like unit in thelike unit in the

structurestructure

(+)(+)

The pyroxeneThe pyroxene

structure is thenstructure is then

composed ofcomposed of

alternating I-beamsalternating I-beams

Clinopyroxenes haveClinopyroxenes have

all I-beams orientedall I-beams oriented

the same: all are (+)the same: all are (+)

in this orientationin this orientation

(+)(+)

(+)(+)(+)(+)

Note that M1 sites areNote that M1 sites are

smaller than M2 sites, sincesmaller than M2 sites, since

they are at the apices of thethey are at the apices of the

tetrahedral chainstetrahedral chains

The pyroxeneThe pyroxene

structure is thenstructure is then

composed ofcomposed of

alternation I-beamsalternation I-beams

Clinopyroxenes haveClinopyroxenes have

all I-beams orientedall I-beams oriented

the same: all are (+)the same: all are (+)

in this orientationin this orientation

OrthopyroxenesOrthopyroxenes have have

alternating (+) and (-)alternating (+) and (-)

orientationsorientations

(+)(+)

(+)(+)(+)(+)

TetrehedraTetrehedra and M1 and M1

octahedra shareoctahedra share

tetrahedral apicaltetrahedral apical

oxygen atomsoxygen atoms

The tetrahedral chainThe tetrahedral chain

above the M1s is thusabove the M1s is thus

offset from that belowoffset from that below

The M2 slabs have aThe M2 slabs have a

similar effectsimilar effect

The result is aThe result is a

monoclinicmonoclinic unit cell, unit cell,

hence hence clinopyroxenesclinopyroxenes

(+) M1(+) M1

(+) M2(+) M2

OrthopyroxenesOrthopyroxenes have have

alternating (+) and (-)alternating (+) and (-)

I-beamsI-beams

the offsets thusthe offsets thus

compensate and resultcompensate and result

in an in an orthorhombicorthorhombic

unit cellunit cell

(+) M1(+) M1

(-) M1(-) M1

(-) M2(-) M2

(+) M2(+) M2

Pyroxenoids“Ideal” pyroxene chains with5.2 Å repeat (2 tetrahedra)become “kinked” as other

cations occupy M sites

Wollastonite CaSiO3

3-tet repeat

RhodoniteMnSiO3

5-tet repeat

Pyroxmangite (Mn, Fe)SiO3

7-tet repeat

PyroxeneMgSiO3

2-tet repeat

7.1 A12.5 A

17.4 A

Pyroxene

Pyroxene GroupThe general pyroxene formula:

XY Z2O6

-X = Ca, Na, Fe2+, Mn, Mg, Li (M2 site)

-Y = Mg, Fe2+, Mn, Ni, Al, Fe3+, Cr, Ti (M1 site)-Z = Si, Al (Tetrahedral site)

Anhydrous (no OH in structure)

Dry conditions favor the crystallization ofpyroxenes over amphiboles

Common Pyroxene End members and solidsolutions

A. OrthopyroxenesEnd members:Enstatite (En):

Mg2Si2O6

Ferrosilite (Fs):Fe2''Si2O6

Complete solid solution betweenEn and Fs. Pure Fs is unstable at1 bar.

Prismatic crystals

En FsStable at P = 1 bar

Stable at P >1 bar

Monoclinic Pyroxenes

Common (Ca-Mg-Fe) monoclinic pyroxenes:End members:

Diopside (Di): CaMgSi2O6

Hedenbergite (Hd): CaFe"Si2O6

Clino-enstatite (C-En): Mg2Si2O6

Clino-ferrosilite (C-Fs): Fe"2Si2O6

Monoclinic pyroxenes solid solutions: Pigeonite: (Mg,Fe)2Si2O6 with some Ca, Al…Augite: Ca(Mg,Fe")Si2O6 with some with some Al, Ti, Cr, Fe"'

Compositions of common Ca-Mg-Fe pyroxenes are usually shown inan En-Di-Hd-Fs quadrilateral (topless triangle).

Pyroxene Chemistry

The pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)

DiopsideDiopside

CaMgSiCaMgSi22OO66

HedenbergiteHedenbergite

CaFeSi CaFeSi22OO66

WollastoniteWollastonite Ca Ca22SiSi

22OO66

EnstatiteEnstatite

MgMg22SiSi

22OO66

FerrosiliteFerrosilite

FeFe22SiSi

22OO66

orthopyroxenes

clinopyroxenes

pigeonite

••OrthopyroxenesOrthopyroxenes –– solid solid solnsoln

between between Enstatite-FerrosiliteEnstatite-Ferrosilite

•Clinopyroxenes –– solid solid solnsoln

between between Diopside-HedenbergiteDiopside-Hedenbergite

Orthopyroxene - Clinopyroxene

OPX and CPX have different crystal structures –results in a complex solvus between them

Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

DiopsideDiopside

CaMgSiCaMgSi22OO66

HedenbergiteHedenbergite

CaFeSi CaFeSi22OO66

WollastoniteWollastonite Ca Ca22SiSi22OO66

EnstatiteEnstatite

MgMg22SiSi22OO66

FerrosiliteFerrosilite

FeFe22SiSi22OO66

orthopyroxenes

clinopyroxenes

pigeonite

(Mg,Fe)(Mg,Fe)22SiSi22OO66 Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

pigeonite clinopyroxenes

orthopyroxenes

SolvusSolvus

12001200ooCC

10001000ooCC

800800ooCC

OPXOPX CPXCPX

CPXCPX

OPXOPX

Pyroxene CompositionThe pyroxene quadrilateral and opx-cpx solvus

Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

Diopside:CaMgSi2O6

Hedenbergite:CaFeSi2O6

WollastoniteCa2Si2O6

Enstatite:Mg2Si2O6

Ferrosilite:Fe2Si2O6

orthopyroxenes

clinopyroxenes

pigeonite

Px in thisregion areunstable atlow P

pigeonite

Pig Aug

T(ºC)

En(Mg2Si2O6)

Di(CaMgSi2O6)

Opx--CpxMiscibility Gap

SOLVUS

solidus

Section along En-Di join

Pyroxenesnot stable

Start with a crystal of pyroxene ofbulk composition X and cool

Subsolidus relations in the common pyroxenes

At T~1200°C the pigeoniteintersects the pigeonite-augitesolvus and begins to exsolve augite(2).

With cooling, pigeonite changescomposition to 3 and augitelamellae change composition to 4

At 1080°C, pigeonite becomeunstable and breaks down toorthopyroxene (5).

Cooling below 1080°C, opxwill exsolve augite

Pig Aug

T(ºC)

En(Mg2Si2O6)

Di(CaMgSi2O6)

Opx--Cpx MiscibilityGap

solidus

Pyroxene Chemistry

“Non-quad” pyroxenesJadeiteJadeite

NaAlSiNaAlSi22OO66

Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

AegirineAegirine

NaFeNaFe3+3+SiSi22OO66

Diopside-HedenbergiteDiopside-Hedenbergite

Ca-TschermackCa-Tschermack’’ss

moleculemolecule CaAl2SiOCaAl2SiO66

Ca / (Ca + Na)Ca / (Ca + Na)

0.20.2

0.80.8

Omphaciteaegirine-

augite

AugiteAugite

Spodumene:Spodumene:

LiAlSiLiAlSi22OO66

Occurrence of pyroxenes

Orthopyroxenes

Major mineral in mafic (basalts-andesites-gabbros) andultramafic (peridotite-pyroxenite) igneous rocks(volcanic and plutonic)

High grade metamorphic rocks (granulites)

Meteorites, lunar rocks and Martian rocks

Ca-Mg-Fe Clinopyroxenes

Major mineral in mafic, ultramafic and intermediateigneous rocks (V and P)

High grade metamorphic rocks (meta-basalts andmeta-carbonates)Formed during the metamorphism of impuredolostones

CaMg(CO3)2 + 2SiO2 CaMgSi2O6 + 2CO2

Jadeite-omphacite

Typical of high pressure metamorphism (blueschistsand eclogites) formed by metamorphism of basaltsduring subduction. Excellent pressure indicator.

Aegirine

Phenocrysts in some siliceous volcanics (rhyolites)

Spodumene

Giant crystals in granite pegmatites (> 14 m)

orthopyroxeneCpx (diopside)

Jadeite (massive) aegirine

Spodumene, var.kunzite

Mantle xenolith with olivine, cpx, garnet

Cpx phenocryst in Hawaiian basalt

Apollo 15 mare basalt: cpx + plag

Twinned + exsolved cpx from nakhlite (MARS)

CPX + leucite phenocrysts from Vesuvius

Pigeonite from Pasamonte meteorite

TEM image ofexsolvedclinopyroxene(pigeonite +augite) fromPasamonte

augpig

Co-existing pyroxenes from Skaergaard Intrusion, Greenland plotted in thepyroxene quadrilateral. Note that the cpx (augite) has less Ca than Di-Hd andthat pigeonite and opx (aka. hypersthene) have some Ca relative to pure En-Fs. Tie lines (dashed lines 1, 2, 3, 4, 5) join pyroxenes from the same rocksample

These images show exsolution features in clinopyroxene. (A) Optical photo of pigeoniteexsolving from augite, (B) a drawing of the photograph. Pigeonite forms lamellae in an augitehost crystal. Pigeonite I forms by exsolution at high T, while pigeonite II and Pigeonite III form atsuccessively lower T.

TEM (transmission electron microscope) image of exsolution of clinopyroxene (cpx)from orthopyroxene (opx). Sample from Stillwater Complex, Montana

Other pyroxenes

Jadeite: NaAlSi2O6

Aegerine: NaFe3+Si2O6

Ureyite: NaCrSi2O6

Spodumene (Kunzite): LiAlSi2O6

Johannsenite: CaMnSi2O6

Omphacite: (Ca,Na)(Mg,Fe",Al)Si2O6

Omphacite is an ordered solid solution of Diopside andJadeite.

Aegirine

Jadeite

spodumene

kunzite

Varieties of Pyroxene

Orthopyroxene – Clinopyroxenesolvus T dependenceComplex solvus – the stability of a particular mineralchanges with T. A different mineral s stability maychange with T differently…OPX-CPX exsolution lamellae Geothermometer…

Miscibility

FsFsEnEn

DiDi HdHd

FsFsEnEn

DiDi HdHd

OPXOPXOPXOPX

CPXCPX CPXCPX

pigeonite

augite

orthopyroxene

Pigeonite + orthopyroxene

orthopyroxene

Subcalcic augite

pigeonite

augite

Miscibility

800800ºCºC 12001200ºCºC

Pyroxenes pdf

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