Download - 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
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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]]
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a s
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Where are the Si-O-Si-O chains??Where are the Si-O-Si-O chains??
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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)
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a s
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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)
bb
a s
ina s
in
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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)
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a s
ina s
in
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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)
bb
a s
ina s
in
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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)
bb
a s
ina s
in
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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)
Perspective viewPerspective view
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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)
SiOSiO44 as polygons as polygons
(and larger area)(and larger area)IV slabIV slab
IV slabIV slab
IV slabIV slab
IV slabIV slab
VI slabVI slab
VI slabVI slab
VI slabVI slab
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a s
ina s
in
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Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
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Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
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Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
(+) type by convention(+) type by convention
(+)
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Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
This is a (-) typeThis is a (-) type
(-)
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Inosilicates: single chains- pyroxenes
TT
M1M1
TT
Creates an Creates an ““I-beamI-beam””
like unit in thelike unit in the
structure.structure.
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Inosilicates: single chains- pyroxenes
TT
M1M1
TT
Creates an Creates an ““I-beamI-beam””
like unit in thelike unit in the
structurestructure
(+)(+)
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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
(+)(+)
(+)(+)(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes
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
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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
(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes
(+)(+)(+)(+)
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TetrehedraTetrehedra and M1 and M1
octahedra shareoctahedra share
tetrahedral apicaltetrahedral apical
oxygen atomsoxygen atoms
Inosilicates: single chains- pyroxenes
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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
Inosilicates: single chains- pyroxenes
cc
aa
(+) M1(+) M1
(+) M2(+) M2
(+) M2(+) M2
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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
Inosilicates: single chains- pyroxenes
cc
aa
(+) M1(+) M1
(-) M1(-) M1
(-) M2(-) M2
(+) M2(+) M2
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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
5.2 A
Pyroxene
c
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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
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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
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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).
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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
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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
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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
Opx
T(ºC)
800
1000
1200
En(Mg2Si2O6)
Di(CaMgSi2O6)
Opx--CpxMiscibility Gap
SOLVUS
1080
solidus
Section along En-Di join
Pyroxenesnot stable
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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
Opx
T(ºC)
800
1000
1200
En(Mg2Si2O6)
Di(CaMgSi2O6)
Opx--Cpx MiscibilityGap
X
1080
solidus
12
3 45
6 7
Di Hd
En Fs
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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
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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
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Mantle xenolith with olivine, cpx, garnet
Cpx phenocryst in Hawaiian basalt
Apollo 15 mare basalt: cpx + plag
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Twinned + exsolved cpx from nakhlite (MARS)
CPX + leucite phenocrysts from Vesuvius
Pigeonite from Pasamonte meteorite
TEM image ofexsolvedclinopyroxene(pigeonite +augite) fromPasamonte
augpig
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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
Wo
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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.
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TEM (transmission electron microscope) image of exsolution of clinopyroxene (cpx)from orthopyroxene (opx). Sample from Stillwater Complex, Montana
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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.
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Aegirine
Jadeite
spodumene
kunzite
Varieties of Pyroxene
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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
Gap
FsFsEnEn
DiDi HdHd
FsFsEnEn
DiDi HdHd
OPXOPXOPXOPX
CPXCPX CPXCPX
pigeonite
augite
orthopyroxene
Pigeonite + orthopyroxene
orthopyroxene
Subcalcic augite
pigeonite
augite
Miscibility
Gap
800800ºCºC 12001200ºCºC