1. Pyroxene group
Fundamental group of mineral
Composition of basalt ~ composition of
pyroxene
Abundantly found from mantle rocks to
rhyolites
Both metamorphic and igneous
2. Inosilicates: single chains- pyroxenes
b
Diopside: CaMg [Si2O6]
a sin
Where are the Si-O-Si-O chains??
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
8. Inosilicates: single chains- pyroxenes
Perspective view
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
9. Inosilicates: single chains- pyroxenes
SiO4 as polygons
IV slab
(and larger area)
VI slab
IV slab
a sin
VI slab
IV slab
VI slab
IV slab
b
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
16. Inosilicates: single chains- pyroxenes
(+) (+)
The pyroxene
structure is then
composed of
alternating I-beams
Clinopyroxenes have
(+)
all I-beams oriented
the same: all are (+)
in this orientation
Note that M1 sites are
(+) (+) smaller than M2 sites, since
they are at the apices of the
tetrahedral chains
17. Inosilicates: single chains- pyroxenes
The pyroxene
structure is then
(+) (+) composed of
alternation I-beams
Clinopyroxenes have
all I-beams oriented
the same: all are (+)
(+) in this orientation
Orthopyroxenes have
alternating (+) and (-)
orientations
(+) (+)
19. Inosilicates: single chains- pyroxenes
The tetrahedral chain
(+) M2 above the M1s is thus
c offset from that below
a
The M2 slabs have a
(+) M1
similar effect
The result is a
(+) M2 monoclinic unit cell,
hence clinopyroxenes
20. Inosilicates: single chains- pyroxenes
Orthopyroxenes have
c
alternating (+) and (-)
(-) M1 I-beams
the offsets thus
(+) M2 compensate and result
in an orthorhombic
a unit cell
(+) M1
(-) M2
21. Pyroxene Pyroxenoids
“Ideal” pyroxene chains with
5.2 Å repeat (2 tetrahedra)
become “kinked” as other
cations occupy M sites
c
17.4 A
12.5 A
7.1 A
5.2 A
Pyroxene Wollastonite Rhodonite Pyroxmangite
MgSiO3 CaSiO3 MnSiO3 (Mn, Fe)SiO3
2-tet repeat 3-tet repeat 5-tet repeat 7-tet repeat
22. Pyroxene Group
The 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 of
pyroxenes over amphiboles
23. Common Pyroxene End members and solid
solutions
A. Orthopyroxenes
End members:
Enstatite (En):
Mg2Si2O6
Ferrosilite (Fs):
Fe2''Si2O6
Prismatic crystals
Complete solid solution between
En and Fs. Pure Fs is unstable at
1 bar.
Stable at P >1 bar
En Stable at P = 1 bar Fs
24. 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 in
an En-Di-Hd-Fs quadrilateral (topless triangle).
25. Pyroxene Chemistry
The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Wollastonite Ca2Si2O6
•Orthopyroxenes – solid soln
between Enstatite-Ferrosilite
•Clinopyroxenes – solid soln
between Diopside-Hedenbergite
Diopside Hedenbergite
CaMgSi2O6 clinopyroxenes CaFeSi2O6
pigeonite
orthopyroxenes
Enstatite Ferrosilite
Mg2Si2O6 Fe2Si2O6
26. 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)
Wollastonite Ca2Si2O6 pigeonite cli
no 1200oC
orthopyroxenes py
ro
xe
ne
s
1000oC
Hedenbergite
Diopside clinopyroxenes CPX
CaMgSi2O6 CaFeSi2O6
Solvus
800oC
pigeonite
orthopyroxenes
(Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6
Enstatite Ferrosilite
Mg2Si2O6
OPX Fe2Si2O6 OPX CPX
27. Pyroxene Composition
The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Wollastonite Section along En-Di join
Ca2Si2O6
solidus
pigeonite
Pig Aug 1200
Pyroxenes
1080
not stable T(ºC)
Diopside: Hedenbergite: 1000
CaMgSi2O6 CaFeSi2O6 Opx--Cpx
clinopyroxenes Opx Miscibility Gap
SOLVUS
Px in this 800
region are
unstable at
low P En Di
pigeonite (Mg2Si2O6) (CaMgSi2O6)
orthopyroxenes
Enstatite: Ferrosilite:
Mg2Si2O6 Fe2Si2O6
28. Subsolidus relations in the common pyroxenes
solidus Start with a crystal of pyroxene of
bulk composition X and cool
X At T~1200°C the pigeonite
Pig
2 Aug 1200 intersects the pigeonite-augite
1 solvus and begins to exsolve augite
(2).
5 1080
3 4
T(ºC)
1000 With cooling, pigeonite changes
Opx--Cpx Miscibility composition to 3 and augite
Opx Gap lamellae change composition to 4
6 7 At 1080°C, pigeonite become
800
unstable and breaks down to
orthopyroxene (5).
En Di
Cooling below 1080°C, opx
(Mg2Si2O6) (CaMgSi2O6)
will exsolve augite
Di Hd
En Fs
30. Occurrence of pyroxenes Cpx (diopside)
Orthopyroxenes orthopyroxene
Major mineral in mafic (basalts-andesites-gabbros) and
ultramafic (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 intermediate
igneous rocks (V and P) Jadeite (massive) aegirine
High grade metamorphic rocks (meta-basalts and
meta-carbonates)
Formed during the metamorphism of impure
dolostones
CaMg(CO3)2 + 2SiO2 CaMgSi2O6 + 2CO2
Jadeite-omphacite
Typical of high pressure metamorphism (blueschists
and eclogites) formed by metamorphism of basalts
during subduction. Excellent pressure indicator.
Aegirine
Phenocrysts in some siliceous volcanics (rhyolites)
Spodumene Spodumene, var.
Giant crystals in granite pegmatites (> 14 m) kunzite
31. Cpx phenocryst in Hawaiian basalt
Mantle xenolith with olivine, cpx, garnet Apollo 15 mare basalt: cpx + plag
32. Pigeonite from Pasamonte meteorite
Twinned + exsolved cpx from nakhlite (MARS)
CPX + leucite phenocrysts from Vesuvius
pig
aug
TEM image of
exsolved
clinopyroxene
(pigeonite +
augite) from
Pasamonte
33. Co-existing pyroxenes from Skaergaard Intrusion, Greenland plotted in the
pyroxene quadrilateral. Note that the cpx (augite) has less Ca than Di-Hd and
that 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 rock
sample
Wo
34. These images show exsolution features in clinopyroxene. (A) Optical photo of pigeonite
exsolving from augite, (B) a drawing of the photograph. Pigeonite forms lamellae in an augite
host crystal. Pigeonite I forms by exsolution at high T, while pigeonite II and Pigeonite III form at
successively lower T.
35. TEM (transmission electron microscope) image of exsolution of clinopyroxene (cpx)
from orthopyroxene (opx). Sample from Stillwater Complex, Montana
36. 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 and
Jadeite.
38. Orthopyroxene – Clinopyroxene
solvus T dependence
Complex solvus – the stability of a particular mineral
changes with T. A different mineral s stability may
change with T differently…
OPX-CPX exsolution lamellae Geothermometer…
Di CPX Hd Di CPX Hd
augite
augite
Miscibility Miscibility Subcalcic augite
Gap Gap pigeonite
pigeonite
orthopyroxene orthopyroxene
En OPX Fs En Fs
OPX
800ºC 1200ºC Pigeonite + orthopyroxene
![Inosilicates: single chains- pyroxenes
b
Diopside: CaMg [Si2O6]
a sin
Where are the Si-O-Si-O chains??
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)