ERTH 2002

1
ERTH 2002

• Mineral of the Week (MoW)
• olivine
• 2. pyroxene
• 3. feldspar (some review)
• 4. garnet
• 5. amphibole
• 6. mica
• 7. quartz
• 8. clays
• 9. carbonates (some review)
• 10. Fe-Ti oxides
• 11. sulphides (some review)

properties chemical composition crystal
structure phase diagrams occurrence use
2
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
photos mindat.org
3
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
a single-chain silicate with the general formula
XYSi2O6
clinopyroxene Ca(Mg,Fe)Si2O6
orthopyroxene (Mg,Fe)2Si2O6
opx
cpx
interference colours
4
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
a single-chain silicate with the general formula
XYSi2O6
clinopyroxene Ca(Mg,Fe)Si2O6
orthopyroxene (Mg,Fe)2Si2O6
distinguishing optical properties (opx vs
cpx) colour in PPL birefringence extinction angle
5
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
clinopyroxene in thin section photos from
OM-series, ERTH 2001
PPL
PPL
OM-15 (4.5mm) parallel to c-axis
OM-15 (4.5mm) normal to c-axis
XN
XN
6
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
orthopyroxene in thin section photos from
OM-series, ERTH 2001
PPL
PPL
OM-10 (3.5mm) normal to c-axis
OM-10 (4.5mm) parallel to c-axis
XN
XN
7
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
SiO44- chain
crystal structure single-chain
silicate (inosilicate)
chains linked by cations
M1, M2 octahedral sites
linked I-beam structures and pyroxene cleavage
T O (M1) T
I-beam structure
Nesse, Fig. 14.1
8
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
a chain silicate with the general formula XYSi2O6
clinopyroxene Ca(Mg,Fe)Si2O6
complete solid solution between Mg, Fe
end-members diopside CaMgSi2O6 hedenbergite
CaFeSi2O6 intermediate Fe/Mg members of the
series include augite Ca(Mg,Fe)Si2O6 ferro-augi
te Ca(Fe,Mg)Si2O6 in addition, Na, Li, Al, Ti,
Fe3, Mn can also substitute for Ca, Mg, Fe
monoclinic biaxial ve
M1 ? Mg,Fe M2 ? Ca
9
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
a chain silicate with the general formula XYSi2O6
complete solid solution between Mg, Fe
end-members enstatite Mg2Si2O6
(MgSiO3) ferrosilite Fe2Si2O6
(FeSiO3) intermediate Fe/Mg members of the
series include bronzite (Mg,Fe)2Si2O6 hypersthe
ne (Fe,Mg)2Si2O6 in addition, minor Ca, Al,
Fe3, Mn can also substitute for Mg,
Fe pigeonite (Mg,Fe,Ca)2Si2O6 sub-calcic
clinopyroxene with Ca lt 30 of M2 site
orthopyroxene (Mg,Fe)2Si2O6
orthorhombic biaxial /-ve
10
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
pyroxene compositions can be plotted in the
Wo-En-Fs triangle where wollastonite
(Ca2Si2O6) is a pyroxenoid a chain silicate with
a related but not identical structure
Wo
Fs
En
11
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
more commonly only the bottom half is used,
forming the pyroxene quadrilateral
Hd
Di
Fs
En
12
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
more commonly only the bottom half is used,
forming the pyroxene quadrilateral
Hd
Di
grey zones show the limits of solid solution
between clino- and ortho-pyroxene
tie-lines can be used to show the compositions
of coexisting pyroxenes
Fs
En
13
OM-10 (4.5 mm, XN) bright lines/patches exsolutio
n lamellae of cpx in opx formed during cooling
Hd
Di
grey zones show the limits of solid solution
between clino- and ortho-pyroxene more extensive
at high T than at low T (? exsolution)
tie-lines can be used to show the compositions
of coexisting pyroxenes
Fs
En
14
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
exsolution lamellae in pyroxenes
host augite opx
pigeonite opx after pig
15
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
exsolution lamellae in pyroxenes
OM-10 (4.5 mm, XN) bright lines/patches exsolutio
n lamellae of cpx in opx formed during cooling
host augite opx
pigeonite opx after pig
16
MoW 2 Pyroxene (Nesse, Ch. 14, p. 261-269)
occurrence a) ultramafic and mafic igneous
rocks cpx an essential mineral in basalt,
gabbro, pyroxenite wehrlite, lherzolite common
in andesite, diorite opx essential in
pyroxenite, harzburgite, lherzolite,
norite common in basalt, gabbro b)
high-grade metamorphic rocks cpx common in
metabasites (granulite, amphibolite) also
impure marbles, calc-silicates opx essential in
mafic granulite use no industrial uses
rare gem-quality crystals