Back to silicate structures:

1
Back to silicate structures
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates
2

• Nesosilicates independent SiO4 tetrahedra

b
c
projection
Olivine (100) view blue M1 yellow M2
3

• Inosilicates single chains- pyroxenes

T M1 T Creates an I-beam like unit in the
structure
4
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
5
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
6
Inosilicates single chains- pyroxenes
The pyroxene structure is then composed of
alternation I-beams Orthoopyroxenes have I-beams
oriented in alternate direction in different
layers
7
Inosilicates single chains- pyroxenes
The tetrahedral chain above the M1s is thus
offset from that below The M2 slabs have a
similar effect The result is a monoclinic unit
cell, hence clinopyroxenes
() M2
c
a
() M1
() M2
8
Inosilicates single chains- pyroxenes
Orthopyroxenes have alternating () and (-)
I-beams the offsets thus compensate and result
in an orthorhombic unit cell
c
(-) M1
() M2
a
() M1
(-) M2
9
Pyroxene Chemistry

• The general pyroxene formula
• W1-P (X,Y)1P Z2O6
• Where
• W Ca Na
• X Mg Fe2 Mn Ni Li
• Y Al Fe3 Cr Ti
• Z Si Al
• Anhydrous so high-temperature or dry
  conditions favor pyroxenes over amphiboles

10
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

Hedenbergite CaFeSi2O6
Diopside CaMgSi2O6
clinopyroxenes
Joins lines between end members limited
mixing away from join
pigeonite
orthopyroxenes
Ferrosilite Fe2Si2O6
Enstatite Mg2Si2O6
11
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)

pigeonite
1200oC
orthopyroxenes
clinopyroxenes
1000oC
CPX
Solvus
800oC
(Mg,Fe)2Si2O6
Ca(Mg,Fe)Si2O6
OPX
OPX
CPX
12
Orthopyroxene Clinopyroxenesolvus T dependence

• Complex solvus the stability of a particular
  mineral changes with T. A different minerals
  stability may change with T differently
• OPX-CPX exsolution lamellae ? Geothermometer

CPX
CPX
Hd
Di
Di
Hd
augite
augite
Subcalcic augite
Miscibility Gap
Miscibility Gap
pigeonite
pigeonite
orthopyroxene
orthopyroxene
Fs
En
Fs
En
OPX
OPX
800ºC
1200ºC
Pigeonite orthopyroxene
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Pyroxene Chemistry

• Non-quad pyroxenes

Jadeite
Aegirine
NaAlSi2O6
NaFe3Si2O6
0.8
Omphacite
aegirine- augite
Spodumene LiAlSi2O6
Ca / (Ca Na)
Ca-Tschermacks molecule
0.2
CaAl2SiO6
Augite
Ca(Mg,Fe)Si2O6
Diopside-Hedenbergite
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Pyroxenoids
Ideal pyroxene chains with 5.2 A repeat (2
tetrahedra) become distorted as other cations
occupy VI sites
Wollastonite (Ca ? M1) ? 3-tet repeat
Rhodonite MnSiO3 ? 5-tet repeat
Pyroxmangite (Mn, Fe)SiO3 ? 7-tet repeat
Pyroxene 2-tet repeat
15
Back to silicate structures
nesosilicates
phyllosilicates
sorosilicates
inosilicates
cyclosilictaes
tectosilicates
16

• Inosilicates double chains- amphiboles

b
Tremolite Ca2Mg5 Si8O22 (OH)2
a sin?
Tremolite (001) view blue Si purple M1
rose M2 gray M3 (all Mg) yellow M4 (Ca)
17

• Inosilicates double chains- amphiboles

b
Hornblende (Ca, Na)2-3 (Mg, Fe, Al)5
(Si,Al)8O22 (OH)2
a sin?
Hornblende (001) view dark blue Si, Al
purple M1 rose M2 light blue M3 (all
Mg, Fe) yellow ball M4 (Ca) purple ball
A (Na) little turquoise ball H
18

• Inosilicates double chains- amphiboles

Hornblende (Ca, Na)2-3 (Mg, Fe, Al)5
(Si,Al)8O22 (OH)2
Same I-beam architecture, but the I-beams are
fatter (double chains)
Hornblende (001) view dark blue Si, Al
purple M1 rose M2 light blue M3 (all
Mg, Fe)
19

• Inosilicates double chains- amphiboles

b
Hornblende (Ca, Na)2-3 (Mg, Fe, Al)5
(Si,Al)8O22 (OH)2
Same I-beam architecture, but the I-beams are
fatter (double chains)
a sin?
All are () on clinoamphiboles and alternate in
orthoamphiboles
Hornblende (001) view dark blue Si, Al
purple M1 rose M2 light blue M3 (all
Mg, Fe) yellow ball M4 (Ca) purple ball
A (Na) little turquoise ball H
20

• Inosilicates double chains- amphiboles

Hornblende (Ca, Na)2-3 (Mg, Fe, Al)5
(Si,Al)8O22 (OH)2 M1-M3 are small sites M4 is
larger (Ca) A-site is really big Variety of
sites ? great chemical range
Hornblende (001) view dark blue Si, Al
purple M1 rose M2 light blue M3 (all
Mg, Fe) yellow ball M4 (Ca) purple ball
A (Na) little turquoise ball H
21

• Inosilicates double chains- amphiboles

Hornblende (Ca, Na)2-3 (Mg, Fe, Al)5
(Si,Al)8O22 (OH)2 (OH) is in center of
tetrahedral ring where O is a part of M1 and M3
octahedra
(OH)
Hornblende (001) view dark blue Si, Al
purple M1 rose M2 light blue M3 (all
Mg, Fe) yellow ball M4 (Ca) purple ball
A (Na) little turquoise ball H
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Amphibole Chemistry
See handout for more information General
formula W0-1 X2 Y5 Z8O22 (OH, F, Cl)2 W
Na K X Ca Na Mg Fe2 (Mn Li) Y Mg
Fe2 Mn Al Fe3 Ti Z Si Al Again, the
great variety of sites and sizes ? a great
chemical range, and hence a broad stability
range The hydrous nature implies an upper
temperature stability limit
23
Amphibole Chemistry
Ca-Mg-Fe Amphibole quadrilateral (good analogy
with pyroxenes)
Tremolite
Ferroactinolite
Actinolite
Ca2Mg5Si8O22(OH)2
Ca2Fe5Si8O22(OH)2
Clinoamphiboles
Cummingtonite-grunerite
Anthophyllite
Fe7Si8O22(OH)2
Mg7Si8O22(OH)2
Orthoamphiboles
Al and Na tend to stabilize the orthorhombic form
in low-Ca amphiboles, so anthophyllite ? gedrite
orthorhombic series extends to Fe-rich gedrite
in more Na-Al-rich compositions
24
Amphibole Chemistry
Hornblende has Al in the tetrahedral
site Geologists traditionally use the term
hornblende as a catch-all term for practically
any dark amphibole. Now the common use of the
microprobe has petrologists casting hornblende
into end-member compositions and naming
amphiboles after a well-represented
end-member. Sodic amphiboles Glaucophane
Na2 Mg3 Al2 Si8O22 (OH)2 Riebeckite Na2
Fe23 Fe32 Si8O22 (OH)2 Sodic amphiboles are
commonly blue, and often called blue amphiboles
25
Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates Actin
olite occurs in low-grade metamorphosed basic
igneous rocks Orthoamphiboles and
cummingtonite-grunerite (all Ca-free, Mg-Fe-rich
amphiboles) are metamorphic and occur in
meta-ultrabasic rocks and some meta-sediments.
The Fe-rich grunerite occurs in
meta-ironstones The complex solid solution called
hornblende occurs in a broad variety of both
igneous and metamorphic rocks Sodic amphiboles
are predominantly metamorphic where they are
characteristic of high P/T subduction-zone
metamorphism (commonly called blueschist in
reference to the predominant blue sodic
amphiboles Riebeckite occurs commonly in sodic
granitoid rocks
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Inosilicates







a











-
-
-
-
-
-
Clinopyroxene
Clinoamphibole





a

-
-
-
-
-
-
Orthopyroxene
Orthoamphibole

• Pyroxenes and amphiboles are very similar
• Both have chains of SiO4 tetrahedra
• The chains are connected into stylized I-beams by
  M octahedra
• High-Ca monoclinic forms have all the T-O-T
  offsets in the same direction
• Low-Ca orthorhombic forms have alternating ()
  and (-) offsets

27

• Inosilicates

pyroxene
amphibole
b
a
Cleavage angles can be interpreted in terms of
weak bonds in M2 sites (around I-beams instead of
through them) Narrow single-chain I-beams ? 90o
cleavages in pyroxenes while wider double-chain
I-beams ? 60-120o cleavages in amphiboles
28
Tectosilicates
After Swamy and Saxena (1994) J. Geophys. Res.,
99, 11,787-11,794.
29
Tectosilicates

• Low Quartz

001 Projection Crystal Class 32
30
Tectosilicates

• High Quartz at 581oC

001 Projection Crystal Class 622
31
Tectosilicates

• Cristobalite

001 Projection Cubic Structure
32
Tectosilicates

• Stishovite

High pressure ? SiVI
33
Tectosilicates

• Low Quartz
  Stishovite

SiIV
SiVI
34
Igneous Minerals

• Quartz, Feldspars (plagioclase and alkaline),
  Olivines, Pyroxenes, Amphiboles
• Accessory Minerals mostly in small quantities
  or in special rocks
• Magnetite (Fe3O4)
• Ilmenite (FeTiO3)
• Apatite (Ca5(PO4)3(OH,F,Cl)
• Zircon (ZrSiO4)
• Titanite (CaTiSiO5)
• Pyrite (FeS2)
• Fluorite (CaF2)