Mineral Structures

1
Mineral Structures

• Silicates are classified on the basis of Si-O
  polymerism

the SiO44- tetrahedron
2
Mineral Structures

• Silicates are classified on the basis of Si-O
  polymerism

SiO44- Independent tetrahedra
Nesosilicates Examples olivine
garnet Si2O76- Double tetrahedra
Sorosilicates Examples lawsonite
epidote nSiO32- n 3, 4, 6
Cyclosilicates Examples benitoite
BaTiSi3O9 beryl
Be3Al2Si6O18
3
Mineral Structures

• Inosilicates

SiO32- single chains
Inosilicates Si4O114- Double
chains pryoxenes pyroxenoids
amphiboles
4
Mineral Structures

• Phyllosilicates

Si2O52- Sheets of tetrahedra
Phyllosilicates micas
talc clay minerals serpentine
5
Mineral Structures

• Tectosilcates

low-quartz
SiO2 3-D frameworks of tetrahedra fully
polymerized Tectosilicates quartz
and the silica minerals feldspars
feldspathoids zeolites
6

• Nesosilicates independent SiO4 tetrahedra

b
c
M1 and M2 as polyhedra
Olivine (100) view blue M1 yellow M2
7

• Nesosilicates Olivine (Mg,Fe)2SiO4
• Olivine Occurrences
• Principally in mafic and ultramafic igneous
  rocks- Typically 60 of mantle source for
  basalts-
• Fayalite in meta-ironstones and in some alkalic
  granitoids
• Forsterite in some siliceous dolomitic marbles

8

• Nesosilicates Garnet

Garnet A23 B32 SiO43 Pyralspites - B
Al Pyrope Mg3 Al2 SiO43 Almandine Fe3 Al2
SiO43 Spessartine Mn3 Al2 SiO43
Ugrandites - A Ca Uvarovite Ca3 Cr2
SiO43 Grossularite Ca3 Al2 SiO43
Andradite Ca3 Fe2 SiO43 Occurrence Mostly
metamorphic Some high-Al igneous Also in some
mantle peridotites
Garnet (001) view blue Si purple A
turquoise B
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• 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)
10

• Inosilicates single chains- pyroxenes

b
a sin?
Diopside (001) view blue Si purple M1 (Mg)
yellow M2 (Ca)
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Inosilicates single chains- pyroxenes
The tetrahedral chain above the M1s is offset
from that below The result is a monoclinic
unit cell, hence clinopyroxenes e.g. Diopside,
Augite
() M2
c
a
() M1
() M2
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Inosilicates single chains- pyroxenes
Orthopyroxene an orthorhombic unit
cell Enstatite (Mg2Si2O6)
c
(-) M1
() M2
a
() M1
(-) M2
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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

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Pyroxene Chemistry

• The pyroxene quadrilateral and opx-cpx solvus
• Coexisting opx cpx in many rocks (pigeonite
  only in volcanics)

Wollastonite
pigeonite
1200oC
orthopyroxenes
clinopyroxenes
1000oC
Diopside
Hedenbergite
clinopyroxenes
Solvus
800oC
pigeonite
(Mg,Fe)2Si2O6
Ca(Mg,Fe)Si2O6
orthopyroxenes
Ferrosilite
Enstatite
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Pyroxene Chemistry

• Non-quad pyroxenes

Jadeite
Aegirine
NaAlSi2O6
NaFe3Si2O6
0.8
Omphacite
aegirine- augite
Ca / (Ca Na)
Ca-Tschermacks molecule
0.2
CaAl2SiO6
Augite
Ca(Mg,Fe)Si2O6
Diopside-Hedenbergite
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• 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)
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• 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
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Amphibole Chemistry
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
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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
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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
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Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates Actino
lite occurs in low-grade metamorphosed basic
igneous rocks 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
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• Inosilicates

pyroxene
amphibole
b
a
Cleavage angles can be interpreted in terms of
weak bonds in M2 sites Narrow single-chain
I-beams ? 90o cleavages in pyroxenes while wider
double-chain I-beams ? 60-120o cleavages in
amphiboles
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Phyllosilicates
SiO4 tetrahedra polymerized into 2-D sheets
Si2O5 Apical Os are unpolymerized and are
bonded to other constituents
24
Phyllosilicates
Tetrahedral layers are bonded to octahedral
layers (OH) pairs are located in center of T
rings where no apical O
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Phyllosilicates
a2
a1
Gibbsite Al(OH)3 Layers of octahedral Al in
coordination with (OH) Al3 means that only 2/3
of the VI sites may be occupied for
charge-balance reasons Brucite-type layers may
be called trioctahedral and gibbsite-type
dioctahedral
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Phyllosilicates
T O T K T O T K T O T
Muscovite K Al2 Si3AlO10 (OH)2 (coupled K -
AlIV) T-layer - diocathedral (Al3) layer -
T-layer - K
K between T - O - T groups is stronger than vdw
27
Phyllosilicates
T O T K T O T K T O T
Phlogopite K Mg3 Si3AlO10 (OH)2 T-layer -
triocathedral (Mg2) layer - T-layer - K
K between T - O - T groups is stronger than vdw
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Phyllosilicates
Chlorite (Mg, Fe)3 (Si, Al)4O10 (OH)2 (Mg,
Fe)3 (OH)6 T - O - T - (brucite) - T - O - T
- (brucite) - T - O - T - Very hydrated (OH)8,
so low-temperature stability (low-T metamorphism
and alteration of mafics as cool)
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Tectosilicates
After Swamy and Saxena (1994) J. Geophys. Res.,
99, 11,787-11,794.
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Tectosilicates

• Low Quartz
  Stishovite

SiIV
SiVI
31
Tectosilicates

• Feldspars

Substitute Al3 for Si4 allows Na or K to be
added
Substitute two Al3 for Si4 allows Ca2 to be
added
Albite NaAlSi3O8