silicate structures

Mineral Structures
Silicates are classified on the basis of Si-O polymerism
the [SiO4]4- tetrahedron


Mineral Structures
Silicates are classified on the basis of Si-O polymerism
[SiO4]4-

Independent tetrahedra Nesosilicates

Examples: olivine garnet

[Si2O7]6-

Double tetrahedra

Sorosilicates

Examples: lawsonite epidote

n[SiO3]2- n = 3, 4, 6
Examples: benitoite BaTi[Si3O9]
beryl Be3Al2[Si6O18]

Cyclosilicates


Mineral Structures
Inosilicates

[SiO3]2pryoxenes

single chains
pyroxenoids

Inosilicates

[Si4O11]4-

Double chains
amphiboles


Mineral Structures
Phyllosilicates

[Si2O5]2-

Sheets of tetrahedra

micas talc clay minerals serpentine

Phyllosilicates


Mineral Structures
Tectosilcates

low-quartz

[SiO2]

3-D frameworks of tetrahedra: fully polymerized

quartz and the silica minerals feldspars feldspathoids zeolites

Tectosilicates


Nesosilicates: independent SiO4 tetrahedra

b

c

M1 and M2 as polyhedra

Olivine (100) view blue = M1 yellow = M2


Nesosilicates: Olivine (Mg,Fe)2SiO4
Olivine Occurrences:
Principally in mafic and ultramafic igneous rocksTypically ~60+% of mantle source for basaltsFayalite in meta-ironstones and in some alkalic
granitoids
Forsterite in some siliceous dolomitic marbles


Nesosilicates: Garnet
Garnet: A2+3 B3+2 [SiO4]3
“Pyralspites” - B = Al

Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites

Garnet (001) view blue = Si purple = A turquoise = B


Inosilicates: single chains- pyroxenes
b

a sinβ

Diopside: CaMg [Si2O6]

Where are the Si-O-Si-O chains??

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates: single chains- pyroxenes

a sinβ

b

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates: single chains- pyroxenes

(+) M2

The tetrahedral chain
above the M1s is
offset from that below

c
a

(+) M1

(+) M2

The result is a
monoclinic unit cell,
hence clinopyroxenes

e.g. Diopside, Augite


Inosilicates: single chains- pyroxenes
Orthopyroxene

c
(-) M1

an orthorhombic unit
cell
(+) M2
Enstatite (Mg2Si2O6)

a
(+) M1

(-) M2


Pyroxene Chemistry
The general pyroxene formula:
W1-P (X,Y)1+P 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


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

Wollastonite
pigeonite
orthopyroxenes

Diopside

clinopyroxenes

cli
no
py
ro
xe
ne
s

1200oC

1000oC

Hedenbergite

Solvus
800oC

pigeonite

(Mg,Fe)2Si2O6

orthopyroxenes

Enstatite

Ferrosilite

Ca(Mg,Fe)Si2O6


Pyroxene Chemistry
“Non-quad” pyroxenes
Jadeite
NaAlSi2O6

Aegirine
NaFe3+Si2O6

0.8
Omphacite

aegirineaugite

Ca / (Ca + Na)

0.2

Ca-Tschermack’s
molecule CaAl2SiO6
Augite

Diopside-Hedenbergite

Ca(Mg,Fe)Si2O6


Inosilicates: double chains- amphiboles
b

a sinβ

Tremolite:
Ca2Mg5 [Si8O22] (OH)2

Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)
yellow = M4 (Ca)


Inosilicates: double chains- amphiboles
b

a sinβ

Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5
[(Si,Al)8O22] (OH)2

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


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


Amphibole Chemistry
Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)

Tremolite
Ca2Mg5Si8O22(OH)2

Anthophyllite
Mg7Si8O22(OH)2

Actinolite

Cummingtonite-grunerite

Orthoamphiboles

Ferroactinolite
Ca2Fe5Si8O22(OH)2
Clinoamphiboles
Fe7Si8O22(OH)2

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 Fe2+3 Fe3+2 [Si8O22] (OH)2
Sodic amphiboles are commonly blue, and often called “blue amphiboles”


Amphibole Occurrences
Tremolite (Ca-Mg) occurs in meta-carbonates
Actinolite 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


pyroxene

Inosilicates

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 doublechain I-beams → 60-120o cleavages in amphiboles


Phyllosilicates
SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents


Phyllosilicates
Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O


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