Download Lecture 8_0204_silicates 2

Earth and Planetary Materials
Spring 2013
Lecture 8
2013.02.04
XtalDraw
Free software to view crystal structures (with database)
www.geo.arizona.edu/xtal/group/software.htm
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Phyllosilicates (sheet or layer silicates)
Serpentine group
Clay mineral group
Mica group
Chlorite group
Phyllosilicates
Phyllon (Greek): leaf
Generally soft, low specific
gravity
Platy/flaky habit with one
prominent cleavage
Examples: clay minerals, mica
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Phyllosilicates – Structure
Contain sheets of tetrahedra
that share 3 oxygens with
other tetrahedra
Composition of the
tetrahedral sheet: Si2O52 Each sheet has 6-fold
symmetry
Si2O52-
Infinitely extended sheet of SiO4
tetrahedra
Most phyllosilicates have hydroxyl (OH) groups
In the center of the tetrahedral ring
At the same height as the unshared apical oxygens
The apical oxygens (non-bridging) connect to an octahedral
sheet
Unshared apical oxygen
Similar to the size of triangular
faces of XO6 octahedra (X
commonly Al, Mg)
Mg2+, Fe2+, or Al3+
O or OHOH
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The octahedral sheet resembles brucite (trioctahedral) or
gibbsite (dioctahedral)
brucite Mg(OH)2
trioctahedral sheet
Cation +2
gibbsite Al(OH)3
Cation +3
dioctahedral sheet
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The structure of phyllosilicates thus can be:
1:1 tetrahedral sheet to octahedral sheet (TO TO TO TO ...)
2:1 tetrahedral to octahedral (TOT TOT TOT TOT ... )
The TO or TOT layers are held together by weak van der Waals
forces or by large cations
1:1 phyllosilicates
1:1 tetrahedral sheet to octahedral sheet (TO TO TO TO ...)
Examples
Kaolinite Al2Si2O5(OH)4 – clay mineral group
Lizardite Mg3Si2O5(OH)4 – serpentine mineral group
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brucite Mg(OH)2
gibbsite Al(OH)3
OH
Al
Mg
Mg3
(OH)3
(OH)3
1:1 phyllosilicates
Mg3
(OH)Si2O5
(OH)3
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Kaolinite Al2Si2O5(OH)4
Clay mineral group
1:1 dioctahedral phyllosilicate
tetrahedral sheet
all basal oxygens
shared by two
aluminosilicate
tetrahedra
apical oxygens
connect to the
overlying
octahedral sheet
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dioctahedral (gibbsite) sheet: 2/3 of all octahedral sites filled
kaolinite: tetrahedral and octahedral sheets fit together
1:1, dioctahedral
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In kaolinite, the TO layers are electrically neutral, so they are
held together only weakly by van der Waal's forces
kaolinite Al2(OH)4Si2O5
T
O
T
O
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Lizardite Mg3Si2O5(OH)4
Serpentine group
1:1 trioctahedral phyllosilicates
lizardite (serpentine group): 1:1, trioctahedral
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2:1 phyllosilicates
2:1 tetrahedral sheet to octahedral sheet (TOT TOT TOT...)
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Talc – 2:1 trioctahedral
T
O
T
T
O
T
Mica
The micas have some Al3+ substitution for Si4+ in the tetrahedral
sheet. This charge is balanced by alkali or alkaline earth cations
between the TOT layers.
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2:1 phyllosilicates
micas
Muscovite KAl2(OH)2(Si3Al)O10
T
O
T
K
T
O
T
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Margarite CaAl2(Al2Si2O10)(OH)2
Chlorite
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brucite-like layer
O
T
O
phlogopite-like layer
T
Important phyllosilicate minerals
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Serpentine group
In reality, there is considerable mismatch between a brucite
sheet and an undistorted Si2O5 hexagonal rings
Edges of an Mg(OH)6 octahedron in the brucite sheet are
greater than the distances between apical oxygens in the Si2O5
or (Si,Al)2O5 sheet
Antigorite and chrysotile: this misfit is compensated for by a
bending of the tetrahedral sheet
Antigorite:
Continuous bending
Corrugated
Chrysotile:
Continuous bending into cylindrical tubes
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Antigorite and lizardite: massive and fine grained
Chrysotile: fibrous
Asbestos
Health effects of asbestos
Asbestosis (type of pulmonary fibrosis) where air sacs in the
lung become scarred, inflamed, and the tissue hardens
Pleural Mesothelioma – a cancer that affects the lining around
the lungs (Pleura) – can have a 30 year latency before
presenting.
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Serpentine minerals occurrence
(Hydration) alteration product of Mg-rich silicates (e.g.
pyroxene, olivine)
2Mg2SiO4 + 3H2O ↔ Mg3Si2O5(OH)4 + Mg(OH)2
olivine water
serpentine
brucite
Commonly associated with minerals such as magnesite
(MgCO3), chromite (FeCr2O4), magnetite (Fe3O4)
Serpentinite: rocks made up almost entirely of serpentine
Clay mineral group
Clay mineral: a small number of minerals that occur with a
grain size < 2 µm in the largest dimension
Mostly hydrous aluminum silicates with layered structures
1:1 (T-O) or 2:1 (T-O-T) phyllosilicates, some with interlayer
cations
Generally plastic behavior when wet; harden when dried/fired
Highly reactive surfaces, high cation exchange capacities,
catalytic activity
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Clay mineral vs clay
Clay
A rock term indicating that the grain size of the various minerals
comprising the rock is < 2 µm
No compositional implications
Clay mineral
Implications for both size and composition
Kaolinite
Very common. Primary component of kaolin or clay
Forms in sedimentary rocks (low T/P), or by the chemical
weathering of aluminum silicate minerals such as feldspar
Uses
Filler in paper (largest use)
Ceramics, bricks, drain tile, sewer pipes
Toothpaste, cosmetics, etc
Kaolinite mine
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Talc
Secondary mineral from the alteration of Mg-silicates (e.g.
olivine, pyroxenes, and amphiboles)
Characteristically in low-grade metamorphic rocks: massive
form soapstone
Mostly used in powdered form: paint, ceramics, rubber, paper,
cosmetics etc.
Mica group
Dioctahedral micas
Trioctahedral micas
Muscovite, paragonite, margarite
White mica
Biotite, clintonite
Black (or brown) mica
Most common: muscovite, biotite
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Muscovite
Very common in metamorphic rocks, forming the primary
constituent of mica schists
Use:
High dielectric and heat-resisting properties single cleavage plates
(sheet mica) are used as insulating material in the manufacture of
electrical apparatus
Ground mica: wallpapers (shiny luster), lubricant when mixed with
oils, a filler, cosmetics
Biotite
Composition similar to phlogopite but with considerable
substitution of Fe2+ for Mg2+
phlogopite
annite
KMg3AlSi3O10(OH)2
KFe3AlSi3O10(OH)2
Found in a variety of geological environments, e.g. igneous
rocks, metamorphic rocks
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Chlorite group
Chlorite (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6
A common mineral in low grade metamorphic rocks
Recognized in hand specimen by its green color, micaceous
habit and cleavage
Reading for next class
Tectosilicates
Klein p467-482, 534-552
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