Download Sheet Silicates

Sheet Silicates
• Abundant and common minerals
• Throughout upper 20 km of crust
• Felsic to intermediate rocks, metamorphic, and sedimentary rocks
• All are hydrous
- contain H
- bonded to O to form OH• Also called “Phyllosilicates” and “Sheet Silicates”
- all have flaky or platy habit
• Si/O ratio of 2/5
Classification – based on structure
• Two different kinds of “sheets”
- T sheets: tetrahedral layers
- O sheets: octahedral layers
• T and O layers are joined to form sheets
- the sheets are repeated in vertical direction
- the layers between sheets may be vacant or filled with interlayer cations, water, or other
sheets
• Primary characteristic is the basal cleavage
- single perfect cleavage
- occurs because bonds between sheets are very weak
• Octahedral Sheets
- two planes of OH- anionic groups
- cations are two types:
* divalent (Fe2+ or Mg2+)
* trivalent (Al3+ or Fe3+)
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• Divalent cations fill 3 of 3 sites
- form trioctahedral sheets
- ideal formula is Mg3(OH)6
- mineral called brucite (a hydroxide – not silicate)
• Trivalent cations fill 2 of 3 sites
- form dioctahedral sheets
- ideal formula is Al2(OH)6
- mineral called gibbsite (a hydroxide – not silicate)
• Charges balance
- cations = 6+
- anions = 6• Tetrahedral sheets
- sheets of tetrahedrally coordinated cations
- formula represented by Z2O5
- Z usually Si4+, Al3+, less commonly Fe3+
• Tetrahedron are in mesh of 6-fold rings
- three oxygen on each tetrahedron shared by adjacent tetrahedron
- these three are basal oxygen
- the fourth, unshared oxygen is the apical oxygen
• Tetrahedral layers are two oxygen thick
• Tetrahedral sheet composition is Si2O52- may have Al3+ or Fe3+ substitute for Si4+
- increases net negative charge
• Symmetry of rings is hexagonal
- symmetry of sheet silicates close to hexagonal
- depends on arrangement of stacking
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• Tetrahedral and octahedral sheets always joined
- apical oxygen of tetrahedral sheets formed part of octahedral sheets
- oxygen replaces one of the OH- in octahedral sheets
• Sheets joined in two ways:
- TO layers, called 1:1 layers
- TOT layers, called 2:1 layers
• 1:1 layers
- consists of three planes of anions
- one plane is basal plane of shared tetrahedral oxygen
- other side is the OH- anionic group of the octahedral sheet
- middle layer is OH- anionic group with some OH- replaced by oxygen.
• Can build TO layer as follows:
• 2:1 layers
- this layer has tetrahedral layer joined to both sides of octahedral layer
- TOT structure has 4 layers of anions
- both sides (outermost) are planes of basal, shared oxygen
- middle planes contain original OH- from octahedral layers and apical oxygen from
tetrahedron
• Can build TOT layers as follows:
• 1:1 layer silicates
- unit structure is repeating TO layers
- dioctahedral = kaolinite,
Al2Si2O5(OH)4
- trioctahedral = serpentine, Mg3Si2O5(OH)4
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• Bonding between layers are weak
- electrostatic bonds: van der Waals and hydrogen
- means minerals are very soft
- thickness of TO layers around 7Å, so c unit cell dimension around 7 to 14Å
• 2:1 Layer Silicates
- unit structure is repeating TOT layers
- TOT layers can be electrically neutral
- substitution can give TOT layers a net charge
- most common example is substitute Al3+ for Si4+ in tetrahedral layers
•Variety of 2:1 structures depend on
- charge on TOT layers
- what is place in interlayer site to balance charge
• TOT structure
- Si4+ occupies entire structure
- electrically neutral, no interlayer cations
- TOT layers bonded weakly by van der Wall and hydrogen bonds
- Soft (talc = 1) and greasy feel
- TOT layers 9 to 9.5Å thick, this (or twice) is unit cell dimension in c direction
• TOT + c structure
- these are the mica minerals
- also less common are “brittle micas”
- structure is TOT layers with some tetrahedral sites occupied by Si4+
• Micas
- Si/Al ratio in the tetrahedral layers is 1:3
- Dioctahedral TOT layers = Al2(AlSi3O10(OH)21- Trioctahedral TOT layers = Mg3(AlSi3O10(OH)21- negative charge balanced by large monovalent cation, usually K+
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• Dioctahedral mica (muscovite):
• Trioctahedral mica (phlogopite):
• Interlayer cation:
- generally located in center of ring, between apical oxygen
- Bonding is ionic, so micas generally are harder
- Repeat distance is 9.5 to 10Å – larger than TOT structure because of cation
• Brittle micas
- Similar to micas, but more Al3+ substitution
- margarite – half of sites have Al3+ substituted
* Charge balance by Ca2+
- clintonite – 1/3 of sites have Al3+ substitution
* Charge balance by Ca2+
• TOT + O Structures
- most common members are in the chlorite group
- consider structure to be like talc, but with brucite interlayer
- Can build chlorite according to:
• thickness of layers about 14Å, ~10Å for TOT layer and 4Å for O layer
• TOT layers often have net negative charge
- substitution of Al3+ for Si4+
• O layers often have net positive charge
- substitution of Al3+ and Fe3+ for divalent cations
• Net negative charge balances net positive charge
- results in harder minerals than expected
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• Polytypism
- caused by way layers are stacked
• micas are good example
- in TOT layers, one T layer offset from other
- done to accommodate the apical oxygen in the octahedral layer
- the interlayer cation keys the TOT layers together
- polytypes depend on the direction of the offset
TO structures
• Serpentine (var. Antigorite, Chrysotile, Lizardite)
- all are trioctahedral
- for trioctahedral sheets, a = 5.4Å; b = 9.3Å
- for tetrahedral sheets, a = 5.0Å, b = 8.7Å
- mismatch in size needs to be accommodated causing variations
• Occurrence
- hydrothermal alteration of mafic and ultramafic rocks (peridotite and pyroxonite)
- serpentine rich rocks are called serpentinite
- Fe commonly goes to magnetite
• Use – chrysotile is ~98% of world’s asbestos
- insulation
- also brake and clutch facings
• Health risks of asbestos?
- cancer, asbestosis etc.; largely found in chronic exposures
- expensive removal from buildings
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TOT Structures
• Talc – trioctahedral (a common useful mineral)
• Pyrophyllite – dioctahedral (an uncommon mineral)
TOT + c minerals
• Muscovite – dioctahedral
• Biotite – trioctahedral
- very common mineral
- solid solution of Fe and Mg
- K(Fe,Mg)3AlSi3O10(OH)2
- common end member is pure Fe, Phlogopite
• Weathered biotite forms vermiculite
- interlayer K replaced with Ca, Mg, and H20
- water in interlayer site makes useful for keeping plants hydrated
Clay minerals
• Clay has two meanings:
- particles < 0.002 mm
- a group of sheet silicate minerals that are commonly clay-sized
- original description from not being able to identify small grain size material
- now use X-ray diffraction to determine clays
• Problems:
- clay-size fraction can contain other minerals (quartz, carbonates, zeolites etc)
- clay mineral commonly used to define size fraction, but size not mineralogical
definition
- several “clay” minerals can be larger than the size requirement
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• Structure and classification
- divided like the sheet silicates
- 1:1 layer clays
- 2:1 layer clays
- also mixed layer clays with different combinations of components
- TO layers generally around 7Å
- TOT layers generally 10Å or 14Å
• 1:1 clay minerals
- typically kaolinite and serpentine
2:1 clay minerals
- constructed of repeated TOT layers
- have net negative charge, but less than one per formula unit
- require cations to maintain charge balance
- two kinds:
* 10Å where K, Na, or Ca interlayer cation, similar to mica
* 14Å where interlayer is brucite layer, similar to chlorite
• Two types of 10Å clays:
- low charge – smectites
- high charge – illites
- intermediate charge - vermicullite
• Low charge – smectites (swell clays)
- net negative charge is 0.2 to 0.6 per formula unit, typically 0.33
- Ca and Na are typical interlayer ions
- may be dioctahedral or trioctahedral
- charge results from
* Al substitute for Si in tetrahedron
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* Mg for Al in octahedron (in dioctahedral)
• Low charge means water easily moves in and out of interlayer sites
- expands interlayer spacing
- one water layer = 12.5Å
- two water layer – 15.2Å
• water moves in and out at room temperature depending on moisture
- when no water, the interlayers = 10Å
 High charge group – illites
- net negative charge of 0.8 to 1 per formula unit
- mostly substitution of Al3+ for Si4+
- all are dioctahedral
- interlayer ion is K+
- very similar to muscovite, thus called “mica-like”
- relatively high number of K+ means interlayers bonded strongly
- difficult for water molecules to enter interlayers
- non-swelling clays
• intermediate charge – vermiculite
- higher net negative charge than smectites; usually0.6 per formula unit
- comes from oxidation of Fe2+ to Fe3+ in biotite
- decreases the net negative charge
- loses some K+
- addition K+ exchanges for Ca2+ and Mg2+ plus water
- makes it a swell clay
- with water interlayer spacing about 14.4Å
Mixed layer clays
• Natural clays rarely similar to the end-members
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• typically contain parts of different types of clays
• actually “mixtures” at unit cell level – not physical mixtures
• named by combining names – most common illite/smectite, also chlorite/smectite
Geology of clay minerals
• Clays form by hydrous alteration of silicate-mineral bearing rocks
rock + water + dissolved ions = clay + other minerals + ions in solution
• example of k-spar weathering:
• Clay formed in environments at earth’s surface
• most abundant in weathered rock
• also in soils and fine grained clastic sediments
• type of clay formed depends on:
- nature (type) of parent rock
- temperature
- availability and chemistry of water
• once formed clays prone to diagenesis
- e.g. smectite converts to illite with burial
- clays formed by weathering tend to be rich in smectite
- with burial (increase T) smectite converts to illite
- most conversion at 50 to 100º C, over range of 30 to 100º C
- conversion requires K, usually comes from K-spar that’s dissolved
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Identification
• Small size means microscopic techniques useless
• Require chemical pretreatment and X-ray diffractometry
- disaggregate rock (not crush)
- Separate by decanting from liquid
- chemically remove oxides
- saturate clays with interlayer cation (Mg)
- particle size separation by centrifugation
- x-ray slide preparation
- glycolate sample
- measure interlayer spacing
* air dried
* glycolated
* oven dried
Uses
• Paper
• Drilling mud
• ceramics
• filler
• cosmetics
• refractory products
• building products
• Portland cement
• absorbants
• pharmaceuticals
• food
Clays in environment
• Plants use because of water and ion absorption and release
• Swell clays
- altered volcanic ash layers – bentonite
- slope stability problems
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• Environmental protection – land fill liners and caps
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