Sheet SilicatesSheet Silicates Abundant and common minerals Abundant and common minerals

throughout upper 20 km of crustthroughout upper 20 km of crust Felsic to intermediate igneous, Felsic to intermediate igneous,

metamorphic, and sedimentary rocksmetamorphic, and sedimentary rocks All are hydrousAll are hydrous

Contain HContain H Bonded to O to form OH-Bonded to O to form OH-

Z/O ratio of 2/5Z/O ratio of 2/5 2 Major groups: Micas & Clays2 Major groups: Micas & Clays

GroupingsGroupings

Based on structureBased on structure Two kinds of “layers” within the Two kinds of “layers” within the

“sheets”“sheets” ““T” layers – T” layers – tetrahedral layerstetrahedral layers

Tetrahedral coordination of Si and AlTetrahedral coordination of Si and Al ““O” sheets – O” sheets – octahedral layersoctahedral layers

Octahedral coordination of mostly Al and Octahedral coordination of mostly Al and Mg, occasionally FeMg, occasionally Fe

T and O layers bonded to form sheetsT and O layers bonded to form sheets The sheets are repeated in vertical directionThe sheets are repeated in vertical direction The spaces between the sheets may be:The spaces between the sheets may be:

VacantVacant Filled with interlayer cations, water, or other sheetsFilled with interlayer cations, water, or other sheets

Primary characteristic - Primary characteristic - basal cleavagebasal cleavage Single perfect cleavageSingle perfect cleavage Occurs because bonds between sheets are Occurs because bonds between sheets are

very weakvery weak

Construction of T-O-T Construction of T-O-T SheetsSheets

Octahedral layers:Octahedral layers: Two planes of OHTwo planes of OH-- anionic groups anionic groups Cations are two types:Cations are two types:

Divalent (FeDivalent (Fe2+2+ or Mg or Mg2+2+)) Trivalent (AlTrivalent (Al3+3+ or Fe or Fe3+3+)) Mg and Al most commonMg and Al most common

DivalentDivalent cations fill 3 of 3 sites cations fill 3 of 3 sites Form Form trioctahedraltrioctahedral sheets sheets Ideal formula is MgIdeal formula is Mg33(OH)(OH)66

This formula is bruciteThis formula is brucite A hydroxide, not a silicate mineralA hydroxide, not a silicate mineral

All sites All sites filled with filled with divalent divalent cationscations

Charge Charge neutralneutral

TrivalentTrivalent cations fill 2 of 3 sites cations fill 2 of 3 sites Form Form dioctrahedraldioctrahedral sheets sheets Ideal formula is AlIdeal formula is Al22(OH)(OH)66

Mineral called gibbsiteMineral called gibbsite A hydroxide, not silicate mineralA hydroxide, not silicate mineral

2/3 of sites 2/3 of sites filled with filled with trivalent trivalent cationscations

Charge Charge neutralneutral

Tetrahedral sheetsTetrahedral sheets Sheets of tetrahedrally coordinated Sheets of tetrahedrally coordinated

cationscations Formula represented by ZFormula represented by Z22OO55: Z/O = 2/5: Z/O = 2/5 Z usually SiZ usually Si4+4+, Al, Al3+3+, less commonly Fe, less commonly Fe3+3+

• Symmetry of rings is Symmetry of rings is hexagonalhexagonal• Symmetry of sheet Symmetry of sheet

silicates is close to silicates is close to hexagonalhexagonal

• Depends on Depends on arrangement of arrangement of stackingstacking

Fig. 11-2Fig. 11-2

Tetrahedron are meshes of 6-fold Tetrahedron are meshes of 6-fold ringsrings Three basal oxygen on each tetrahedron Three basal oxygen on each tetrahedron

shared by adjacent tetrahedronshared by adjacent tetrahedron The fourth, unshared oxygen is the The fourth, unshared oxygen is the

apical oxygenapical oxygen Tetrahedral layers are two oxygen Tetrahedral layers are two oxygen

thickthick

Fig. 13-1Fig. 13-1

•Tetrahedral sheet composition is SiTetrahedral sheet composition is Si22OO552-2-

•May have AlMay have Al3+3+ or Fe or Fe3+3+ substitute for Si substitute for Si4+4+

•Increases net negative chargeIncreases net negative charge

Tetrahedral and octahedral sheets Tetrahedral and octahedral sheets always joinedalways joined Apical oxygen of tetrahedral sheets formed Apical oxygen of tetrahedral sheets formed

part of octahedral sheetspart of octahedral sheets Apical oxygen replaces one of the OH- in the Apical oxygen replaces one of the OH- in the

octahedral sheetsoctahedral sheets Sheets joined in two waysSheets joined in two ways

TOTO layers, called layers, called 1:11:1 layer silicates layer silicates TOTTOT layers, called layers, called 2:12:1 layer silicates layer silicates

T layer on topOH in middle of rings

Al3+ (dioctahedral) orMg2+ (trioctahedral)

Basal Oxygen

(an example of 1:1 layer type)Fig. 13.1

1:1 layer summary1:1 layer summary

Consists of Consists of 3 planes of anions3 planes of anions One plane is basal plane of shared One plane is basal plane of shared

tetrahedral oxygentetrahedral oxygen Other side is the OH- anionic group of Other side is the OH- anionic group of

the octahedral sheetthe octahedral sheet Middle layer is the OH- anionic group Middle layer is the OH- anionic group

with some OH- replaced by oxygenwith some OH- replaced by oxygen

1:1 layering in 1:1 layering in phyllosilicatesphyllosilicates

OH- onlyOH- + oxygen

Oxygen only

Al2Si2O5(OH)4 = kaolinite, dioctahedral 1:1 sheet silicateMg3Si2O5(OH)4 = serpentine, trioctahedral, 1:1 sheet silicate

2:1 layer silicates2:1 layer silicates

2 tetrahedral layers on both sides of 2 tetrahedral layers on both sides of octahedral layeroctahedral layer

TOT structure has TOT structure has 4 layers of anions4 layers of anions Both sides (outermost) are planes of Both sides (outermost) are planes of

basal, shared oxygenbasal, shared oxygen Middle planes contain original OH- Middle planes contain original OH-

from octahedral layers and apical from octahedral layers and apical oxygen from tetrahedronoxygen from tetrahedron

2:1 layering in 2:1 layering in phyllosilicatesphyllosilicates

Oxygen onlyOH- + oxygen

Oxygen onlyOH- + oxygen

Al2Si4O10(OH)2 = Pyrophyllite, dioctahedral 2:1 sheet silicateMg3Si4O10(OH)2 = Talc, trioctahedral 2:1 sheet silicate

How are sheets stacked?How are sheets stacked?I.I. 1:1 layer1:1 layer

I.I. ……T-O…T-O…T-O…T-O…T-O…T-O…

II.II. 2:1 layer2:1 layerI.I. ……T-O-T…T-O-T…T-O-T…T-O-T…T-O-T…T-O-T…

II.II. c…T-O-T…c…T-O-T…c…T-O-T…c…c…T-O-T…c…T-O-T…c…T-O-T…c…

III.III. O…T-O-T…O…T-O-T…OO…T-O-T…O…T-O-T…O

Four types of layers, each dioctahedral or Four types of layers, each dioctahedral or trioctahedraltrioctahedral

Each one may be dioctahedral or trioctahedralEach one may be dioctahedral or trioctahedral

Kaolinite (dioctahedral)Serpentine (trioctahedral)

Pyrophyllite (dioctahedral)Talc (trioctahedral)

1:1 layer silicates1:1 layer silicates

Kaolinite and SerpentineKaolinite and Serpentine Bonding between sheets very weakBonding between sheets very weak

Electrostatic bonds – van der Waals and Electrostatic bonds – van der Waals and hydrogenhydrogen

Results in very soft mineralsResults in very soft minerals Thickness of TO layers around 7 ÅThickness of TO layers around 7 Å

C unit cell dimension about 7 Å

Fig. 13.3

2:1 layer silicates2:1 layer silicates Unit structure is repeating TOT layers, two Unit structure is repeating TOT layers, two

ways:ways: (1) TOT layers can be (1) TOT layers can be electrically neutralelectrically neutral

Nothing in between sheets van der Waal forcesNothing in between sheets van der Waal forces Pyrophyllite & TalcPyrophyllite & Talc

(2) substitution in TOT layers can give a (2) substitution in TOT layers can give a net net chargecharge

Most common substitution is AlMost common substitution is Al3+3+ for Si for Si4+4+ in tetrahedral in tetrahedral layerslayers

Charge balance maintained with substitution between Charge balance maintained with substitution between the sheetsthe sheets

TOT structureTOT structure If there is only SiIf there is only Si4+4+ in T layers (no Al in T layers (no Al3+3+ or or

FeFe3+3+)) Electrically neutral, no interlayer cationsElectrically neutral, no interlayer cations TOT layers weakly bonded by van der TOT layers weakly bonded by van der

Waal and hydrogen bondsWaal and hydrogen bonds Soft (Talc), greasy feelSoft (Talc), greasy feel

C unit cell dimension about 9 to 9.5 Å Nothing in

interlayer site

Fig. 13.3

c…T-O-T…c…T-O-T…cc…T-O-T…c…T-O-T…c These are the These are the mica mineralsmica minerals Also less common are “Also less common are “brittle micasbrittle micas”” Structure is TOT layers with 1 out of 4 Structure is TOT layers with 1 out of 4

tetrahedral sites occupied by Altetrahedral sites occupied by Al3+3+

MicasMicas Al/Si ratio in the tetrahedral layer is 1/3Al/Si ratio in the tetrahedral layer is 1/3 Dioctahedral TOT layer = AlDioctahedral TOT layer = Al22((AlSiAlSi33OO1010)(OH))(OH)22

1-1-

Remember Pyrophyllite: Al(SiRemember Pyrophyllite: Al(Si22OO55)(OH))(OH)22

Trioctahedral TOT layer = MgTrioctahedral TOT layer = Mg33((AlSiAlSi33OO1010)(OH))(OH)221-1-

Remember Talc: MgRemember Talc: Mg33(Si(Si22OO55)(OH))(OH)22

Negative charge balance by large monovalent Negative charge balance by large monovalent cation, usually cation, usually KK++

Bonds are ionic, fairly strong, harder mineralsBonds are ionic, fairly strong, harder minerals

C unit cell C unit cell dimension dimension about 9.5 about 9.5 to 10 Åto 10 Å

KK++ in in interlayer interlayer sitesite

Dioctahedral mica – muscoviteDioctahedral mica – muscovite KKAlAl22(AlSi(AlSi33OO1010)(OH))(OH)22

Trioctahedral mica – PhlogopiteTrioctahedral mica – Phlogopite KKMgMg33(AlSi(AlSi33OO1010)(OH))(OH)22

Brittle MicasBrittle Micas

Similar to micas, but more AlSimilar to micas, but more Al3+3+ substitutionsubstitution

Charge balanced by CaCharge balanced by Ca2+2+

MargariteMargarite – half of tetrahedral sites – half of tetrahedral sites have Alhave Al3+3+ substitution substitution

ClintoniteClintonite – ¾ of tetrahedral sites – ¾ of tetrahedral sites have Alhave Al3+3+ substitution substitution

MargariteMargarite DioctahedralDioctahedral CaCaAlAl22((AlAl22SiSi22OO1010)(OH))(OH)44

ClintoniteClintonite TrioctahedralTrioctahedral CaCaMgMg22AlAl((AlAl33SiSiOO1010)(OH))(OH)22

Now charge balance in part from Al Now charge balance in part from Al substitution in octahedral layersubstitution in octahedral layer

……O…T-O-T…O…T-O-T…O…O…T-O-T…O…T-O-T…O… Most common members are in the chlorite groupMost common members are in the chlorite group Structure like Talc, but with brucite (Structure like Talc, but with brucite (MgMg33(OH)(OH)66) )

interlayerinterlayer T layers with small negative chargeT layers with small negative charge

Substitute small amounts of AlSubstitute small amounts of Al3+3+ for Si for Si4+4+

O layers often have net positive chargeO layers often have net positive charge Substitute Substitute AlAl3+3+ or or FeFe3+3+ for divalent cations for divalent cations

Minerals harder than expectedMinerals harder than expected

C unit cell dimension about 14 Å

TOT layers have slight negative charge, substitute Al3+ for Si4+

O layers often have net positive charge

Fig. 13.3

Some Al3+ for Si4+

Some Al3+ for Mg2+

Varieties of sheet silicatesVarieties of sheet silicates

TO structuresTO structures Serpentine (var. Antigorite, Chrysotile, Serpentine (var. Antigorite, Chrysotile,

Lizardite)Lizardite) All are trioctahedralAll are trioctahedral Trioctahedral sheets, Trioctahedral sheets, a = 5.4 Å; b = 9.3 a = 5.4 Å; b = 9.3

ÅÅ Tetrahedral sheets, Tetrahedral sheets, a = 5 Å; b = 8.7 Åa = 5 Å; b = 8.7 Å Mismatched size leads to variationsMismatched size leads to variations

Fig. 13-5Fig. 13-5

Chrysotile (curved tubes)

Antigorite (reversed direction)

Lizardite (distorted tetrahedral mesh)

Clay MineralsClay Minerals

Clay has two meanings:Clay has two meanings: Particles < 1/256 mm, or 0.0039 mmParticles < 1/256 mm, or 0.0039 mm A group of sheet silicate minerals (not A group of sheet silicate minerals (not

micas) that are commonly clay-sizedmicas) that are commonly clay-sized Original description from not being able Original description from not being able

to identify small grain size materialto identify small grain size material Now can use X-ray diffraction to Now can use X-ray diffraction to

determine claysdetermine clays

TerminologyTerminology ClayClay: Sediment composed of particles that : Sediment composed of particles that

are < 0.002 mmare < 0.002 mm ClaystoneClaystone: Rock composed of clay-sized : Rock composed of clay-sized

particlesparticles Clay mineralsClay minerals: 1:1 and 2:1 Phyllosilicate : 1:1 and 2:1 Phyllosilicate

minerals without Kminerals without K++ or Ca or Ca2+ 2+ bonding sheets bonding sheets (those are Micas)(those are Micas)

ArgillaceousArgillaceous: Rock or sediment containing : Rock or sediment containing large amounts of clay and clay mineralslarge amounts of clay and clay minerals

ProblemsProblems Clay size fraction can contain other Clay size fraction can contain other

minerals (quartz, carbonates, zeolites minerals (quartz, carbonates, zeolites etc.)etc.)

““Clay” used to define size fraction – size Clay” used to define size fraction – size not mineralogicalnot mineralogical

Several clay minerals can be larger than Several clay minerals can be larger than the size requirements the size requirements

Clay classificationClay classification

1:1 layer clays1:1 layer clays 7 Å type7 Å type TO layersTO layers

Kaolinite (dioctahedral)Kaolinite (dioctahedral) Serpentine (trioctahedral)Serpentine (trioctahedral)

2:1 layer clays:2:1 layer clays: End members:End members:

10 Å – Pyrophyllite (dioctahedral) & talc 10 Å – Pyrophyllite (dioctahedral) & talc (trioctahedral)(trioctahedral)

10 Å – Charge imbalance: with K as 10 Å – Charge imbalance: with K as interlayer: Mica: Muscovite (trioctahedral) & interlayer: Mica: Muscovite (trioctahedral) & Biotite (dioctahedral) Biotite (dioctahedral)

14 Å type (Chlorite)14 Å type (Chlorite)

Intermediate 2:1 claysIntermediate 2:1 clays Have net negative charge, but less than Have net negative charge, but less than

one per formulaone per formula Requires less interlayer cations to Requires less interlayer cations to

charge balancecharge balance Mixed layer clays – combined 1:1 and Mixed layer clays – combined 1:1 and

1:21:2

Three types of intermediate 10 Å claysThree types of intermediate 10 Å clays Low charge imbalance – Low charge imbalance – smectitesmectite clays clays High charge imbalance – High charge imbalance – illiteillite clays clays Intermediate charge imbalance – Intermediate charge imbalance – vermiculitevermiculite

Charge imbalance controlled byCharge imbalance controlled by ““interlayer” cationsinterlayer” cations They move in and out – Cation Exchange They move in and out – Cation Exchange

Capacity (CEC)Capacity (CEC) Surface adsorptionSurface adsorption

Low chargeLow charge SmectiteSmectite approximately = Caapproximately = Ca0.170.17(Al,Mg,Fe)(Al,Mg,Fe)22(Si,Al)(Si,Al)44OO1010(OH)(OH)22•nH•nH22OO Net negative charge is 0.2 to 0.6 per formula unit, Net negative charge is 0.2 to 0.6 per formula unit,

typically 0.33typically 0.33 Ca and Na are typical interlayer ionsCa and Na are typical interlayer ions

ExchangableExchangable May be dioctahedral or trioctahedralMay be dioctahedral or trioctahedral Charge results from Charge results from

Al substitution for Si in tetrahedronAl substitution for Si in tetrahedron Mg for Al in octahedron (in dioctahedral)Mg for Al in octahedron (in dioctahedral)

Low charge means water and cations Low charge means water and cations (Na, K, Ca, Mg) easily move in and (Na, K, Ca, Mg) easily move in and out of interlayer sitesout of interlayer sites No water = 10 ÅNo water = 10 Å One water layer = 12.5 ÅOne water layer = 12.5 Å Two water layer = 15.2 ÅTwo water layer = 15.2 Å

Water moves in and out depending Water moves in and out depending on moisture in environmenton moisture in environment

High chargeHigh charge Illite/glauconiteIllite/glauconite

Approximately = KApproximately = K0.80.8AlAl22(Al(Al0.80.8SiSi3.23.2)(OH))(OH)22

Net negative charge of 0.8 to 1 per formulaNet negative charge of 0.8 to 1 per formula Very similar to muscovite – called mica-likeVery similar to muscovite – called mica-like Mostly substitute of AlMostly substitute of Al3+3+ for Si for Si4+4+

All are dioctahedral; Glauconite has FeAll are dioctahedral; Glauconite has Fe3+3+

Interlayer ion is KInterlayer ion is K++

High K concentration means strong bondHigh K concentration means strong bond Difficult for water to enterDifficult for water to enter Non-swelling clayNon-swelling clay

Intermediate chargeIntermediate charge VermiculiteVermiculite

Approximately = Approximately = (Mg,Ca)(Mg,Ca)0.30.3(Mg,Fe(Mg,Fe2+2+,Fe,Fe3+3+,Al),Al)33(Si,Al)(Si,Al)44)O)O1010(OH)(OH)22

About 0.6 charge per formula unitAbout 0.6 charge per formula unit Comes from oxidation of FeComes from oxidation of Fe2+2+ to Fe to Fe3+3+ in biotite in biotite

Reduces the negative charge on TOT layer from -1 to -0.6Reduces the negative charge on TOT layer from -1 to -0.6 Less KLess K++ than mica, can exchange for Ca than mica, can exchange for Ca2+2+ and Mg and Mg2+2+

and waterand water Swell claySwell clay With water interlayer spacing is 14.4 Å With water interlayer spacing is 14.4 Å

Mixed layer claysMixed layer clays

Natural clays rarely similar to the end Natural clays rarely similar to the end membersmembers

Typically contain parts of different Typically contain parts of different types of claystypes of clays

Actually mixtures at unit cell level – Actually mixtures at unit cell level – not physical mixturesnot physical mixtures

Nomenclature – combined namesNomenclature – combined names Illite/smectite or chlorite/smectiteIllite/smectite or chlorite/smectite

1:1 layer clays

2:1 layer clays – low charge, smectite2:1 layer clays – high charge, illite

2:1 layer clays – Chlorite gp

Mixed layer

7 Å

10 Å

14 Å

Figure 13-15

Burial DiagenesisBurial Diagenesis

Smectite converts to illite with burialSmectite converts to illite with burial Most conversion at 50 to 100º CMost conversion at 50 to 100º C Conversion requires K, usually comes Conversion requires K, usually comes

from dissolution of K sparfrom dissolution of K spar

Mineralogy of Miocene/Oligocene sediments Gulf Coast

1. K-spar dissolves2. Smectite converts to illite

(with extra K)3. Releases interlayer water4. Increase pore pressures5. T corresponds to “oil

window”6. Forces oil from pore spaces

into reservoirs

Figure 13-16