Sheet Silicates Abundant and common minerals throughout upper 20 km of crust Felsic to intermediate igneous, metamorphic, and sedimentary rocks All are hydrous Contain H Bonded to O to form OH- Z/O ratio of 2/5 2 Major groups: Micas & Clays

Groupings Based on structure Two kinds of “layers” within the “sheets” “T” layers – tetrahedral layers Tetrahedral coordination of Si and Al “O” sheets – octahedral layers Octahedral coordination of mostly Al and Mg, occasionally Fe

T and O layers join to form sheets The sheets are repeated in vertical direction The spaces between the sheets may be: Vacant Filled with interlayer cations, water, or other sheets Primary characteristic - 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+)

Divalent cations fill 3 of 3 sites Form trioctahedral sheets Ideal formula is Mg3(OH)6 This formula is brucite A hydroxide, not a silicate mineral All sites filled with divalent cations Charge neutral

Trivalent cations fill 2 of 3 sites Form dioctrahedral sheets Ideal formula is Al2(OH)6 Mineral called gibbsite A hydroxide, not silicate mineral 2/3 of sites filled with trivalent cations Charge neutral

Tetrahedral sheets Sheets of tetrahedrally coordinated cations Formula represented by Z2O5: Z/O = 2/5 Z usually Si4+, Al3+, less commonly Fe3+ Symmetry of rings is hexagonal Symmetry of sheet silicates is close to hexagonal Depends on arrangement of stacking Fig. 11-2

Tetrahedron are meshes of 6-fold rings Three basal oxygen on each tetrahedron shared by adjacent tetrahedron 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 Fig. 13-1

Tetrahedral and octahedral sheets always joined Apical oxygen of tetrahedral sheets formed part of octahedral sheets Apical oxygen replaces one of the OH- in the octahedral sheets Sheets joined in two ways TO layers, called 1:1 layer silicates TOT layers, called 2:1 layer silicates

Al3+ (dioctahedral) or Mg2+ (trioctahedral) OH in middle of rings Basal Oxygen T layer on top (an example of 1:1 layer type)

1:1 layer summary Consists of 3 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 the OH- anionic group with some OH- replaced by oxygen

OH- only OH- + oxygen Oxygen only Al2Si2O5(OH)4 = kaolinite, dioctahedral 1:1 sheet silicate Mg3Si2O5(OH)4 = serpentine, trioctahedral, 1:1 sheet silicate

2:1 layer silicates 2 tetrahedral layers on 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

Oxygen only OH- + oxygen OH- + oxygen Oxygen only Al2Si4O10(OH)2 = Pyrophyllite, dioctahedral 2:1 sheet silicate Mg3Si4O10(OH)2 = Talc, trioctahedral 2:1 sheet silicate

How are layers stacked? 1:1 layer 2:1 layer …T-O…T-O…T-O… 2:1 layer …T-O-T…T-O-T…T-O-T… c…T-O-T…c…T-O-T…c…T-O-T…c… O…T-O-T…O…T-O-T…O Four types of layers, each dioctahedral or trioctahedral

1:1 layer silicates Kaolinite and Serpentine Bonding between layers very weak Electrostatic bonds – van der Waals and hydrogen Results in very soft minerals Thickness of TO layers around 7 Å

C unit cell dimension about 7 Å

2:1 layer silicates Unit structure is repeating TOT layers, two ways: (1) TOT layers can be electrically neutral (2) substitution in TOT layers can give a net charge Most common substitution is Al3+ for Si4+ in tetrahedral layers

TOT structure Only Si4+ in T layers (no Al3+ or Fe3+) Electrically neutral, no interlayer cations TOT layers weakly bonded by van der Wall and hydrogen bonds Soft (Talc), greasy feel

C unit cell dimension about 9 to 9.5 Å Nothing in interlayer site

c…T-O-T…c…T-O-T…c These are the mica minerals Also less common are “brittle micas” Structure is TOT layers with some tetrahedral sites occupied by Al3+

Micas Al/Si ratio in the tetrahedral layer is 1/3 Dioctahedral TOT layer = Al2(AlSi3O10)(OH)21- Trioctahedral TOT layer = Mg3(AlSi3O10)(OH)21- Negative charge balance by large monovalent cation, usually K+ Bonds are ionic, fairly strong, harder minerals

C unit cell dimension about 9.5 to 10 Å K+ in interlayer site

Dioctahedral mica – muscovite KAl2(AlSi3O10)(OH)2 Trioctahedral mica – Phlogopite KMg3(AlSi3O10)(OH)2

Brittle Micas Similar to micas, but more Al3+ substitution Charge balanced by Ca2+ Margarite – half of tetrahedral sites have Al3+ substitution Clintonite – ¾ of tetrahedral sites have Al3+ substitution

Margarite Clintonite Dioctahedral CaAl2(Al2Si2O10)(OH)4 Trioctahedral CaMg2Al(Al3SiO10)(OH)2

T layers with small negative charge …O…T-O-T…O…T-O-T…O… Most common members are in the chlorite group Structure like Talc, but with brucite (Mg3(OH)6) interlayer T layers with small negative charge Substitute small amounts of Al3+ for Si4+ O layers often have net positive charge Substitute Al3+ or Fe3+ for divalent cations Minerals 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

Varieties of sheet silicates TO structures Serpentine (var. Antigorite, Chrysotile, Lizardite) All are trioctahedral Trioctahedral sheets, a = 5.4 Å; b = 9.3 Å Tetrahedral sheets, a = 5 Å; b = 8.7 Å Mismatched size leads to variations

Chrysotile (curved tubes) Antigorite (reversed direction) Lizardite (distorted tetrahedral mesh) Fig. 13-5

Clay Minerals Clay has two meanings: Particles < 1/256 mm, or 0.0039 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 can use X-ray diffraction to determine clays

Problems Clay size fraction can contain other minerals (quartz, carbonates, zeolites etc.) Clay minerals used to define size fraction – size not mineralogical Several clay minerals can be larger than the size requirements

Clay classification 1:1 layer clays 7 Å type, TO layers 2:1 layer clays – 10 Å or 14 Å type Have net negative charge, but less than one per formula Requires less interlayer cations to charge balance Mixed layer clays – combined 1:1 and 1:2

Charge imbalance controlled by Three types of 10 Å clays Low charge imbalance – smectite clays High charge imbalance – illite clays Intermediate charge imbalance – vermiculite Charge imbalance controlled by “interlayer” cations They move in and out – Cation Exchange Capacity (CEC) Surface adsorption

Low charge Smectite 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 substitution for Si in tetrahedron Mg for Al in octahedron (in dioctahedral)

Water moves in and out depending on moisture in environment Low charge means water and cations (Na, K, Ca, Mg) easily move in and out of interlayer sites No water = 10 Å One water layer = 12.5 Å Two water layer = 15.2 Å Water moves in and out depending on moisture in environment

High charge Illite/glauconite Net negative charge of 0.8 to 1 per formula Almost mica Mostly substitute of Al3+ for Si4+ All are dioctahedral Interlayer ion is K+ Very similar to muscovite – called mica-like High K concentration means strong bond Difficult for water to enter non=swelling clay

Intermediate charge Vermiculite About 0.6 charge per formula unit Comes from oxidation of Fe2+ to Fe3+ in biotite Reduces the negative charge on TOT layer from -1 to -0.6 Less K+ than mica, can exchange for Ca2+ and Mg2+ and water Swell clay With water interlayer spacing is 14.4 Å

Mixed layer clays Natural clays rarely similar to the end members Typically contain parts of different types of clays Actually mixtures at unit cell level – not physical mixtures Nomenclature – combined names Illite/smectite or chlorite/smectite

7 Å 1:1 layer clays 2:1 layer clays – low charge, smectite 10 Å 2:1 layer clays – high charge, illite 2:1 layer clays – Chlorite gp 14 Å Mixed layer

Burial Diagenesis Smectite converts to illite with burial Most conversion at 50 to 100 C Conversion requires K, usually comes from dissolution of K spar

Mineralogy of Miocene/Oligocene sediments Gulf Coast Release structural water of smectite; corresponds to “oil window”