Chapter 27 The Phyllosilicates N. MacDonald

Outline Introduction Introduction Phyllosilicates Phyllosilicates Basic structural units Basic structural units Structure and chemistry of: Structure and chemistry of: Micas Micas Chlorites Chlorites Clay minerals Clay minerals Other sheet silicates Other sheet silicates

Introduction Phyllosilicates (Sheet silicates) Sheets consists of tetrahedral (T) and octahedral (O) sheets : Sheets consists of tetrahedral (T) and octahedral (O) sheets : T: Sheets of SiO 4 tetrahedrons - all in same orientation T: Sheets of SiO 4 tetrahedrons - all in same orientation O: Sheets of octahedrons sharing O 2- anions; main octahedral cations are O: Sheets of octahedrons sharing O 2- anions; main octahedral cations are Mg 2+ (brucite), Al 3+ (gibbsite), Fe 2+, Fe 3+ Mg 2+ (brucite), Al 3+ (gibbsite), Fe 2+, Fe 3+ Two dimensional (planar) structure forms hexagonal network Two dimensional (planar) structure forms hexagonal network

Basic Structural units Consist of two distinct structural units. Consist of two distinct structural units nm aluminium or magnesium hydroxyl or oxygen Aluminium Octahedron 0.26 nm oxygen silicon Silica tetrahedron Neutral sheets bonded weak dipolar & vd Waals forces.

Basic Structural units

The octahedral layer can be: The octahedral layer can be: Dioctahedral Dioctahedral Every third octahedral space unoccupied Every third octahedral space unoccupied Trivalent cations (Al 3+, Fe 3+ ) occupy octahedral spaces – every third space vacant to maintain charge balance Trivalent cations (Al 3+, Fe 3+ ) occupy octahedral spaces – every third space vacant to maintain charge balance Real structure: octahedra distorted; tetrahedra rotated relative to idealized structure Real structure: octahedra distorted; tetrahedra rotated relative to idealized structure Trioctahedral Trioctahedral All 3 octahedral spaces occupied All 3 octahedral spaces occupied Divalent cations (Mg 2+, Fe 2+ ) occupy every octahedral space Divalent cations (Mg 2+, Fe 2+ ) occupy every octahedral space More symmetrical than dioctahedral micas More symmetrical than dioctahedral micas

Structure and chemistry: General sheet silicates Serpentine Mg 3 Si 2 O 5 (OH) 4 Serpentine Mg 3 Si 2 O 5 (OH) 4 Talc Mg 3 Si 4 O 10 (OH) 2 Talc Mg 3 Si 4 O 10 (OH) 2 Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Basis for most sheet silicate structures Basis for most sheet silicate structures

Serpentine Mg 3 Si 2 O 5 (OH) 4 Antigorite, chrysotile and lizardite Antigorite, chrysotile and lizardite Consists of tetrahedral layer and Mg-octahedral layer called the brucite layer Consists of tetrahedral layer and Mg-octahedral layer called the brucite layer Basis for structure of double-layer clay minerals Basis for structure of double-layer clay minerals

Talc Mg 3 Si 4 O 10 (OH) 2 Trioctahedral; TOT Trioctahedral; TOT Consists of 2 tetrahedral layers separated by a brucite layer Consists of 2 tetrahedral layers separated by a brucite layer Basis for structure of: Basis for structure of: trioctahedral micas – no interlayer trioctahedral micas – no interlayer Triple layer clay minerals Triple layer clay minerals

Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Dioctahedral; TOT Dioctahedral; TOT Consists of 2 tetrahedral layers separated by an Al- octahedral layer called the gibbsite layer Consists of 2 tetrahedral layers separated by an Al- octahedral layer called the gibbsite layer Basis for structure of dioctahedral micas – no interlayer Basis for structure of dioctahedral micas – no interlayer

Structure and chemistry: Micas Stacking of two T-O-T units by means of an interlayer Stacking of two T-O-T units by means of an interlayer Part of tetrahedral Si 4+ replaced by Al 3+ ; large Na +, K +, Ca 2+ incorporated to maintain charge balance Part of tetrahedral Si 4+ replaced by Al 3+ ; large Na +, K +, Ca 2+ incorporated to maintain charge balance Large cations in cuboctahedrons: Large cations in cuboctahedrons: eg.: 1 K + : 12 O 2- - coordination number of 12 eg.: 1 K + : 12 O 2- - coordination number of 12 This is the ideal close-packed coordination number for ion-pairs with similar radii This is the ideal close-packed coordination number for ion-pairs with similar radii

Important dioctahedral micas Ordinary: Ordinary: Muscovite KAl 2 Si 3 AlO 10 (OH) 2 Muscovite KAl 2 Si 3 AlO 10 (OH) 2 Paragonite NaAl 2 Si 3 AlO 10 (OH) 2 Paragonite NaAl 2 Si 3 AlO 10 (OH) 2 Interlayer-deficient Interlayer-deficient (Pyrophyllite)No interlayer at all (Pyrophyllite)No interlayer at all Glauconite Glauconite K 0.8 (Fe Mg 0.67 )(Si 3.87 Al 0.13 )O 10 (OH) 2 Brittle Brittle Margarite CaAl 2 Si 2 Al 2 O 10 (OH) 2 Margarite CaAl 2 Si 2 Al 2 O 10 (OH) 2

Important dioctahedral micas Muscovite Muscovite Paragonite Paragonite Glauconite Glauconite

Important trioctahedral micas Ordinary Ordinary ‘Biotite’K(Mg,Fe 2+,Al) 3 (Si,Al) 3 (Al,Fe 3+ )O 10 (OH) 2 ‘Biotite’K(Mg,Fe 2+,Al) 3 (Si,Al) 3 (Al,Fe 3+ )O 10 (OH) 2 Phlogopite Phlogopite Annite Annite Siderophyllite Siderophyllite Eastonite Eastonite ‘Zinnwaldite’ ‘Zinnwaldite’ K(Fe 2+,Al,Li)Si 2 (Al,Si)O 10 F 2 ‘Lepidolite’ ‘Lepidolite’ PolylithioniteKLi 2 AlSi 4 O 10 F 2 PolylithioniteKLi 2 AlSi 4 O 10 F 2 TrilithioniteK(Li, Al) 3 (Si,Al) 4 O 10 (OH) 2 TrilithioniteK(Li, Al) 3 (Si,Al) 4 O 10 (OH) 2 Brittle: Brittle: Clintonite CaMg 2 AlSiAl 3 O 10 (OH) 2 Clintonite CaMg 2 AlSiAl 3 O 10 (OH) 2

Important trioctahedral micas ‘Biotite’ ‘Biotite’ ‘Zinnwaldite’ ‘Zinnwaldite’ ‘ Lepidolite’ ‘ Lepidolite’

Structure and chemistry: Chlorites Trioctrahedral sheet silicates Trioctrahedral sheet silicates TOT-brucite-TOT: TOT-brucite-TOT: Brucite layer replaces large cations in interlayers of dioctahedral micas Brucite layer replaces large cations in interlayers of dioctahedral micas Two major members: clinochloreMg-richGreen Two major members: clinochloreMg-richGreen chamositeFe-richBrown Low T alteration of olivine, pyroxenes, hornblendes Low T alteration of olivine, pyroxenes, hornblendes (serpentine, talc and brucite also forms during alteration of above minerals)

Clay minerals: Introduction Hydrous aluminium phyllosilicates. Hydrous aluminium phyllosilicates. Contains variable amounts of iron, water, magnesium, alkali metals and other cations. Contains variable amounts of iron, water, magnesium, alkali metals and other cations. Structures similar to micas thus they have flat hexagonal sheets. Structures similar to micas thus they have flat hexagonal sheets. Common in fine grained sedimentary rocks and metamorpic rocks- shale, mudstone, siltstone, slate and phyllite. Common in fine grained sedimentary rocks and metamorpic rocks- shale, mudstone, siltstone, slate and phyllite.

Clay Minerals: Introduction Specific surface & ion exchange capacities Specific surface & ion exchange capacities Variety of applications Variety of applications Difficult to study: size & composition Difficult to study: size & composition Gibbsite-dioctahedral-Al 2 (OH) 6 Gibbsite-dioctahedral-Al 2 (OH) 6 Brucite-trioctahedral-Mg 3 (OH) 6 Brucite-trioctahedral-Mg 3 (OH) 6 Composition varies Composition varies Crystalline, amorphous, platy or acicular Crystalline, amorphous, platy or acicular

Structure and chemistry: Clay minerals

Double-layer clay minerals – serpentine-type structure Kaolinite group

Kaolinite Gibbsite & single tetrahedron layer Gibbsite & single tetrahedron layer Not expand hydroxyl position Not expand hydroxyl position Six-sided little flakes Six-sided little flakes Ceramic Ceramic

Triple-layer clay minerals – talc-type structure Montmorillonite group Illite

Montmorillonite group (Smectites ) Dioctahedral & trioctahedral Dioctahedral & trioctahedral Bonds are weak Bonds are weak High Si & Mg High Si & Mg Brucite inter-layer replaced by: Brucite inter-layer replaced by: water & exchangeable cations Ideal endmembers: Ideal endmembers: Saponite Saponite Beidellite Beidellite Nontronite Nontronite

Illite Non-expanding, dioctahedral clay minerals Non-expanding, dioctahedral clay minerals Unit: silica tetrahedral sheets; central octahedral sheet Unit: silica tetrahedral sheets; central octahedral sheet More Si, Mg, Fe & water than muscovite More Si, Mg, Fe & water than muscovite Less tetrahedral Al & interlayer K than muscovite Less tetrahedral Al & interlayer K than muscovite

Vermiculite Mg-vermiculite resembles talc Mg-vermiculite resembles talc Separated by water molecules Separated by water molecules Arranged in distorted hexagonal fashion Arranged in distorted hexagonal fashion Electrically neutral; weak cohesion Electrically neutral; weak cohesion

Mixed-layered clays Different clays alternate with each other Different clays alternate with each other Vertical stacking Vertical stacking Illite-vermiculite, illite-smectite, chlorite-vermiculite, chlorite- smectite & kaolinite-smectite Illite-vermiculite, illite-smectite, chlorite-vermiculite, chlorite- smectite & kaolinite-smectite Formed by: Formed by: removal/uptake of cations removal/uptake of cations hydrothermal alteration removal of hydroxide interlayers hydrothermal alteration removal of hydroxide interlayers

Other sheet silicates Prehnite Prehnite Paligorskite Paligorskite Sepiolite Sepiolite

Prehnite Ca 2 AlSi 3 AlO 10 (OH) 2 Low-grade metamorphic rocks Low-grade metamorphic rocks

Sepiolite and palygorskite Similar fibrous/lath-like morphologies Similar fibrous/lath-like morphologies Palygorskite less Mg more Al Palygorskite less Mg more Al Both require alkaline conditions Both require alkaline conditions Commercially: carriers, fillers Commercially: carriers, fillers clarifying agents clarifying agents lub. recovery lub. recovery

Structure of sheet silicates

Interest & Importance of clay minerals Ultimate fate of rocks Ultimate fate of rocks Global biogeochemical cycling Global biogeochemical cycling Role in natural hazards Role in natural hazards Human health Human health Civil engineering Civil engineering Nuclear waste repositories Nuclear waste repositories

Formation conditions Mostly low T, low P Only the following present in igneous rocks: Muscovite, phlogopite, biotite and Li-micas Endogenetic: Micas, talc, pyrophyllite, serpentines, chlorites Exogenetic: Kaolinite group, montmorillonites, hydromicas and some serpentines and chlorites Clay minerals: precipitate from seawater or alteration product of primary minerals Main constituents of clays at surface or submarine conditions

Weathering Alteration of minerals and rocks: On earth surface Influence of physical, chemical, biological processes Alteration of pre-existing rocks often display zoning Mechanical decomposition zone Clay mineral zone Kaolinite zone Bauxite-latterite zone (oxides and hydroxides)

Clay minerals in soils Clay minerals in soil – very NB for sustaining life Very fine grained minerals in soil Negatively charged clay minerals attached on surfaces to soil solution Amount of negative charge influences capacity to hold water and other soil ions Vary according to particle size of clay minerals Also non-clay minerals in soils: halite, calcite, gypsum (in evaporite environments)