Chapter 27The Phyllosilicates

N. MacDonald

Outline Introduction

Phyllosilicates Basic structural units

Structure and chemistry of: Micas Chlorites Clay minerals Other sheet silicates

IntroductionPhyllosilicates (Sheet silicates) Sheets consists of tetrahedral (T) and

octahedral (O) sheets : T: Sheets of SiO4 tetrahedrons - all in same

orientation O: Sheets of octahedrons sharing O2- anions;

main octahedral cations are Mg2+ (brucite), Al3+ (gibbsite), Fe2+, Fe3+

Two dimensional (planar) structure forms hexagonal network

Basic Structural units Consist of two distinct structural units.

0.29 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

Basic Structural units The octahedral layer can be:

Dioctahedral Every third octahedral space unoccupied Trivalent cations (Al3+, Fe3+) occupy octahedral spaces – every

third space vacant to maintain charge balance Real structure: octahedra distorted; tetrahedra rotated relative

to idealized structure Trioctahedral

All 3 octahedral spaces occupied Divalent cations (Mg2+, Fe2+) occupy every octahedral space More symmetrical than dioctahedral micas

Structure and chemistry:General sheet silicates

Serpentine Mg3Si2O5(OH)4

Talc Mg3Si4O10(OH)2

Pyrophyllite Al2Si4O10(OH)2

Basis for most sheet silicate structures

Serpentine Mg3Si2O5(OH)4

Antigorite, chrysotile and lizardite Consists of tetrahedral layer and

Mg-octahedral layer called the brucite layer

Basis for structure of double-layer clay minerals

Talc Mg3Si4O10(OH)2

Trioctahedral; TOT Consists of 2

tetrahedral layers separated by a brucite layer

Basis for structure of: trioctahedral micas –

no interlayer Triple layer clay

minerals

Pyrophyllite Al2Si4O10(OH)2

Dioctahedral; TOT Consists of 2

tetrahedral layers separated by an Al-octahedral layer called the gibbsite layer

Basis for structure of dioctahedral micas – no interlayer

Structure and chemistry: Micas

Stacking of two T-O-T units by means of an interlayer Part of tetrahedral Si4+ replaced by Al3+; large Na+, K+,

Ca2+ incorporated to maintain charge balance Large cations in cuboctahedrons:

eg.: 1 K+: 12 O2- - coordination number of 12 This is the ideal close-packed coordination number for

ion-pairs with similar radii

Important dioctahedral micas Ordinary:

Muscovite KAl2Si3AlO10(OH)2

Paragonite NaAl2Si3AlO10(OH)2

Interlayer-deficient (Pyrophyllite) No interlayer at all Glauconite

K0.8(Fe3+1.33Mg0.67)(Si3.87Al0.13)O10(OH)2

Brittle Margarite CaAl2Si2Al2O10(OH)2

Important dioctahedral micas Muscovite

Paragonite

Glauconite

Important trioctahedral micas Ordinary

‘Biotite’ K(Mg,Fe2+,Al)3(Si,Al)3(Al,Fe3+)O10(OH)2 Phlogopite Annite Siderophyllite Eastonite

‘Zinnwaldite’ K(Fe2+,Al,Li)Si2(Al,Si)O10F2

‘Lepidolite’ Polylithionite KLi2AlSi4O10F2

Trilithionite K(Li, Al)3(Si,Al)4O10(OH)2

Brittle: Clintonite CaMg2AlSiAl3O10(OH)2

Important trioctahedral micas‘Biotite’

‘Zinnwaldite’

‘Lepidolite’

Structure and chemistry: Chlorites

Trioctrahedral sheet silicates TOT-brucite-TOT:

Brucite layer replaces large cations in interlayers of dioctahedral micas

Two major members: clinochlore Mg-rich Green

chamosite Fe-rich Brown Low T alteration of olivine, pyroxenes, hornblendes

(serpentine, talc and brucite also forms during alteration of above minerals)

Clay minerals:Introduction

Hydrous aluminium phyllosilicates. Contains variable amounts of iron, water,

magnesium, alkali metals and other cations.

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.

Clay Minerals:Introduction

Specific surface & ion exchange capacities Variety of applications Difficult to study: size & composition Gibbsite-dioctahedral-Al2(OH)6 Brucite-trioctahedral-Mg3(OH)6

Composition varies Crystalline, amorphous, platy or acicular

Structure and chemistry:Clay minerals

Double-layer clay minerals –

serpentine-type structure

Kaolinite group

Kaolinite Gibbsite & single tetrahedron

layer Not expand hydroxyl position Six-sided little flakes Ceramic

Triple-layer clay minerals – talc-type

structure

Montmorillonite groupIllite

Montmorillonite group (Smectites )

Dioctahedral & trioctahedral Bonds are weak High Si & Mg Brucite inter-layer replaced by:

water & exchangeable cations Ideal endmembers:

Saponite Beidellite Nontronite

Illite

Non-expanding, dioctahedral clay minerals Unit: silica tetrahedral sheets; central

octahedral sheet More Si, Mg, Fe & water than muscovite Less tetrahedral Al & interlayer K than

muscovite

Vermiculite Mg-vermiculite

resembles talc Separated by water

molecules Arranged in distorted

hexagonal fashion Electrically neutral;

weak cohesion

Mixed-layered clays Different clays alternate with each

other Vertical stacking Illite-vermiculite, illite-smectite,

chlorite-vermiculite, chlorite-smectite & kaolinite-smectite

Formed by: removal/uptake of cations hydrothermal alteration removal of

hydroxide interlayers

Other sheet silicates Prehnite Paligorskite Sepiolite

PrehniteCa2AlSi3AlO10(OH)2

Low-grade metamorphic rocks

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

Structure of sheet silicates

Interest & Importance of clay minerals

Ultimate fate of rocks Global biogeochemical cycling Role in natural hazards Human health Civil engineering 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)