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)