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Chapter 27 The Phyllosilicates

Chapter 27 The Phyllosilicates. N. MacDonald. Outline. Introduction Phyllosilicates Basic structural units Structure and chemistry of: Micas Chlorites Clay minerals Other sheet silicates. Introduction Phyllosilicates (Sheet silicates).

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Chapter 27 The Phyllosilicates

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  1. Chapter 27The Phyllosilicates N. MacDonald

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

  3. IntroductionPhyllosilicates (Sheet silicates) • Sheets consists of tetrahedral (T) and octahedral (O) sheets : • T: Sheets of SiO4tetrahedrons - 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

  4. hydroxyl or oxygen oxygen aluminium or magnesium silicon 0.26 nm 0.29 nm Aluminium Octahedron Silica tetrahedron Basic Structural units • Consist of two distinct structural units. Neutral sheets bonded weak dipolar & vd Waals forces.

  5. Basic Structural units

  6. 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

  7. Structure and chemistry:General sheet silicates Serpentine Mg3Si2O5(OH)4 Talc Mg3Si4O10(OH)2 Pyrophyllite Al2Si4O10(OH)2 Basis for most sheet silicate structures

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Important dioctahedral micas • Muscovite • Paragonite • Glauconite

  14. 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

  15. Important trioctahedral micas • ‘Biotite’ • ‘Zinnwaldite’ • ‘Lepidolite’

  16. Structure and chemistry: Chlorites • Trioctrahedralsheet 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)

  17. 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.

  18. 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

  19. Structure and chemistry:Clay minerals

  20. Double-layer clay minerals – serpentine-type structureKaolinite group

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

  22. Triple-layer clay minerals – talc-type structure Montmorillonite groupIllite

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

  24. 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

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

  26. 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

  27. Other sheet silicates Prehnite Paligorskite Sepiolite

  28. PrehniteCa2AlSi3AlO10(OH)2 Low-grade metamorphic rocks

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

  30. Structure of sheet silicates

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

  32. 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

  33. 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)

  34. 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)

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