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Sheet Silicates

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

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Sheet Silicates

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  1. 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

  2. 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

  3. T and O layers bonded 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

  4. Construction of T-O-T Sheets • Octahedral layers: • Two planes of OH- anionic groups • Cations are two types: • Divalent (Fe2+ or Mg2+) • Trivalent (Al3+ or Fe3+) • Mg and Al most common

  5. 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

  6. 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

  7. Fig. 11-2 • 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

  8. 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

  9. Fig. 13-1 • Tetrahedral sheet composition is Si2O52- • May have Al3+ or Fe3+ substitute for Si4+ • Increases net negative charge

  10. 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

  11. Al3+ (dioctahedral) or Mg2+ (trioctahedral) OH in middle of rings Basal Oxygen T layer on top (an example of 1:1 layer type) Fig. 13.1

  12. 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

  13. 1:1 layering in phyllosilicates OH- only OH- + oxygen Oxygen only Al2Si2O5(OH)4 = kaolinite, dioctahedral 1:1 sheet silicate Mg3Si2O5(OH)4 = serpentine, trioctahedral, 1:1 sheet silicate

  14. 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

  15. 2:1 layering in phyllosilicates 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

  16. How are sheets stacked? Kaolinite (dioctahedral) Serpentine (trioctahedral) • 1:1 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 • Each one may be dioctahedral or trioctahedral Pyrophyllite (dioctahedral) Talc (trioctahedral)

  17. 1:1 layer silicates • Kaolinite and Serpentine • Bonding between sheets very weak • Electrostatic bonds – van der Waals and hydrogen • Results in very soft minerals • Thickness of TO layers around 7 Å

  18. C unit cell dimension about 7 Å Fig. 13.3

  19. 2:1 layer silicates • Unit structure is repeating TOT layers, two ways: • (1) TOT layers can be electrically neutral • Nothing in between sheets van der Waal forces • Pyrophyllite & Talc • (2) substitution in TOT layers can give a net charge • Most common substitution is Al3+ for Si4+ in tetrahedral layers • Charge balance maintained with substitution between the sheets

  20. TOT structure • If there is only Si4+ in T layers (no Al3+ or Fe3+) • Electrically neutral, no interlayer cations • TOT layers weakly bonded by van der Waal and hydrogen bonds • Soft (Talc), greasy feel

  21. C unit cell dimension about 9 to 9.5 Å Nothing in interlayer site Fig. 13.3

  22. 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 1 out of 4 tetrahedral sites occupied by Al3+

  23. Micas • Al/Si ratio in the tetrahedral layer is 1/3 • Dioctahedral TOT layer = Al2(AlSi3O10)(OH)21- • Remember Pyrophyllite: Al(Si2O5)(OH)2 • Trioctahedral TOT layer = Mg3(AlSi3O10)(OH)21- • Remember Talc: Mg3(Si2O5)(OH)2 • Negative charge balance by large monovalent cation, usually K+ • Bonds are ionic, fairly strong, harder minerals

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

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

  26. 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

  27. Margarite • Dioctahedral • CaAl2(Al2Si2O10)(OH)4 • Clintonite • Trioctahedral • CaMg2Al(Al3SiO10)(OH)2 • Now charge balance in part from Al substitution in octahedral layer

  28. …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

  29. Some Al3+ for Si4+ C unit cell dimension about 14 Å Some Al3+ for Mg2+ TOT layers have slight negative charge, substitute Al3+ for Si4+ O layers often have net positive charge Fig. 13.3

  30. 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

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

  32. Clay Minerals • Clay has two meanings: • Particles < 1/256 mm, or 0.0039 mm • A group of sheet silicate minerals (not micas) 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

  33. Terminology • Clay: Sediment composed of particles that are < 0.002 mm • Claystone: Rock composed of clay-sized particles • Clay minerals: 1:1 and 2:1 Phyllosilicate minerals without K+ or Ca2+ bonding sheets (those are Micas) • Argillaceous: Rock or sediment containing large amounts of clay and clay minerals

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

  35. Clay classification • 1:1 layer clays • 7 Å type • TO layers • Kaolinite (dioctahedral) • Serpentine (trioctahedral)

  36. 2:1 layer clays: • End members: • 10 Å – Pyrophyllite (dioctahedral) & talc (trioctahedral) • 10 Å – Charge imbalance: with K as interlayer: Mica: Muscovite (trioctahedral) & Biotite (dioctahedral) • 14 Å type (Chlorite)

  37. Intermediate 2:1 clays • 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

  38. Three types of intermediate 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

  39. Low charge • Smectite • approximately = Ca0.17(Al,Mg,Fe)2(Si,Al)4O10(OH)2•nH2O • Net negative charge is 0.2 to 0.6 per formula unit, typically 0.33 • Ca and Na are typical interlayer ions • Exchangable • May be dioctahedral or trioctahedral • Charge results from • Al substitution for Si in tetrahedron • Mg for Al in octahedron (in dioctahedral)

  40. 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

  41. High charge • Illite/glauconite • Approximately = K0.8Al2(Al0.8Si3.2)(OH)2 • Net negative charge of 0.8 to 1 per formula • Very similar to muscovite – called mica-like • Mostly substitute of Al3+ for Si4+ • All are dioctahedral; Glauconite has Fe3+ • Interlayer ion is K+ • High K concentration means strong bond • Difficult for water to enter • Non-swelling clay

  42. Intermediate charge • Vermiculite • Approximately = (Mg,Ca)0.3(Mg,Fe2+,Fe3+,Al)3(Si,Al)4)O10(OH)2 • 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

  43. 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

  44. 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 Figure 13-15

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

  46. Mineralogy of Miocene/Oligocene sediments Gulf Coast • K-spar dissolves • Smectite converts to illite (with extra K) • Releases interlayer water • Increase pore pressures • T corresponds to “oil window” • Forces oil from pore spaces into reservoirs Figure 13-16

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