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3.0 Sheet silicates - Sheet Silicates Abundant and common...

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Unformatted text preview: Sheet Silicates Abundant and common minerals throughout upper 20 km of crust Felsic to intermediate igneous, metamorphic, and sedimentary rocks All are hydrous Z/O ratio of 2/5 2 Major groups: Micas & Clays Contain H Bonded to O to form OH Groupings Based on structure Two kinds of "layers" within the "sheets" "T" layers tetrahedral layers "O" sheets octahedral layers Tetrahedral coordination of Si and Al Octahedral coordination of mostly Al and Mg, occasionally Fe T and O layers join to form sheets The sheets are repeated in vertical direction The spaces between the sheets may be: Primary characteristic basal cleavage Vacant Filled with interlayer cations, water, or other sheets Single perfect cleavage Occurs because bonds between sheets are very weak Octahedral Sheets Two planes of OH anionic groups Cations are two types: Divalent (Fe2+ or Mg2+) Trivalent (Al3+ or Fe3+) 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 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 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 Fig. 112 Tetrahedron are meshes of 6fold rings Tetrahedral layers are two oxygen thick Three basal oxygen on each tetrahedron shared by adjacent tetrahedron The fourth, unshared oxygen is the apical oxygen Tetrahedral sheet composition is Si2O52 May have Al3+ or Fe3+ substitute for Si4+ Increases net negative charge Fig. 131 Tetrahedral and octahedral sheets always joined Sheets joined in two ways Apical oxygen of tetrahedral sheets formed part of octahedral sheets Apical oxygen replaces one of the OH in the octahedral sheets TO layers, called 1:1 layer silicates TOT layers, called 2:1 layer silicates Al3+ (dioctahedral) or Mg2+ (trioctahedral) Basal Oxygen OH in middle of rings T layer on top (an example of 1:1 layer type) 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 OH only OH + oxygen Oxygen only Al2Si2O5(OH)4 = kaolinite, dioctahedral 1:1 sheet silicate Mg3Si2O5(OH)4 = serpentine, trioctahedral, 1:1 sheet silicate 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 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 How are layers stacked? I. I. 1:1 layer 2:1 layer I. II. III. ...TO...TO...TO... ...TOT...TOT...TOT... c...TOT...c...TOT...c...TOT...c... O...TOT...O...TOT...O II. Four types of layers, each dioctahedral or trioctahedral 1:1 layer silicates Kaolinite and Serpentine Bonding between layers very weak Electrostatic bonds van der Waals and hydrogen Results in very soft minerals Thickness of TO layers around 7 C unit cell dimension about 7 2:1 layer silicates Unit structure is repeating TOT layers, two ways: (1) TOT layers can be electrically neutral (2) substitution in TOT layers can give a net charge Most common substitution is Al3+ for Si4+ in tetrahedral layers TOT structure Only Si4+ in T layers (no Al3+ or Fe3+) Electrically neutral, no interlayer cations TOT layers weakly bonded by van der Wall and hydrogen bonds Soft (Talc), greasy feel C unit cell dimension about 9 to 9.5 Nothing in interlayer site c...TOT...c...TOT...c These are the mica minerals Also less common are "brittle micas" Structure is TOT layers with some tetrahedral sites occupied by Al3+ Micas Al/Si ratio in the tetrahedral layer is 1/3 Dioctahedral TOT layer = Al2(AlSi3O10)(OH)21 Trioctahedral TOT layer = Mg3(AlSi3O10)(OH)21 Negative charge balance by large monovalent cation, usually K+ Bonds are ionic, fairly strong, harder minerals C unit cell dimension about 9.5 to 10 K+ in interlayer site Dioctahedral mica muscovite KAl2(AlSi3O10)(OH)2 Trioctahedral mica Phlogopite KMg3(AlSi3O10)(OH)2 Brittle Micas Similar to micas, but more Al3+ substitution Charge balanced by Ca2+ Margarite half of tetrahedral sites have Al3+ substitution Clintonite of tetrahedral sites have Al3+ substitution Margarite Clintonite Dioctahedral CaAl2(Al2Si2O10)(OH)4 Trioctahedral CaMg2Al(Al3SiO10)(OH)2 ...O...TOT...O...TOT...O... T layers with small negative charge Most common members are in the chlorite group Structure like Talc, but with brucite (Mg3(OH)6) interlayer Substitute small amounts of Al3+ for Si4+ O layers often have net positive charge Minerals harder than expected Substitute Al3+ or Fe3+ for divalent cations C unit cell dimension about 14 TOT layers have slight negative charge, substitute Al3+ for Si4+ O layers often have net positive charge Varieties of sheet silicates TO structures Serpentine (var. Antigorite, Chrysotile, Lizardite) All are trioctahedral Trioctahedral sheets, a = 5.4 ; b = 9.3 Tetrahedral sheets, a = 5 ; b = 8.7 Mismatched size leads to variations Chrysotile (curved tubes) Antigorite (reversed direction) Lizardite (distorted tetrahedral mesh) Fig. 135 Clay Minerals Clay has two meanings: Original description from not being able to identify small grain size material Now can use Xray diffraction to determine clays Particles < 1/256 mm, or 0.0039 mm A group of sheet silicate minerals that are commonly claysized Problems Clay size fraction can contain other minerals (quartz, carbonates, zeolites etc.) Clay minerals used to define size fraction size not mineralogical Several clay minerals can be larger than the size requirements Clay classification 1:1 layer clays 2:1 layer clays 10 or 14 type 7 type, TO layers Mixed layer clays combined 1:1 and 1:2 Have net negative charge, but less than one per formula Requires less interlayer cations to charge balance Three types of 10 clays Charge imbalance controlled by Low charge imbalance smectite clays High charge imbalance illite clays Intermediate charge imbalance vermiculite "interlayer" cations They move in and out Cation Exchange Capacity (CEC) Surface adsorption Low charge Smectite Net negative charge is 0.2 to 0.6 per formula unit, typically 0.33 Ca and Na are typical interlayer ions May be dioctahedral or trioctahedral Charge results from Al substitution for Si in tetrahedron Mg for Al in octahedron (in dioctahedral) Low charge means water and cations (Na, K, Ca, Mg) easily move in and out of interlayer sites Water moves in and out depending on moisture in environment No water = 10 One water layer = 12.5 Two water layer = 15.2 High charge Illite/glauconite Net negative charge of 0.8 to 1 per formula Almost mica Mostly substitute of Al3+ for Si4+ All are dioctahedral Interlayer ion is K+ Very similar to muscovite called micalike High K concentration means strong bond Difficult for water to enter non=swelling clay Intermediate charge Vermiculite About 0.6 charge per formula unit Comes from oxidation of Fe2+ to Fe3+ in biotite Less K+ than mica, can exchange for Ca2+ and Mg2+ and water Swell clay With water interlayer spacing is 14.4 Reduces the negative charge on TOT layer from 1 to 0.6 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 7 10 14 1:1 layer clays 2:1 layer clays low charge, smectite 2:1 layer clays high charge, illite 2:1 layer clays Chlorite gp Mixed layer Burial Diagenesis Smectite converts to illite with burial Most conversion at 50 to 100 C Conversion requires K, usually comes from dissolution of K spar Mineralogy of Miocene/Oligocene sediments Gulf Coast Release structural water of smectite; corresponds to "oil window" ...
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