"Phyllosilicates": Sheet silicates - Si₂O₅

Tetrahedra share 3 "basal" oxygens with neighboring tetrahedra: Remaining "apical" oxygen points out toward you This apical oxygen generally bonds with other, octahedrally coordinated cations       Si:O ratio is 2 : 5

• Different stacking arrangements of tetrahedral sheets and octahedral sheets, along with the type of cation that occupies the octahedral site, allow for the variety of phyllosicilicates that occur in nature.

  These stacking patterns include:

  1 : 1 Phyllosilicates
  have a "T-O" stacking pattern - 1 Tetrahedral Sheet for every 1 Octahedral Sheet
  

  2 : 1 Phyllosilicates

  have a "T-O-T" stacking pattern - 2 Tetrahedral Sheets for every 1 Octahedral Sheet
  

  Octahedral varieties include:

  Dioctahedral
  - cations in octahedral sheet are trivalent (+3) (usually Al⁺³)
  - the -6 charge from the anions is satisfied by only two trivalent cations
      so one out of every three cation sites is vacant
  - each O or OH is bonded to TWO cations

  Trioctahedral
  - cations in octahedral sheet are divalent (+2) (usually Mg or Fe⁺²)
  - the -6 charge from the anions is satisfied by three divalent cations
      so all cation sites are filled
  - each O or OH is bonded to THREE cations

  Examples of Phyllosilicates: 
  

  1 : 1 Phyllosilicates: (Si₂O₅)
  Dioctahedral: Kaolinite Al₂Si₂O₅(OH)₄ 
  Trioctahedral: Serpentine (Mg,Fe)₃Si₂O₅(OH)₄

  2 : 1 Phyllosilicates: (Si₄O₁₀)

  Dioctahedral: Pyrophyllite Al₂Si₄O₁₀(OH)₂ 
  Trioctahedral: Talc (Mg,Fe)₃Si₄O₁₀(OH)₂
  Micas: 
  Dioctahedral: Muscovite KAl₂(Si₃Al₁)O₁₀(OH)₂ 
  Trioctahedral: Biotite K(Mg,Fe)₃(Si₃Al₁)O₁₀(OH)₂

   

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