"Tectosilicates": Framework silicates - SiO₂

Tetrahedra share all 4 oxygens with neighboring tetrahedra:             Si:O ratio is 1 : 2

• The tectosilicate group includes the most abundant minerals composing the Earth's crust.

  These include:

  Plagioclase Feldspar - 39% 
  Orthoclase Feldspar - 12%

  Quartz - 12%

  Examples of Tectosilicates: 
  

  Quartz - SiO₂ 
  Orthoclase KAlSi₃O₈ 
  Albite NaAlSi₃O₈ 
  Labradorite (Ca,Na)(Al,Si)₄O₈ 
  Anorthite CaAl₂Si₂O₈

  SiO₂ Group

  2 Polymorphs of Quartz: 
  
  Low Quartz (low pressure/low temp. conditions)- Hex/rhomb system 
  High Quartz (low pressure/high temp. conditions) - hex/hex system  
  

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  Tridymite 
  
  High Tridymite - hex.hex system
  Low Tridymite - orthorhombic system 
  
  Cristobalite 
  
  High Cristobalite - isometric system
  Low Cristobalite - tetragonal system 
  
  Coesite 
  Stishovite
  Stability Fields for the SiO₂ Group

  

  One method that the SiO₂ structure changes from one polymorph to another is by: 
  Reconstructive Transformation 
  
  - this involves the breaking and reforming of bonds 
  - the process is slow (may take a million years) and irreversible

   

  Feldspar Group - (SiO₂)₄ = Si₄O₈

  - Al⁺³ substitutes for 1 - 2 Si⁺⁴

  - the charge difference is balanced by the addition of other cations

  For example:

  K⁺¹AlSi₃O₈ 
  Ca⁺²Al₂Si₂O₈

  Click here for a triangular compositional diagram of the feldspars

   

  Back to Main Silicate Page

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