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

to balance the charge difference and allow 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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