• Si-tetrahedra: 50/50 mix ionic and covalent bonds
• tetrahedra are never links to one another by more than one oxygen
  -if more than one O-edge sharing - Si ions would be too close together=unstable

• very resistant to weathering

-olivine: cleavage is irregular - no preferred plane of weakness
*alters to serpentine (a phyllosilicate)

Sorosilicates: double tetrahedra linked by common O - Si₂O₇
ex: epidote -actually mix of isolated tetrahedra and "soro-linked" tetrahedra

Cyclosilicates: 3, 4, 6, or 12 tetrahedra linked - Si₆O₁₈
ex: tourmaline, beryl

tourmaline - trigonal/rhombohedral

beryl - hexagonal

Inosilicates:

• "thread" - single chain - linked by 2 O's of base - higher temp.
• double chain - two linked chains with a large hole between them
  -filled by large anions: OH^-, Cl^-, F^-
  -amphiboles contain water (OH^-) in holes - found at lower temps
  -can drive water out of structure of amphiboles with heating - form pyroxenes

Phyllosilicates: "flaky"
-2 components: tetrahedral sheet & octahedral sheet
-Clay minerals: derived as weathering products
kaolinite, illite, vermiculite, smectite

Tectosilicates: framework - tetrahedra linked by every one of 4 oxygens
ex: SiO₂
-low quartz - trigonal or rhombohedral
-high quartz - hexagonal
Varieties:

• cryptocrystalline: flint, chert, agate, jasper, chalcedony, onyx
  -very low temp environments - even sedimentary
• crystalline: (macrocrystalline): quartz, tridymite, coesite, cristobalite, stishovite
  -occurrence depends on temp and press conditions
  -displacive transformation: *573^oC - cooling through this temp (high to low), get rapid and reversible transformation which involves a warping or twisting of bonds rather than breaking of bonds
  -reconstructive transformation: slow & irreversible - bonds broken and reformed

Stability Field Diagram:

http://classes.colgate.edu/rapril/geol201/ Questions to: rapril@mail.colgate.edu Copyright 1997 © Colgate University.

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