Basic Building Block = Silica Tetrahedron

• Composed of one silica atom bonded to four oxygen atoms 
• 50/50 mix ionic and covalent bonds - mixed bond  
  
• oxygens form isodesmic bonds, meaning that they equally distribute their charge between the central Si and some other ion. (i.e., -1 to Si and -1 to other = total -2)
• tetrahedra are never linked to one another by more than one shared oxygen because the structure would be unstable  
  i.e., if 2 oxygen (edge sharing) or 3 oxygen (face sharing) are shared by adjacent tetrahedra, then the Si ions would be too close together and would begin to repel each other

 

* Silicates account for 91.6% of the Earth's crust *

 

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Silicates are subdivided into 6 groups based on the arrangement of Si-tetrahedra within their structure:

Subgroup*	Structure	Si:O Ratio
Nesosilicates	single tetrahedrons (no oxygens shared between neighboring tetrahedra) joined by bonds with other cations	1:4
Sorosilicates	two neighboring tetrahedra share one point	2:7
Cyclosilicates	all tetrahedra share 2 oxygens, each one with a different neighbor, building up 3, 4, 6 or 12 tetrahedra into a ring structure	1:3
Inosilicates Single Chain	all tetrahedra share 2 oxygens, each one with a different neighbor, building a chain structure	1:3
Inosilicates Double Chain	alternate tetrahedra share 2 then 3 oxygens, each one with a different neighbor, building a side-by-side double chain structure	4:11
Phyllosilicates	each tetrahedron shares 3 basal oxygens, each one with a different neighbor, building a sheet structure	2:5
Tectosilicates	each tetrahedron shares all 4 oxygens, each one with a different neighbor, building a 3-D framework structure	1:2

*Click on Subgroup Name for more information on that group.

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