RR = radius of cation/radius of anion (usually oxygen)

Radius Ratio (RR)	Cation Coord. #	Configuration	Example
<0.155	2	linear	CO2
0.155-0.225	3	trigonal planar	CO32-
0.225-0.414	4	tetrahedral or square	SiO4
0.414-0.732	6	octahedral	NaCl
0.732-0.99	8	cubic	CsCl
1.0	12	close-packing (cubic or hexagonal)	metals

Notes:

• Coordination numbers may vary for a particular element due to external influences such as pressure and temperature:
  • high pressure/low temperature - higher coordination number favored.
  • low pressure/high temperature - lower coordination number favored
    Example: SiO₂
    RR (Si:O) = 0.42/1.40 = 0.3 which is about in the middle of 4CN range.
    
    Therefore, Si⁺⁴ almost ALWAYS has a Coordination # = 4
    
    Exception - Stishovite (high pressure form of SiO₂) has a Coordination # = 6

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