RatiocoordinationrkrO2numbers

0.225

C4+ B3+

0.16 0.16

3, 4

Si4+

Al3+

0.25 0.30 0.36

4, 6

0.732

Mg2+

Fe2+

Ca2+

0.46 0.47 0.49 0.53 0.69 0.71

6, 8

1.000

Ba2+

0.80 0.95 0.96

8-12 8-12

12

Cs+

1.19

12

The radius ratio rule is only applicable to ionic compounds. In silicate minerals, however, it is the bonds between oxygen and silicon and between oxygen and aluminium (Al) which are structurally important. These bonds are almost equally ionic and covalent in character and the radius ratio rule predicts the coordination of these ions adequately.

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