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radius ratio Radius of a cation divided by that of an anion. In silicate minerals, the radius of a cation divided by that of the oxygen anion. Most minerals have bonds that are mainly ionic, therefore radius ratios are most useful in studying mineral structures. Ions tend to pack closely together in a low-energy, stable state and the most stable arrangements are classified in terms of the cation-to-anion radius ratio. Oxygen is the major anion in most minerals, so the radius ratio of various cations to oxygen is particularly important. This controls the co-ordination numbers and therefore the structures of silicates, e.g. chains, frameworks, etc.