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radius sum (R ∑ )

electrostatic valency (EV)

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ssss1 Common coordination polyhedra: (a) cubic closest packing, (b) cubic, (c) octahedral, (d) tetrahedral, (e) triangular, (f) linear.

Source: Wenk and Bulakh (2004). © Cambridge University Press.

ssss1 Variations in ionic radius (in angstroms) with coordination number (CN) for some common cations.

Ion CN = 4 CN = 6 CN = 8 Na⁺¹ 0.99 1.02 1.18 K⁺¹ 1.38 1.51 Rb⁺¹ 1.52 1.61 Cs⁺¹ 1.67 1.74 Mg⁺² 0.57 0.72 Al⁺³ 0.39 0.48 Si⁺⁴ 0.26 0.40 P⁺⁵ 0.17 0.38 S⁺⁶ 0.12 0.29

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ssss1 A silica tetrahedron is formed when four oxygen ions (O⁻²) bond to one silicon ion (Si⁺⁴) in the form of a tetrahedron. The electrostatic valency of each silicon–oxygen bond in the silica tetrahedron is one charge unit, which fully neutralizes the charge on the central silicon ion (four = four), while leaving the charge on the oxygen ions only partially neutralized (one is one‐half of two).

ssss1 Mineral classification based on the major anion groups.