Why the emphasis on cations?
Because cations are small, and electrostatic attraction is an inverse-square force. Any inverse-square force exerted by a sphere can be regarded as coming from a point at the center of the sphere. If we consider an oxygen ion with -2 charge and ionic radius 1.4 Angstroms, and a magnesium ion of +2 charge and radius 0.66 Angstroms, the electrostatic attraction exerted by the magnesium ion on its “surface” is more than four times that of the oxygen ion. Cations attract anions more strongly than anions attract cations. • In a crystal containing different cations, cations with large charge and small coordination number tend not to share polyhedral elements. Think of Bowen’s Series: among the ferromagnesian minerals, olivine (isolated tetrahedra) tends to form first, followed by pyroxene (single chains), amphibole (double chains) and biotite (sheets). The silica tetrahedra don’t link until they have no other choice. (In the feldspars, the other ingredient in Bowen’s Series, the Ca, Na and K
