It happens most often among the sulfides and semi-metals where covalent bonding and ellipsoidal ions frequently are found. Also, after major ions are accounted for, minor ions sometimes tend to get stuffed into whatever spaces are left over. Finally, if ions in surrounding polyhedra aren't arranged perfectly symmetrically, polyhedra may be distorted somewhat. Since 4 oxygens surround a silicon, there must be a charge of -1 on each oxygen to be balanced by the Mg and Fe atoms. The repulsion is especially great for cations with large charge and small coordination number, like Si.
Silica tetrahedra almost never link any other way than at their corners.
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. 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. The silica tetrahedra don't link until they have no other choice.