corner sharing

Silicate structures are built by one simple move, repeated over and over: two neighbouring tetrahedra reach out and hold the very same oxygen atom at a shared corner. This is corner-sharing. Think of two pyramids just touching at a single point rather than being glued together along whole faces. It is a light, open way to connect that lets long chains, broad sheets, and airy three-dimensional frameworks grow, while still leaving space between them for other atoms.

An oxygen that sits at a corner belonging to two tetrahedra at once is called a bridging oxygen; one that belongs to only a single tetrahedron, and instead bonds to a metal cation, is a non-bridging oxygen. Sharing a corner lets each of the two silicons keep its own full cage of oxygens while the two of them split the cost of the shared atom, which is precisely why the Si:O ratio drops as more corners are shared. Crucially, silicate tetrahedra share corners but almost never edges or whole faces. Pauling's rules explain why: sharing an edge or a face would drag the two small, highly charged Si4+ ions dangerously close, and their electrostatic repulsion would tear the structure apart.

The number of corners each tetrahedron shares, zero, one, two, three, or all four, is the single dial that sets the whole silicate classification, from isolated islands up to fully connected frameworks. In a silica glass the very same corner-sharing rule builds a disordered three-dimensional network; the difference from crystalline quartz is only the presence or absence of long-range order, not the bond itself. Corner-sharing is therefore the shared grammar of silicate chemistry, crystalline and amorphous alike.