silicate structures

Silicon and oxygen are the two most common elements in the Earth's crust, so most rocks, sand, clay, and glass are silicates: materials built from one repeating brick. That brick is the SiO4 tetrahedron, a silicon atom with four oxygens sitting at the corners of a little triangular pyramid. All the enormous variety of silicate minerals comes from how many corners of these bricks are shared between neighbours.

Count the sharing and you have classified the silicates. Share no corners and you get isolated tetrahedra held together by metal ions in between (island silicates such as the mineral olivine). Share two corners each and the bricks link into chains or rings (as in pyroxenes and asbestos). Share three corners and you get flat sheets (as in mica and clay, which is why they peel into thin flakes). Share all four corners and the tetrahedra link in every direction into a rigid 3D framework, which is pure silica, SiO2. Each shared oxygen is a bridge between two silicons, so the formula ratio slides from SiO4 groups toward SiO2 as sharing increases.

This one idea explains a startling range of behaviour. Clay's sheets slide over one another when wet, which is why you can throw a pot; heat it and the sheets fuse. The framework silicates are hard and high-melting because you must break strong Si-O bonds in three dimensions to move anything. And when the framework is frozen in a disordered, unshared-here-and-there arrangement rather than a neat crystal, you get glass. Silicates are the backbone of both traditional ceramics and window glass.