the silica tetrahedron

This is the single Lego brick from which nearly all of the ceramic and glass world is built. Put one silicon atom in the middle and four oxygen atoms at the four corners of a tiny pyramid (a tetrahedron, the shape of a caltrop or a four-sided die). That little group, written SiO4, is the silica tetrahedron. Learn this one shape and half of ceramics falls into place.

Why exactly four corners, and why a pyramid? Silicon has 4 outer electrons and shares one with each of four oxygens by strong, directional covalent bonds; four things repelling each other around a centre spread out into a tetrahedron, with corners about 109.5 degrees apart. Each Si-O bond is roughly half covalent and half ionic, so it is very strong: this is why anything built from these tetrahedra is stiff, hard, and high-melting. Each oxygen at a corner has one bond left over, which it uses either to grab a metal ion or to bridge to a second tetrahedron. When every corner bridges, the whole formula averages out to SiO2 because each oxygen is shared by two silicons.

The tetrahedron is the atom of the silicate alphabet: link the bricks into chains, sheets, or a 3D framework and you get every silicate mineral, clay, quartz, and glass. When these tetrahedra tile in a perfectly repeating pattern you get crystalline quartz; when they connect just as strongly but in a jumbled, non-repeating net you get glass. So the very same building block gives you both a sharp mineral crystal and a clear windowpane, depending only on whether the tiling is ordered.