a nesosilicate

The simplest silicate of all: lonely tetrahedra floating like islands in a sea of metal cations, sharing no oxygens with one another. That is a nesosilicate (from the Greek nesos, island). Each SiO4 unit keeps all four of its oxygens to itself, and the units are held together not by silicon-oxygen bonds between them but entirely by the metal ions packed in the gaps, which bond to the corner oxygens of several neighbouring tetrahedra at once.

Because every oxygen is a non-bridging oxygen tied to a cation, the Si:O ratio is exactly 1:4 and the formula reads like a salt: the isolated (SiO4)4- anion balanced by cations, for example Mg2SiO4 (two Mg2+ per tetrahedron) or the garnet and zircon (ZrSiO4) minerals. There is no chain or sheet to peel or cleave along, so nesosilicates tend to be hard, dense, equant crystals rather than fibrous or platy ones, and they usually have relatively high, sharp melting points because the tightly bonded cation-oxygen network reinforces the strong Si-O tetrahedra in every direction.

For a ceramist the headline nesosilicate is olivine, whose magnesium end-member forsterite makes an excellent basic refractory. More broadly, nesosilicates matter because they are the least polymerised silicates, the reference point from which chains, sheets, and frameworks are built by progressively sharing more corners. A common confusion is to expect isolated tetrahedra to give weak, low-melting solids; in fact the surrounding cation bonds make many of them tough and refractory.