pyroxene

Pyroxene is the standard single-chain silicate, one of the most common rock-forming mineral groups in the crust and mantle. Its skeleton is a single, infinitely long chain of SiO4 tetrahedra, each one sharing two of its corners with the neighbour ahead and the neighbour behind, like a row of people each holding the hands of the two beside them. Rows of these chains lie parallel, tied together by metal cations packed between them.

Because every tetrahedron shares exactly two corners, the repeating unit is SiO3 and the Si:O ratio is 1:3; a common way to write the formula is XYSi2O6, where X and Y are the octahedral cations. Familiar members include enstatite (MgSiO3), diopside (CaMgSi2O6), and the aluminium-bearing augite of basalt. The two non-bridging oxygens on each tetrahedron point sideways and bond to those cations, so the chain-plus-cation packing sets the density and colour, while the strong Si-O backbone runs unbroken along the chain.

The chain direction leaves a clear fingerprint: pyroxenes cleave along two planes that meet at close to 90 degrees, the classic way to distinguish them from the roughly 60/120-degree cleavage of their double-chain cousins, the amphiboles. Pyroxenes are only minor ingredients of most fired ceramics, but they matter geologically as the parents from which clays and other ceramic raw materials ultimately weather, and they crystallise in some slags, basalt castings, and glass-ceramics. They are the single-chain rung of the polymerisation ladder, sitting between the isolated islands of olivine and the double chains of amphibole.