an inosilicate

Now let each tetrahedron hold hands with two neighbours, one on each side, and the islands grow into ropes: infinitely long chains of corner-sharing tetrahedra. That is an inosilicate (from the Greek inos, thread or fibre). Picture a paper chain of pyramids running off to the horizon, then bundles of these chains laid side by side and tied together by rows of metal cations. The one-directional chains give many of these minerals a stubby, prismatic, sometimes fibrous habit.

There are two great chain types. In a single chain each tetrahedron shares two corners, so the repeating unit is SiO3 and the Si:O ratio is 1:3; this is the pyroxene structure, for example enstatite MgSiO3. In a double chain, two single chains are cross-linked so that half the tetrahedra share two corners and half share three, averaging out to the repeating unit Si4O11 and a ratio of 4:11; this is the amphibole structure, for example the tremolite-hornblende family, which also carries hydroxyl (OH) groups tucked in the ribbon. The cations between the chains (Mg2+, Fe2+, Ca2+, and others) both balance charge and set the mineral's density and colour.

Because the strong bonding runs along the chains but the weaker cation bonds hold the chains together sideways, inosilicates cleave neatly parallel to the chains, giving pyroxene its roughly 90-degree cleavage and amphibole its roughly 60/120-degree cleavage, a classic way to tell them apart under a hand lens. Inosilicates are the middle rung of the polymerisation ladder, more connected than islands, less than sheets, and they are rock-forming heavyweights of the Earth's crust and mantle even though they are minor players in most fired ceramics.