Inosilicate is one of the principal structural classes of silicate minerals, defined by the linkage of silicon-oxygen tetrahedra into one-dimensional chains. Where the chains are single, the resulting structure characterises the pyroxene group; where two single chains are cross-linked side by side, the resulting double-chain structure characterises the amphibole group. Together pyroxenes and amphiboles account for a substantial portion of the rock-forming mineralogy of the Earth's crust and a significant share of the world's gem and ornamental species.

Structure

In a single-chain inosilicate the SiO4 tetrahedra share two of their four oxygens with neighbouring tetrahedra, producing a continuous chain that runs through the structure. Cations including calcium, magnesium, iron, sodium, lithium and aluminium occupy octahedral sites between the chains and balance the negative charge on the silicate framework. Substitutions among these cations produce the chemical variability that distinguishes individual pyroxene species.

Double-chain inosilicates share one additional oxygen between adjacent chains, giving an Si4O11 stoichiometry per repeat unit and accommodating hydroxyl groups in cavities between the chains. The hydroxyl is essential to amphibole stability and is what distinguishes amphiboles from pyroxenes at the chemical level.

Pyroxene-group gems

The pyroxenes contribute several economically important gem species:

• Spodumene, the lithium aluminium pyroxene LiAlSi2O6, source of kunzite (pink-violet, manganese-coloured) and hiddenite (green, chromium-coloured), with major production from Brazil, Afghanistan and Madagascar.
• Diopside, the calcium magnesium pyroxene CaMgSi2O6, source of chrome diopside from Russian Yakutia and other localities.
• Jadeite, the sodium aluminium pyroxene NaAlSi2O6, the principal jade species and the source of Imperial jade, lavender jade and the broader Burmese jade trade.
• Enstatite-orthopyroxene group, sources of brown and yellow-brown gemstones from a range of localities.

Amphibole-group gems

The amphiboles contribute equally significant material:

• Nephrite, a fibrous tremolite-actinolite, the second jade species, with sources in China, New Zealand, British Columbia, Russia, Australia and elsewhere.
• Chatoyant gem amphiboles such as cat's-eye actinolite and certain richterite varieties, the latter sometimes mistaken for nephrite at the bench.
• Hornblende and pargasite as occasional faceted-stone curiosities for collectors.

Trade context

The inosilicate class also embraces tanzanite, although tanzanite (zoisite) is technically a sorosilicate, and the term "inosilicate" should not be used as a marketing label since it is a structural-mineralogy classification rather than a trade descriptor. For gemmological reasoning the practical importance of the inosilicate framework is that it explains the strong cleavage characteristic of pyroxenes and amphiboles, the fibrous habit of nephrite and the directional polish behaviour that lapidaries handle in jadeite, kunzite and chrome diopside.