Inosilicate

An inosilicate is a silicate mineral in which the silicon-oxygen tetrahedra are linked into chains. They form two main groups: single-chain inosilicates (pyroxenes) and double-chain inosilicates (amphiboles).

Single-chain inosilicates, or pyroxenes, consist of SiO4 tetrahedra that share two oxygens with neighboring tetrahedra, creating an infinite one-dimensional chain with the repeating unit SiO3. These chains are balanced by metal cations in various coordinations, producing minerals such as enstatite (MgSiO3), ferrosilite (FeSiO3), diopside (CaMgSi2O6), and jadeite (NaAlSi2O6). Pyroxenes typically exhibit two cleavage directions at or near 90 degrees.

Double-chain inosilicates, or amphiboles, form when two single chains join by sharing additional oxygens, producing longer chains with repeating units such as Si4O11. Amphiboles have a general formula written as A0-1B2C5T8O22(OH)2 and include minerals like hornblende, tremolite, and actinolite. They generally display two cleavage directions at about 56 and 124 degrees and tend to crystallize in monoclinic or orthorhombic systems.

Occurrence and significance: Inosilicates are major constituents of many igneous and metamorphic rocks. Pyroxenes are common in mafic rocks such as basalt and peridotite, while amphiboles are abundant in intermediate to felsic rocks and in high-grade metamorphic environments. Some inosilicates are used as gemstones (for example jadeite and diopside) or as industrial minerals. Among amphiboles, certain forms have been associated with asbestos and are subject to health and safety regulations.

In summary, inosilicates are distinguished by chain arrangements of SiO4 tetrahedra, encompassing the pyroxene and amphibole groups, and they play a key role in the mineralogy of many rocks.