Mineralogy of Cyclosilicates and Inosilicates

Crystal Chemistry of Cyclosilicates and Inosilicates

Cyclosilicates and inosilicates are two subclasses of silicate minerals. They include numerous mineral groups and important mineral species. The cyclosilicates most familiar to gemologists are beryls and the tourmaline supergroup. Inosilicates are subdivided into single-chain silicates, which includes the pyroxene supergroup, and double-chain silicates, which includes the amphibole minerals. The gem material gemologists call jade actually includes both pyroxene (jadeite) and amphibole (nephrite) members.

Introduction to Cyclosilicates

Cyclosilicate units form by joining three, four, or six SiO₄ tetrahedrons into closed rings.

• Three joined tetrahedrons create a three-member ring with a [Si₃O₉]^6- structural unit.
• Four joined tetrahedrons create a four-member ring with a [Si₄O₁₂]^8- structural unit.
• Six joined tetrahedrons create a six-member ring with a [Si₆O₁₈]^12- structural unit. Six-member rings are the most common type of cyclosilicates.

Benitoite (BaTi[Si₃O₉]), the rare state gem of California, is an example of a