cyclosilicate

Cyclosilicates, or ring silicates, are silicate minerals in which the silica tetrahedra share oxygen to form closed rings. These ring structures link together to create crystalline frameworks that are more polymerized than isolated tetrahedra (nesosilicates) but less so than framework silicates. The repetitive ring units give cyclosilicates their characteristic geometry and often well-formed crystals.

The most familiar cyclosilicates belong to the beryl and tourmaline groups. The beryl group has the general formula Be3Al2Si6O18 and includes gemstones such as emerald and aquamarine. In these minerals, the six-membered rings of silica tetrahedra are linked with aluminum and beryllium to form a hexagonal framework. The tourmaline group is boron-rich and exhibits a wide range of colors; its structure also features ring-like arrangements of SiO4 tetrahedra in a complex, layered framework, with substantial substitution by other elements producing diverse chemistries.

Cyclosilicates occur in diverse geological environments, including granitic pegmatites, metamorphic rocks such as schists, and hydrothermal veins. While many cyclosilicates are valuable as gemstones, some also serve as sources of trace elements or industrial materials. Economic importance is dominated by beryl minerals (notably for beryllium) and the gemstone varieties of tourmaline.

In summary, cyclosilicates are a distinct silicate class defined by ring-based networks of tetrahedra, with notable representatives in the beryl and tourmaline groups, and with significant roles in geology and gemology.