What is a cyclosilicate in mineralogy ?

Cyclosilicates : definition

The cyclosilicate subclass comprises about 95 mineral species. Its basic structure consists of rings formed by the sequence of 3, 4 or 6 [SiO4]4- tetrahedra.
The general formula of cyclosilicates is : SixO3x (with x = 3, 4 or 6). 

There are therefore 3 possible groups : Si3O96- (very rare), Si4O128- (very rare) and Si6O1812-, the latter case bringing together the main minerals of this subclass : beryl (photo on the right) and the tourmaline group.

The rings frequently overlap to give tubular structures capable of retaining alkaline cations (see for example the page about pezzottaite).