section_path: Classification/Silicates/Cyclosilicates
section_name: Cyclosilicates
paragraph_text: Cyclosilicates
  
  Cyclosilicates, or ring silicates, have a ratio of silicon to oxygen of 1:3. Six-member rings are most common, with a base structure of; examples include the tourmaline group and beryl. Other ring structures exist, with 3, 4, 8, 9, 12 having been described. Cyclosilicates tend to be strong, with elongated, striated crystals.
  
  Tourmalines have a very complex chemistry that can be described by a general formula XY Z (BO) T O V W. The T O is the basic ring structure, where T is usually Si, but substitutable by Al or B. Tourmalines can be subgrouped by the occupancy of the X site, and from there further subdivided by the chemistry of the W site. The Y and Z sites can accommodate a variety of cations, especially various transition metals; this variability in structural transition metal content gives the tourmaline group greater variability in colour. Other cyclosilicates include beryl, Al Be Si O, whose varieties include the gemstones emerald (green) and aquamarine (bluish). Cordierite is structurally similar to beryl, and is a common metamorphic mineral.