page_title: Mineral
section_title: Cyclosilicates
hierarchical_section_title: Mineral / Classification / Silicates / Cyclosilicates
caption_attribution_description: English: Elbaite Locality: Minas Gerais, Southeast Region, Brazil (Locality at mindat.org) Size: 1.9 x 1.5 x 1.4 cm. A beautiful partial Tourmaline crystal showing a vibrant popsicle blue color at the cap and a rich green shade below. There are several gemmy areas in the piece, and is rather lustrous. The crystal is not complete, but the color is outstanding. Deutsch: Elbait Fundort: Minas Gerais, Southeast Region, Brasilien (Fundort bei mindat.org) Größe: 1.9 x 1.5 x 1.4 cm.
context_page_description: In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.
  The geological definition of mineral normally excludes compounds that occur only in living beings. However some minerals are often biogenic or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.
  The concept of mineral is distinct from rock, any bulk solid geologic material that is relatively homogeneous at a large enough scale. A rock may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases.
  Some natural solid substances without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur naturally with different crystal structures, each structure is considered a different mineral species. Thus, for example, quartz and stishovite are two different minerals consisting of the same compound, silicon dioxide.
context_section_description: 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 [Si₆O₁₈]¹²⁻; 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 XY₃Z₆(BO₃)₃T₆O₁₈V₃W. The T₆O₁₈ is the basic ring structure, where T is usually Si⁴⁺, but substitutable by Al₃₊ or B³⁺. 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, Al₂Be₃Si₆O₁₈, whose varieties include the gemstones emerald (green) and aquamarine (bluish). Cordierite is structurally similar to beryl, and is a common metamorphic mineral.
caption: An example of elbaite, a species of tourmaline, with distinctive colour banding.
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