page_title: Mineral
section_title: Silicates
hierarchical_section_title: Mineral / Classification / Silicates
caption_attribution_description: English: Aegirine Locality: Langesundsfjorden, Porsgrunn, Telemark, Norway (Locality at mindat.org) Size: 5.4 x 2.0 x 1.8 cm. This sharp, moderately lustrous, doubly terminated, black aegirine crystal is from the Type Locality area in Norway - Langesundsfjorden. The blocky crystal has an interesting, complex termination and the little sidecar crystal is a nice accent. This is a pristine, floater crystal. Classic, old-time material from the John Ydren Collection. Deutsch: Aegirin Fundort: Langesundsfjorden, Porsgrunn, Telemark, Norwegen (Fundort bei mindat.org) Größe: 5.4 x 2.0 x 1.8 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: The base unit of a silicate mineral is the [SiO₄]⁴⁻ tetrahedron. In the vast majority of cases, silicon is in four-fold or tetrahedral coordination with oxygen. In very high-pressure situations, silicon will be in six-fold or octahedral coordination, such as in the perovskite structure or the quartz polymorph stishovite (SiO₂). In the latter case, the mineral no longer has a silicate structure, but that of rutile (TiO₂), and its associated group, which are simple oxides. These silica tetrahedra are then polymerized to some degree to create various structures, such as one-dimensional chains, two-dimensional sheets, and three-dimensional frameworks. The basic silicate mineral where no polymerization of the tetrahedra has occurred requires other elements to balance out the base 4- charge. In other silicate structures, different combinations of elements are required to balance out the resultant negative charge. It is common for the Si⁴⁺ to be substituted by  Al³⁺ because of similarity in ionic radius and charge; in those cases, the [AlO₄]⁵⁻ tetrahedra form the same structures as do the unsubstituted tetrahedra, but their charge-balancing requirements are different.
  The degree of polymerization can be described by both the structure formed and how many tetrahedral corners (or coordinating oxygens) are shared (for aluminium and silicon in tetrahedral sites). Orthosilicates (or nesosilicates) have no linking of polyhedra, thus tetrahedra share no corners. Disilicates (or sorosilicates) have two tetrahedra sharing one oxygen atom. Inosilicates are chain silicates; single-chain silicates have two shared corners, whereas double-chain silicates have two or three shared corners. In phyllosilicates, a sheet structure is formed which requires three shared oxygens; in the case of double-chain silicates, some tetrahedra must share two corners instead of three as otherwise a sheet structure would result. Framework silicates, or tectosilicates, have tetrahedra that share all four corners. The ring silicates, or cyclosilicates, only need tetrahedra to share two corners to form the cyclical structure.
  The silicate subclasses are described below in order of decreasing polymerization.
caption: Aegirine, an iron-sodium clinopyroxene, is part of the inosilicate subclass.
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