section_path: Classification/Silicates
section_name: Silicates
paragraph_text: Silicates
  
  The base unit of a silicate mineral is the 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 (SiO). In the latter case, the mineral no longer has a silicate structure, but that of rutile (TiO), 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 Si to be substituted by Al because of similarity in ionic radius and charge; in those cases, the 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):
  
  The silicate subclasses are described below in order of decreasing polymerization.