Mineral group

In geology and mineralogy, a mineral group is a set of mineral species with essentially the same crystal structure and composed of chemically similar elements. For example, the amphibole group consists of 15 or more mineral species, most of them with the general unit formula AxByC14-3x-2ySi8O22(OH)2, where A is a trivalent cation such as Fe3+ or Al3+, B is a divalent cation such as Fe2+, Ca2+, or Mg2+, and C is an alkali metal cation such as Li+, Na+, or K+. In all these minerals, the anions consist mainly of groups of four SiO4 tetrahedra connected by shared oxygen corners so as to form a double chain of fused six-member rings. In some of the species, aluminum Al3+ may replace some silicon atoms Si4+ in the backbone, with extra B or C cations to balance the charges.