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.[1]
For example, the amphibole group consists of 15 or more mineral species, most of them with the general unit formula A xB yC 14-3x-2ySi 8O 22(OH) 2, where A is a trivalentcation 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 SiO 4 tetrahedra connected by shared oxygen corners so as to form a double chain of fused six-member rings. In some of the species, aluminumAl3+ may replace some silicon atoms Si4+ in the backbone, with extra B or C cations to balance the charges.
^Stuart J. Mills, Frédéric Hatert, Ernest H. Nickel, and Giovanni Ferraris (2009): "The standardisation of mineral group hierarchies: application to recent nomenclature proposals". European Journal of Mineralogy, volume 21, number 5, pages 1073-1080. doi:10.1127/0935-1221/2009/0021-1994