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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 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 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, aluminum Al3+
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