for example, the amphibole group consists of 15 or more mineral species, most of them with the general unit formula , where a is a trivalent cation such as or , b is a divalent cation such as , , or , and c is an alkali metal cation such as , , or . in all these minerals, the anions consist mainly of groups of four tetrahedra connected by shared oxygen corners so as to form a double chain of fused six-member rings. in some of the species, aluminum may replace some silicon atoms in the backbone, with extra b or c cations to balance the charges.