The Calcite Group is composed of minerals with the general formula of ACO3, where "A" can be one or more of several positive 2 charged metal ions specifically calcium, cobalt, iron, magnesium, zinc, cadmium, manganese and/or nickel. The symmetry of the members of this group is trigonal, bar 3 2/m. The structure consists of layers of A position metal ions alternating with stacks of carbonate layers. The carbonate layers are composed of flat triangular shaped carbonate ions (CO3), with a carbon at the center of the triangle and the three oxygens at each corner. This triangular structural element is the key ingredient in the trigonal symmetry of this group. Of course, the metal ions must also fall into place within the symmetrical arrangement in order to preserve the trigonal symmetry.
The Calcite Group is an interesting contrast to the Aragonite Group of minerals. The structure of the Calcite Group is stable at normal temperatures and pressures only with smaller metal ions than the Aragonite Group. The divide is right at the radius of calcium. If the ion is larger than calcium, the mineral's structure will be of the Aragonite Group, otherwise if the ion is smaller than calcium than the mineral's structure will be of the Calcite Group. Ironically, the mineral aragonite is dimorphous with the mineral calcite in that they have the same calcium carbonate chemistry, but different structures. The size of calcium is the same in both minerals, but different crystallization temperatures, pressures and other parameters will decide the structure of the crystallizing mineral, that being either calcite's or aragonite's.
All members of the Calcite Group are important minerals. Calcite's importance is almost without saying as it is used in cements, the steel industry, chemical industry, optical uses, etc. The others in this group have their varied uses, but all are used as ores for their respective metal content. Only otavite and gaspeite are considered rare.
MINERALS THAT BELONG TO THE CALCITE GROUP: