The Aragonite Group of Minerals
The structure of the Aragonite Group minerals is composed of triangular carbonate ion groups (CO3), with a carbon at the center of the triangle and the three oxygens at each corner. Unlike in the Calcite Group the carbonate ions do not lie in a single plane pointing in the same direction. Instead they lie in two planes that point in opposite directions; destroying the trigonal symmetry that is characteristic of the Calcite Group. To illustrate this, imagine the symmetry of an equilateral triangle; a three fold rotation with three mirror planes that cross in the center. Now join two of these triangles together at their bases and you have a diamond-shaped figure with the symmetry of a two fold rotation with one mirror plane in the middle. This is what the effect of the two carbonate planes with opposite orientations has on the symmetry of this structure. The metal ions on the other hand, are arranged in what is called hexagonal closest packing. This is responsible for this groups pseudohexagonal tendencies. The aragonite structure is responsible for the similar properties in these minerals. Dissimilar properties are then the responsibility or result of the differing metal cations.
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; as it is slightly too large for the calcite structure and slightly too small for the aragonite structure. 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 determine the structure of the crystallizing mineral.
The Aragonite Group minerals are relatively common, with the exception of witherite. Aragonite is valuable in cements and in other uses. The other members of this group are used as minor ores of their respective metal components.
MINERALS THAT BELONG TO THE ARAGONITE GROUP: