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. 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.