In aragonite, the distances between the Ca and O atoms (in the CO3 ion) are substantially greater than the distances between the same atoms in calcite; this relationship would appear to contradict the fact that aragonite (density = 2.947 g/cm3) is denser than calcite (density = 2.7102g/cm3). Worse yet is the greater density of aragonite be loosely attributed to the coordination of calcium by nine oxygen rather than the coordination by six oxygen in calcite, this explanation is unsatisfactory once the mutual involvement of of six neighbors of the Ca is the same in both cases (considering the repetition of the unit cell). In this context, it is interesting to consider the oxygen atoms.
The distances between the oxygen of different groups of CO3 are shorter in aragonite. The distances between the oxygen atoms between different groups of CO3 vary from 3.23 to 2.75Å, depending on the observed edge (in the polyhedron formed by the cell unit). This last value is close to two ionic radii of oxygen, and this proximity between two oxygen isn’t uncommon for a shared edge by two coordinated adjacent polyhedrons. Even with the longer edges included, the average of 3.17Å for the distances of O-O between the different CO3 groups in aragonite is shorter than the shortest O-O (3.26Å) in octahedron CaO6 of calcite.
In conclusion, the greater density of aragonite in relation to calcite is due to the more efficient packing of the CO3groups, rather than the difference in Ca-O coordination. In this way, the shorter bonds of Ca-O in calcite are compensated by more compact groups of CO3 in aragonite.
