The four-center charge distribution of the carbonate ion and the lattice energies of calcite and aragonite

The charge distribution of carbonate ions in calcite and in aragonite have been calculated on the principle that the crystal experiences no net expansive or compressive force and no ion within it experiences a net force or couple. The charges q (in ‖e‖) for calcite are q (C) =+0.95±0.26, q (O) =−0.98±0.09 and for aragonite q (C) =+0.89, q (O1) =−0.94, and q (O2) =−0.98 (errors unknown). These estimates differ widely from those of other authors (not using the zero-force principle) but are in close accord with the best ab initio MO calculations currently available.