X-RAY STUDY OF THE ELECTRON-DENSITY IN CALCITE, CACO3

The electron density in synthetic calcite, CaCO3, has been determined using diffraction data for a naturally faced single crystal measured with X-ray Mo Kalpha (lambda = 0.71073 angstrom) radiation. Extinction corrections that minimize differences between equivalent reflection intensities are closely approximated by the values which optimize the extinction parameter as part of the least-squares structure refinement. Deformation electron densities evaluated with the two techniques are closely similar. There are 0.26 e angstrom-3 high-density maxima in the C-O bonds and 0.28 e angstrom-3 maxima at the O-atom lone pairs. There is no evidence in the least-squares parameters and deformation maps for deviation from CO3-group planarity or disorder. Space group R3cBAR, hexagonal, M(r) = 100.09, a = 4.991 (2), c = 17.062 (2) angstrom, V = 368.1 (3) angstrom3, Z = 6, D(x) = 2.709 Mg m-3, mu(Mo Kalpha) = 2.134 mm-1, F(000) = 300, T = 293 K, R = 0.017, wR = 0.023, S = 4.52 for 328 unique reflections.