X-ray study of the electron density in calcite, CaCo₃

Abstract

The electron density in synthetic calcite, CaCO3, has been determined using diffraction data for a naturally faced single crystal measured with X-ray Mo Ka (A = 0.71073/~) 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 ,~-3 high-density maxima in the C--O bonds and 0.28 e A -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 R3c, hexagonal, Mr= 100.09, a=4.991 (2), c=17.062 (2) ,~, V= 368.1 (3) ,~3, Z = 6, Dx = 2.709 Mg m -3, /,t(Mo gee) =2.134mm -~, F(000) = 300, T=293 K, R=0.017, wR = 0.023, S = 4.52 for 328 unique reflections.