Electron density and optical anisotropy in rhombohedral carbonates. III. Synchrotron X-ray studies of CaCO₃, MgCO₃ and MnCO₃

Diffraction-deformation electron-density (Δp) images for small, naturally faced single crystals of synthetic calcite (CaCO 3 ), magnesite (MgCO 3 ) and mineral rhodochrosite (MnCO 3 ) were measured with focused λ = 0.7 and 0.9 A synchrotron (SR) X-radiation. Mo Kα (λ = 0.71073 A) structure factors were also measured for MnCO 3 . Lattice mode frequencies predicted from eigenvalues of T and L tensors for CO 3 rigid-group motion in these structures are close to spectroscopic values. High approximate Δp symmetry around the cations increases towards 6/mmm in the sequence CaCO 3 , MgCO 3 to MnCO 3 . The Δp topography near the CO 3 groups shows the influence of the cations, and correlates strongly with the refractive indices, as required for a cause and effect relationship between electron density and optical anisotropy. Aspherical electron density around the Mn atom can be attributed to the effect of a non-ideal octahedral crystal field on the 3d electron distribution. The relationship of the Δp topography near the Mn atom with that near the CO 3 group in MnCO 3 is consistent with magnetic interactions. Space group R3c, hexagonal, Z = 6, T= 295K : CaCO 3 , M r = 100.09, a = 4.988(2), c = 17.068(2) A, V = 367.8(3) A 3 , D x = 2.711 Mg m -3 , μ 0.7 = 1.93 mm -1 , F(000) = 300, R = 0.015, wR = 0.012, S = 3.0 for 437 unique reflections ; MgCO 3 , M r = 84.31, a = 4.632(1), c = 15.007 (2) A, V = 278.8 (2) A 3 , D x = 3.013 Mg m -3 , μ 0.9 = 0.99 mm -1 , F(000) = 252, R = 0.015, wR = 0.021, S = 4.34 for 270 unique reflections ; MnCO 3 , M r = 114.95, a = 4.772(3), c = 15.637 (3) A, V = 308.4 (4) A 3 , D x = 3.713 Mg m -3 , p 0.7 = 5.62 m -1 , F(000) = 330, R = 0.015, wR = 0.039, S = 3.38 for 386 unique reflections of the SR data set and a = 4.773 (1), c = 15.642(1) A, V = 308.6 (1) A 3 , D x = 3.711 Mg m -3 , μ(Mo Kα) = 5.86 mm -1 , R = 0.017, wR = 0.024, S = 2.79 for 368 unique Mo Kα reflections.