ELECTRON-DENSITY AND OPTICAL ANISOTROPY IN RHOMBOHEDRAL CARBONATES .3. SYNCHROTRON X-RAY STUDIES OF CACO3, MGCO3 AND MNCO3

Diffraction-deformation electron-density (Delta rho) images for small, naturally faced single crystals of synthetic calcite (CaCO3), magnesi te (MgCO3) and mineral rhodochrosite (MnCO3) were measured with focuse d lambda = 0.7 and 0.9 Angstrom synchrotron (SR) X-radiation. Mo K alp ha (lambda = 0.71073 Angstrom) structure factors were also measured fo r MnCO3. Lattice mode frequencies predicted from eigenvalues of T and L tensors for CO3 rigid-group motion in these structures are close to spectroscopic values. High approximate Delta rho symmetry around the c ations increases towards 6/mmm in the sequence CaCO3, MgCO3 to MnCO3. The Delta rho topography near the CO3 groups shows the influence of th e cations, and correlates strongly with the refractive indices, as req uired for a cause and effect relationship between electron density and optical anisotropy. Aspherical electron density around the Mn atom ca n be attributed to the effect of a non-ideal octahedral crystal held o n the 3d electron distribution. The relationship of the Delta rho topo graphy near the Mn atom with that near the CO3 group in MnCO3 is consi stent with magnetic interactions. Space group R(3) over barc$ 3, hexag onal, Z = 6, T = 295 K: CaCO3, M(r) = 100.09, a = 4.988 (2), c = 17.06 8 (2) Angstrom, V = 367.8 (3) Angstrom(3), D-x = 2.711 Mg m(-3), mu(0. 7) = 1.93 mm(-1), F(000) = 300, R = 0.015, wR = 0.012, S = 3.0 for 437 unique reflections; MgCO3, M(r) = 84.31, a = 4.632 (1), c = 15.007 (2 ) Angstrom, V = 278.8 (2) Angstrom(3), D-x = 3.013 Mg m(-3), mu(0.9) = 0.99 mm(-1), F(000) = 252, R = 0.015, wR = 0.021, S = 4.34 for 270 un ique reflections; MnCO3, M(r) = 114.95, a = 4.772 (3), c = 15.637 (3) Angstrom, V = 308.4 (4) Angstrom(3), D-x = 3.713 Mg m(-3), mu(0.7) = 5 .62 mm(-1), F(000) = 330, R = 0.015, wR = 0.039, S = 3.38 for 386 uniq ue reflections of the SR data set and a = 4.773 (1), c = 15.642 (1) An gstrom, V = 308.6 (1) Angstrom(3), D-x = 3.711 Mg m(-3), mu(Mo K alpha ) = 5.86 mm(-1), R = 0.017, wR = 0.024, S = 2.79 for 368 unique Mo K a lpha reflections.