LATTICE-DYNAMICAL ESTIMATION OF ATOMIC DISPLACEMENT PARAMETERS IN CARBONATES - CALCITE AND ARAGONITE CACO3, DOLOMITE CAMG(CO3)(2) AND MAGNESITE MGCO3

Using crystallographic information and empirical potentials derived fr om fitting the vibrational frequencies of all the substances under stu dy, together with those of a group of silicates and oxides, a Born-von Karman rigid-ion lattice-dynamical model has been applied to the whol e Brillouin zone in calcite, aragonite (alpha- and beta-CaCO3, respect ively), magnesite (MgCO3) and dolomite [CaMg(CO3)(2)]. The Raman and I R spectra are satisfactorily reproduced and interpreted by these calcu lations; there is also very good agreement with atomic anisotropic dis placement parameters (a.d.p.'s) derived from accurate crystal structur e refinement by various authors and with the experimental values of th ermodynamic functions over a wide range of temperatures. On these vibr ational grounds, the stability of calcite with respect to aragonite at high temperature can be accounted for.