Structural variation in the dolomite-ankerite solid-solution series: an X-ray, Mossbauer, and TEM study

Structural variation of the solid-solution series that extends from dolomite to ca. 70 mol% CaFe(CO₃)₂ has been examined by using several techniques. Single-crystal X-ray structure refinements for specimens containing 22, 50, and 68 mol% CaFe(CO₃)₂ demonstrate essentially full ordering of Ca and the remaining divalent cations. With increasing Fe content, the (Fe,Mg) octahedron dilates at a normal rate while the Ca octahedron shows a small contraction. ⁵⁷Fe Mossbauer spectra of four ankerites [containing 17, 29, 54, and 66 mol% CaFe(CO₃)₂] show only one moderately split quadrupole doublet with isomer shift of 1.24(1) mm/s. Transmission-electron-microscope images and electron-diffraction patterns are compatible with homogeneous microstructures in these samples, all of which have stoichiometric Ca contents. Domain microstructures suggested previously are not present. -from Authors