Structural variation in the dolomite-ankerite solid-solution series; an X-ray, Moessbauer, 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 severaltechniques. 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. Both octahedral sites (3 symmetry) are trigonally elongated with this small distortion increasing only very slightly with Fe content.

⁵⁷Fe Möissbauesr pectra 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. Quadrupole splitting decreases only very slightly from 1.48 to 1.44 mm/s over this range of Fe contents. The Mössbauer data are consistent with Fe²⁺ in a single, slightly trigonally distorted, octahedral site whose degree of distortion remains very nearly constant with composrtron.

Transmission-electron-microscopeim agesa nd 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.

The factors that cause natural and synthetic ankerites with compositions exceeding ca. 70 mol% CaFe(CO₃)₂ to be unstable (relative to calcite + siderite solid solutions) cannot be obviously identified with any structural parameters so far investigated.