Constraints on cation order in calcium-rich sedimentary dolomite

Most sedimentary Ca-rich dolomite in pre-Holocene rocks is known to exhibit a fine-scale modulation in TEM images, reflecting a domain structure of wh ich one component is a low-symmetry variant of dolomite. The various struct ural models proposed for the low-symmetry component involve Ca-Mg order pat terns that are different from that in dolomite. Caution is therefore requir ed for interpretation of X-ray diffraction data, which average over the ent ire structure. The average structures of two Ca-rich dolomites having a str uctural modulation are refined using single-crystal intensity data. The res ults indicate a poor fit using a dolomite model, which is consistent with t he findings from TEM and electron diffraction indicating that one component of the domain structure is different than dolomite. Results also indicate that average model A and B cation sites have mixed Ca-Mg occupancy, which p rovides some constraints on possible models for the low-symmetry domains.