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 which one component is a low-symmetry variant of dolomite. The various structural models proposed for the low-symmetry component involve Ca-Mg order patterns that are different from that in dolomite. Caution is therefore required for interpretation of X-ray diffraction data, which average over the entire structure. The average structures of two Ca-rich dolomites having a structural modulation are refined using single-crystal intensity data. The results indicate a poor fit using a dolomite model, which is consistent with the 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 provides some constraints on possible models for the low-symmetry domains.