New insight into structural and compositional variability in some ancient excess-Ca dolomite

Samples of non-stoichiometric dolomite of different age and origin have been studied by powder X-ray diffraction, scanning electron microscopy and electron-microprobe analysis. Most samples containing a high excess of Ca consist of two or even three phases, differing in the degree if excess Ca uptake and degree of structural order. On the basis of Rietveld refinements of samples with two 3D periodic phases, only the B sites have a mixed occupancy of Ca and Mg, and the phases differ from each other only in the content of Ca at the B site. A simulation of the experimental XRD patterns of samples showing a high and a low excess of Ca with a defective layer-structure was carried out in terms of mixed-layer structures in which different proportions of nonstoichiometric dolomite layers are interstratified with a small amount of calcite-like as well as stoichiometric dolomite layers. Electron-microprobe analysis and BSE images show that large crystals in the two-phase excess-Ca dolomite samples have sharply bounded zones with different levels of Ca concentrations. Each BSE contrast zone has the same composition (within error), and in each sample there are three or four non-overlapping groups of such zones differing in composition. Remarkably, fine-grained micritic portions of these samples contain the same discrete sets of Ca concentration zones as the large zoned euhedral crystals. This result supports the hypothesis that crystals of excess-Ca dolomite adopt only certain non-continuous compositions, and that growth and composition are structurally controlled.