Composition-induced microstructures in rhombohedral carbonates

Recent TEM observations of two-phase microstructures and associated crystal defects in selected, rare dolomites have been extended to calcite-structured (R3c) carbonates and to other natural and synthetic carbonates that crystallize with the dolomite (R3) structure. The samples included siderites (FeCO3), smithsonites (ZnCO3) , ankerites (Ca[Mg,Fe](CO3)/), and kutnahorites (Ca[Mn,Fe](CO3)2). TEM methods how that the forms of second phases which result from the presence of common, divalent, metallic impurities are morphologically similar in R3c and Rg carbonates and occur more widely than hitherto realized. The most common form consists of thin ribbons of second phase which are coherent with and have the same crystallographic orientation as the host carbonate. Another form of microstructure, manifest as modulations in diffraction contrast, appears to be associated with incipient breakdown of single-phase carbonate. The results of extensive TEM/EDS microanalyses show that in siderite and ankerite the formation of ribbons is promoted by Ca impurity or Ca excess (with respect to R3c stoichiometry). In smithsonite, Cu and Ca impurities can play similar roles in relation to modulated microstructures. In kutnahorites, the perfection of grains and the absence of second-phase effects is strongly dependent on the ratio of Fe to Mn but is also affected by Ca in excess of the stoichiometric requirement. Electron diffraction results from several of the minerals how c-type spots, which can be interpreted as indicating ordering within basal ayers of cations. The results how that, by correlating analytical TEM data with the study of second phases and incipient two-phase microstructures, it should be possible to determine the limits of solid solubility in carbonate systems