Electron optical investigation of sedimentary dolomites

Transmission electron microscope studies demonstrate a structural distinction between stoichiometric and ancient calcian dolomites of sedimentary origin. A modulated or tweed structure similar in appearance to spinodal microstructures characterizes ancient calcian dolomites, whereas stoichiometric dolomites are homogeneous. Recent calcium-rich dolomites lack a modulated structure, but have a heterogeneous microstructure with high densities of growth defects. Electron and optical diffraction and lattice imaging suggest a fluctuation in interplanar spacing in a coherent lattice. In addition, lattice imaging also reveals discontinuous faults throughout the modulated structure. Ordering and possibly incipient exsolution are suggested, c reflections in SAD patterns of calcian dolomites document a new superstructure in at least part of the lamellar structure and raise questions of different ordering states throughout the structure.The results provide new insight regarding the formation of sedimentary dolomites