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All natural carbonate minerals exhibit crystal defects, either in terms of nonstoichoimetry or structural imperfections. These defects represent a record of the crystallization (or in some places post-crystallization) process and thus provide direct evidence concerning various diagenetic reactions. The characterization of these defects is facilitated in many ways by use of the transmission electron microscope (TEM) and its various modifications.
The diffraction contrast mechanism allows certain defects not visible in other microscopes (e.g., domain structures, stacking faults, modulated structures, etc) to be imaged with very high resolution (approximately 20 A). High-resolution electron microscopy (HREM) utilizes a phase contrast mechanism and is capable of imaging individual lattice planes with a resolution better than 2 A. Depending on the particular planes imaged, information about local ordering and stacking defects is obtained, aspects
especially useful in studying dolomites and magnesian calcites. The capability for simultaneous electron diffraction from regions less than 1 µm allows for identification of phases and evaluation of short-range ordering.
In addition, many modern TEMs have scanning devices allowing scanning-transmission images (STEM) as well as the more conventional secondary-electron images to be formed. The use of energy-dispersive X-ray spectrometers is optimized with such STEM instruments as spatial resolution of roughly 150 A is possible.
Recent studies of carbonate minerals using these techniques have revealed a wide variety of defect microstructures characteristic of different growth and diagenetic conditions. In many places, the microstructures are present in such high densities as to significantly influence the stability and reactivity of the mineral.
Notable examples include the calcium-rich dolomites, both from ancient rocks and Holocene sediments. The ancient calcian dolomites exhibit a complex modulated structure with growth and possibly transformation-induced defects. Holocene dolomites are structurally distinct, characterized by a high-density domain structure associated with growth faults. Pervasive, complex microstructures have also been found in saddle dolomites, magnesian calcites, and low-Mg calcites. In the latter, the defect structure is thought to be associated with local CO3 group disorder.
Since the microstructures are characteristic of different diagenetic growth conditions, their proper characterization and interpretation represents a potential tool for hydrocarbon exploration in carbonate terrains.
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