A Comparison of TEM Microstructures in Natural and Synthetic Dolomites

Despite well-documented textural, mineralogical, and geochemical similarities between high-temperature synthetic dolomites and low-temperature natural dolomites, there remains a lingering concern about the ability of laboratory studies to provide an analog for natural dolomites. In this study, high-temperature replacement dolomites formed in the laboratory are analyzed using transmission electron microscopy (TEM) and the results are compared to published observations from natural dolomites.

In previous studies, modern Ca-rich, poorly-ordered dolomites exhibit heterogeneous and patchy TEM microstructures with high densities of growth defects. Ancient Ca-rich dolomites also exhibit heterogeneous TEM microstructures, but are more modulated. Ancient stoichiometric, well-ordered dolomites, in contrast, exhibit more homogenous microstructures. TEM selected area diffraction (SAED) patterns are also different. Ancient stoichiometric, well-ordered dolomites have sharp, well-defined SAED diffraction spots, whereas Ca-rich dolomites have more diffuse diffraction spots.

Preliminary TEM results show both natural and synthetic dolomites have similar microstructural features. Poorly ordered and/or Ca-rich synthetic dolomites are characterized by heterogeneous and modulated TEM microstructures with diffuse SAED spots and diffraction rings in SAED patterns. In contrast, stoichiometric, well-ordered synthetic dolomites are more homogenous with better-defined SAED diffraction spots. The similarities between natural and synthetic dolomites reported here suggest that laboratory and natural dolomites likely form by similar crystal growth mechanisms, as suggested by previous studies.