A VISUAL JOURNEY THROUGH DOLOMITIZATION: MINERALOGICAL AND TEXTURAL TRANSFORMATIONS OF CA-MG-CARBONATES DURING HIGH-TEMPERATURE EXPERIMENTS

Thick dolomite successions are common throughout the Phanerozoic rock record but absent in modern settings. Without direct observations, scientists rely on high-temperature laboratory experiments to explore the dolomitization process. Despite dolomitization temperatures higher than those inferred for their geological counterparts, there is significant chemical, mineralogical, and textural evidence to suggest that lab synthesized dolomites are useful proxies for natural dolomites. These experiments have shown that dolomitization takes place through a dissolution-precipitation process by which a Ca-carbonate precursor mineral is replaced by a succession of meta-stable Ca-Mg-carbonate mineral phases, ending with stoichiometric, ordered dolomite. Experimental studies most commonly present laboratory data as powder X-ray diffraction patterns (XRD) or reaction plots, which do not lend themselves to an intuitive concept of the dolomitization process. Because a picture is often worth a thousand words, the main objective of this study is to document the dolomitization process through a series of scanning electron microscope (SEM) images. By linking mineralogy and texture, this study attempts to provide a more visually intuitive understanding the process of dolomitization.

Dolomitization of powdered Iceland Spar calcite was accomplished in Mg-rich fluids at 190°C. Experiments were periodically ended during dolomitization and solid products were analyzed with SEM and XRD to create a high-resolution visual record of the stepwise reaction. The reaction begins as the calcite reactants start to dissolves, followed by precipitation of metastable, very high-Mg calcite (VHMC). These VHMC crystals grow in crystallographic continuity with the underlying calcite, forming thick layers as the reaction proceeds. As dissolution of the calcite precursor continues inward, a hollow shell of VHMC is left behind. Following the near-complete replacement of the calcite precursor, the dissolution-precipitation process is repeated as the metastable VHMC dissolves and poorly-ordered dolomite precipitates. The final stage of the dolomitization reaction is recrystallization, whereby poorly-ordered dolomite slowly dissolves at the expense of stoichiometric and well-ordered dolomite.