Towards an improved understanding of Ca-Mg carbonates with nonplanar surfaces: An experimental approach

Saddle dolomite is a Ca-Mg carbonate characterized by curved crystal faces, curved cleavage and sweeping extinction under cross-polarized light. Saddle dolomite and, generally, Ca-Mg carbonates with curved crystal faces, are often assigned to the burial (hydrothermal) realm and serve as proxy archives for hydrogeochemical processes in sedimentary basins and orogens. At present, the physicochemical conditions leading to the formation of these peculiar warped carbonates are unclear, and the mechanisms inducing the curvature of their crystal lattice are debated. This study uses laboratory experiments to assess the factors controlling the formation of carbonates with curved crystal faces as a function of fluid temperature, reactant size, fluid salinity and fluid Mg : Ca. Results show that a range of magnesium calcites with curved surfaces form at elevated temperatures (ca 220 degrees C) from calcium-rich fluids (Mg : Ca = 0.43) within a wide range of fluid salinities (5 to 40 wt.%). Magnesium calcites that nucleate epitaxially on rhombohedral or saddle dolomite substrates exhibit warped surfaces, while those that nucleate on calcite seeds form flat surfaces. Although the two crystal habits can co-occur, Mg-calcites with curved crystal faces (Mg : Ca of 0.35 to 0.40) tend to be more calcium-rich than those with flat faces (Mg : Ca of 0.58 to 0.74). In experiments with higher fluid temperature (230 degrees C), calcite reactants undergo replacement by dolomite exhibiting planar crystal faces. The results collectively indicate that the formation of Ca-Mg carbonates with warped surfaces is likely related to a combination of geochemical and physical parameters and various threshold limits, as opposed to one specific parameter, for example, elevated fluid temperature, as is commonly reported in the literature. Although most of the present experimental precipitates are disordered magnesian calcites with bent crystal faces, it is reasonable, at the level of a tentative working hypothesis, that these represent precursors of many ancient saddle dolomites commonly found in burial settings.