Mg/Ca carbonates with curved crystal surfaces: an experimental study

Dolomite, CaMg(CO₃)₂ and related Mg/Ca carbonates (disordered calcian dolomite, high Mg calcite, and similar) are common rock-forming minerals that occur widely in numerous diagenetic settings. They represent pore-filling precipitates (cements), diagenetic replacement phases of precursor carbonates or hydrothermal/metamorphic phases. Dolomites crystallize in the space group R-3 with trigonal symmetry and quite typical rhombohedral growth habit. A unique subtype of dolomites are saddle-dolomites which are characterized by curved (warped) surfaces. These show sweeping extinction under cross-polarized light which points to internal lattice strain. Saddle dolomites are of significance because, based on empirical evidence, they are considered to be one of the key indicators of warm-to-hot hydrothermal fluids in the burial diagenetic realm. However, saddle dolomites are not present in all natural hydrothermal deposits, and where they occur, they may coexist with Mg/Ca carbonates that often (but not always) lack warped crystal surfaces. Thus, the physicochemical conditions that lead to their formation remain unclear. We have undertaken an experimental study to explore the growth of saddle dolomites systematically as a function of various parameters (temperature, crystal size of reactants, fluid salinity, and Mg/Ca ratio of solutions). It was found that newly formed dolomites that grow epitaxially on rhombohedral or saddle dolomites exhibit warped surfaces whereas Mg-rich calcite crystals that grow on calcite seeds form flat surfaces. High Mg-calcite that forms at high temperatures (~ 220 °C) from calcium-rich fluids (Mg/Ca = 0.43) also have curved surfaces. In all cases, crystals with curved surfaces are characterized by excess calcium (Mg/Ca ~ 0.31 – 0.35). These results suggest that curvature of high Mg calcites and dolomites are related to excess Ca in the structure that causes lattice strain. Availability of dolomite precursors on which the crystals can grow, environments that provide excess Ca (e.g. highly saline ones) and rapid growth kinetics at high temperatures seem to facilitate the formation of crystals with curved surfaces. Thus this study identifies some factors that control the formation of high Mg-carbonates with curved surfaces and will aid to anticipate settings where they may be expected to occur in nature and understand why dolomites with curved as well as flat surfaces may coexist.