THE EXISTENCE OF PROTODOLOMITE: IMPLICATIONS OF THE EXISTENCE OF NANOCRYSTALLINE HYDROUS CALCIUM MAGNESIUM CARBONATE PHASES

Hydrous calcium magnesium carbonate phases presents a feasible reaction pathway for the formation of sedimentary dolomite under certain conditions. Initial nucleation in such hydrous states helps overcome the kinetic barrier of dehydrating magnesium cations at low temperatures. Subsequent episodes of recrystallization form phases that are closer to thermodynamically ideal anhydrous dolomite. This represents dolomite formation via Ostwald Ripening, whereby the thermodynamic equilibrium state is reached via a succession of metastable precursors. Dolomite formation via Ostwald Ripening is not a new idea, and intermediaries with imperfect composition and ordering are well documented. However, if we extend it to include hydrous calcium magnesium carbonate, this model then covers all dolomite-like material, even that which occurs in the early stages of dolomitization. As such, the term 'protodolomite' might be considered redundant. All material with the approximate composition and structure of dolomite might be called dolomite, but importantly this should be supplemented by thorough characterization of samples, primarily in terms of composition (including Ca/Mg ratio, H₂O content and SO₄^2- content), cation ordering and particle size. Small structural differences may cause significant differences in properties such as solubility, so that knowing what a dolomite sample actually is is more important than what we call it.