Effects of sodium and potassium concentrations on dolomite formation rate, stoichiometry and crystallographic characteristics

Abstract

This study uses high temperature (215°C) dolomitization experiments to explore the effects of sodium (Na) and potassium (K), two common constituents of natural fluids, on dolomite formation rate, stoichiometry and crystallographic characteristics. In these experiments, aragonite ooids were dolomitized in Mg–Ca–Cl solutions with either no additional salt, or in solutions containing NaCl or KCl at different concentrations. Results show that Na solutions, and to a lesser extent K solutions, correlate with faster reaction rates and that Na at hypersaline fluid concentrations produces dolomites with higher stoichiometry, higher micro‐strain and lower cation ordering. Potassium has a different effect on the dolomite than Na at similar concentrations. Unlike Na, K does not cause micro‐strain but leads to dolomite with smaller unit cell parameters. It is proposed that Na and K catalyse dolomite precipitation and increase stoichiometry by weakening Mg hydration bonds which consequently facilitates Mg incorporation into dolomite. The decrease in dolomite cation ordering at higher Na concentrations may stem from the incorporation of Na into dolomite which strains the crystal lattice and reduces cation order. On the basis that high‐temperature experiments are applicable to natural dolomites, several implications pertinent to natural dolomites are drawn from the results. Firstly, the data suggest that the higher concentrations of Na and K in evaporative fluids (i.e. higher salinity) can explain why dolomite is generally more abundant and more stoichiometric in evaporative settings. Secondly, the results challenge a key prediction of the mixing zone dolomitization model by showing that higher Na and K concentrations increase, rather than decrease, both dolomitization rate and stoichiometry. Thirdly, the observed decrease in dolomite cation ordering with increasing Na and K concentrations implies that evaporative fluids would produce less stable dolomite that may be more prone to subsequent recrystallization and thus resetting of primary geochemical signatures.