Surface complexation clues to dolomite growth

Calcium and magnesium adsorb in near-stoichiometric proportions to dolomite over wide ranges in [Ca2+]/[Mg2+] ionic strength, and solution composition pointing to minimal mixing metal cations between the CaCO3 and MgCO3 layer edges exposed at the dolomite surface. Near-neutral pH Mg and Ca adsorb as hydrated ions, or, in sulfate-rich solutions, as metal sulfate complexes. Near-stoichiometric adsorption of Ca and Mg points to dehydration and subsequent carbonation of adsorbed Mg as the likely rate-limiting step for dolomite growth at near-Earth surface conditions. We propose that one path for dolomite growth from low-temperature natural waters is through the initial adsorption of Mg-sulfate complexes onto either (1) growing dolomite crystals or (2) rate-limiting dolomite nucleii. Field relations, as well as homogeneous syntheses at low temperatures (25 degrees C < T < 100 degrees C) support this hypothesis and provide a mechanistic explanation for dolomite growth from sulfate-rich natural waters.