The Effect of Carboxylates on the Mg Content of Calcites that Transform from ACC

In some skeletal-forming and sedimentary environments, calcite and other CaCO3 polymorphs are produced from an amorphous calcium carbonate (ACC) phase. This experimental study determined the Mg content of calcites that grew in the presence of ACC with/without carboxylated biomolecules. The Mg content of the initial ACC obeys a simple fractionation for all conditions except in the presence of oxydiacetate. For all solution compositions, this ACC transforms into calcite crystallites that contain zero to 38 mole %MgCO3, without evidence of secondary polymorphs (Mg/Ca = 0-15). Mg is in the calcite structure within the resolution of the XRD method. The biomolecules slow the ACC to calcite transformation in proportion to their selectivity for Ca over Mg. Citrate, tartarate, and oxydiacetate increase the amount of Mg in both ACC and the resulting calcite. When the Mg2+/Ca2+ ratios of initial solutions are less than ∼5-8, the Mg content of the ACC and calcite products are similar.