Chemical controls on the magnesium content of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase that forms in diverse biogeochemical settings. This material can incorporate significant amounts of magnesium and other elements, but the conditions that regulate composition are not established. Using a mixed flow reactor method, we synthesize Mg-free ACC (control) and amorphous magnesium calcium carbonate (ACMC) under controlled chemical conditions to determine the relationship between composition and inorganic solution chemistry. Input solutions contained a constant initial Mg/Ca ratio of 5/1 with variable total carbonate concentration, pH, and supersaturation. Within the reactor, input solution chemistry evolves in proportion to the extent of precipitation whereby the initial Mg/Ca ratio increases to values as high as 14 at steady state conditions. By this approach, we produce reproducible quantities of ACMC with 24 to >70 mol% Mg to give compositions of Mg((0.24-0.72))Ca((0.76-0.28))CO3.1.42-1.63H(2)O. The primary control on ACMC composition is the steady state solution composition that develops in the reactor during precipitation. Analysis of the data shows the Mg content of ACMC is regulated by the interplay of three factors at steady state conditions: (1) Mg/Ca ratio; (2) total carbonate concentration; and (3) solution pH. Using the Henderson Kracek model to estimate the partition coefficients for the Mg content of ACMC, we find K-D is approximately constant at 0.047 +/- 0.003 when steady state pH is less than 9.5, but values of K-D triple as steady state pH increases from 9.5 to 10.3. Our K-D values are lower than previous estimates that are based upon initial solution composition. In contrast, our estimates of K-D are determined from the solution chemistry at steady state conditions and for pH conditions that are less extreme than previous experimental studies. We suggest the approach of using steady state composition to estimate K-D gives a more accurate representation of relationships between ACMC composition and local conditions. The findings demonstrate local pH and total carbonate concentration can be regulated at the time of formation to produce Mg amorphous carbonates of a designated composition. Published by Elsevier Ltd.