CHEMICAL, MINERALOGICAL AND ISOTOPE BEHAVIOR, AND PHASE TRANSFORMATION DURING THE PRECIPITATION OF CALCIUM CARBONATE MINERALS FROM INTERMEDIATE IONIC SOLUTION AT 25°C

Abstract

A set of time-series experiments was performed in which CaCO3 was precipitated (25°C, 1 atm) from aliquots of an NaHCO3 (0.330 M)/CaCl2 (0.0023 M) solution in a closed system to evaluate the effect of phase transformation on calcium carbonate precipitation kinetics and isotope behavior.Monohydrocalcite precipitated initially at a solution saturation state (Ωmhc) slightly greater than one, whereas calcite crystallized later at a significant higher saturation state (Ωcl # 14.4). The precipitation of calcite promoted the dissolution of monohydrocalcite at a rate that exceeded calcite precipitation, producing anomalous behavior in the chemical and isotope composition of the system.The carbon isotope fractionation factors (103 lnα) for monohydrocalcite-HCO3(aq)- and monohydrocalcite-CO2(g) were 0.36 +/- 0.01%% and 8.35 +/- 0.01%%, respectively. The oxygen isotope fractionation factor for monohydrocalcite-H2O was 27.8 +/- 0.1%%.The carbon isotope fractionation factors for calcite-HCO3 (aq)- and calcite-CO2(g) were 0.94 +/- 0.06%% and 8.93 +/- 0.06%%, respectively, whereas the oxygen isotope fractionation factor for calcite-H2O(l) was 28.0 +/- 0.2%%. A carbon isotope fractionation factor of 0.58 +/- 0.07%% was determined for the mineral pair calcite-monohydrocalcite, but no fractionation was observed for oxygen isotopes over time steps when both minerals co-precipitated.Fractionation factors for calcite were independent of precipitation rate over the range in rates of 103.96 to 105.63 μmol/m2h. These results extend the upper limit of characterization for the relationship between precipitation rate and isotope partitioning of carbon between calcite, HCO3 (aq)- and CO2(g), and quantitatively document for the first time the independence between precipitation rate and oxygen isotope partitioning in the calcite-H2O(l) system.