Chemical, mineralogical and isotope behavior, and phase transformation during the precipitation of calcium carbonate minerals from intermediate ionic solution at 25°C

A set of time-series experiments was performed in which CaCO3 was precipitated (25 degreesC, 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 (fl,,h,) slightly greater than one, whereas calcite crystallized later at a significant higher saturation state (Omega (c1) congruent to 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 (10(3) ln alpha) for monohydrocalcite-HCO3(aq)- and monohydrocalcite CO2(g) were 0.36 +/- 0.01 parts per thousand and 8.35 +/-0.01 parts per thousand, respectively. The oxygen isotope fractionation factor for monohydrocalcite-H2O was 27.8 +/- 0.1 parts per thousand. The carbon isotope fractionation factors for calcite-HCO3(aq)- and calcite-CO2(g) were 0.94 +/- 0.06 parts per thousand and 8.93 +/- 0.06 parts per thousand, respectively, whereas the oxygen isotope fractionation factor for calcite-H2O(l) was 28.0 +/- 0.2 parts per thousand. A carbon isotope fractionation factor of 0.58 +/- 0.07 parts per thousand 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 10(3.96) to 10(5.63) mu mol/m(2)h h. 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. Copyright (C) 2001 Elsevier Science Ltd.