SREL Reprint #2566

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

C. Jiménez-López1,2, E. Caballero1, F. J. Huertas1, and C. S. Romanek2

1Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas,
c/Profesor Albareda, 1, 18008 Granada, Spain
2Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802, USA

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)/CaC12 (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 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.

SREL Reprint #2566

Jiménez-López, C., E. Caballero, F. J. Huertas, and C. S. Romanek. 2001. Chemical, mineralogical and isotope behavior, and phase transformation during the precipitation of calcium carbonate minerals from intermediate ionic solution at 25°C. Geochimica et Cosmochimica Acta 65: 3219-3231.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).