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Project Progress

Year three accomplishments center primarily on the inorganic growth of Mg-bearing calcite from solution for the characterization of Mg-isotope fractionation relations in the calcite-aqueous Mg system. Pure calcite seed material was used as a substrate for the heterogeneous growth of Mg-calcite overgrowths from dilute solutions that contained dissolved MgCl₂, CaCl₂ and NaHCO₃. Large volumes (50 L) of concentrated Ca, Mg and HCO₃ stock solution (1 M) were made by dissolving the chloride and bicarbonate salts separately in CO₂-equilibrated (1 atm) de-ionized (DI) water. Small aliquots of these solutions were mixed in varying proportion with CO₂-equilibrated DI water to produce master solutions (500 mL) for individual experimental runs. The pH was controlled by bubbling a specified CO₂-N₂ gas mixture (350 ppm to 30% CO₂) through each solution at a specified temperature (10°, 22° or 40°C). Re-equilibration of the solution at the reduced partial pressure of CO₂ resulted in an increase in pH and the establishment of metastable supersaturated conditions with respect to Mg-calcite. Once pH was stabilized, pure calcite seed (~50 mg) was added to initiate the formation of Mg-calcite overgrowths. The pH was periodically monitored over the length of each experiment (1-3 days), and the duration of each experiment was selected to produce sufficient solid for Mg-isotope measurement while simultaneously maintaining solution chemistry within 10% of the initial solution composition. At the conclusion of each experiment, an aliquot of solution was filtered and immediately analyzed for Ca, Mg and alkalinity, and a second filtered sample was archived for Mg-isotope analysis. The remaining solid was separated by filtration, rinsed briefly in DI water and dried for further physicochemical characterization. Final solid weight was used to estimate the percentage of overgrowth for comparison to changes in solution chemistry. Aliquots of solid were characterized by x-ray diffraction and electron microprobe using EDS and WDS analytical techniques to determine the Mg-content of the overgrowth. The solid was also digested and analyzed for Ca and Mg content to confirm the mass and mole percent MgCO₃ content of the overgrowth.

To date, eight experiments have been completed, with five yielding homogenous Mg overgrowths (2 to 9 mole% MgCO₃) on calcite seed material (Fig. 1). In SEM images, Mg-overgrowths have rough crystal surfaces that display numerous growth steps and dislocations compared pure calcite overgrowths (Fig. 2).

In all of the experiments, the initial solution composition contained ~ 10 mmol Ca, ~45 mmol Mg and ~10 mmol HCO3. At 10°C, no Mg-overgrowth precipitated (Fig. 3), while at 22°C a ~2 mole% Mg overgrowth formed (Fig. 4), and at 40°C the same solution produced ~9 mole% MgCO₃ overgrowth (Fig. 5). Multiple WDS spot analyses of the overgrowths demonstrate that Mg is distributed homogeneously throughout the overgrowth in each case. Maintaining a constant chemical composition for the overgrowth is important because preliminary Mg-isotope analyses suggest that isotope fractionation relations may be critically dependent on the chemical composition of the solid phase.

XRD diffractograms are displayed in Figs. 6 – 8 for these same experimental runs. The overgrowth is identified by a shoulder that develops on the main (104) calcite reflection at ~ 29° 2θ as the percentage of overgrowth increases. The difference in 2θ between the pure calcite and Mg-calcite (104) peak provides an estimation of the mol% MgCO₃ in the overgrowth. These values are similar to those determined by WDS and EDs analysis and they are higher than measurements made by the bulk digestion of the solid phase.

Noteworthy results include:
1) Controlling the partial pressure of CO₂ in solution is an effective means of controlling pH during an experimental run.
2) The use of a relatively large amount of seed material and short reaction time results in the generation of sufficient overgrowth for physicochemical characterization without having the solution chemistry change more than 10% over the course of a run.
3) Temperature is an effective means of controlling precipitation rate and the percentage of overgrowth in an experimental run.
4) The use of calcite seed materials promotes the formation of Mg-calcite overgrowth without cross contamination from other phases such as aragonite or hydromagnesite.
5) The WDS/EDS results suggest that chemically homogeneous overgrowths can be generated using the experimental system described above.

These results will form the basis for a broader suite of experiments to be completed to more fully characterize the effect of mol% MgCO₃ on Mg isotope fractionation in the calcite-aqueous Mg system. Factors to be characterized include: 1) a wider range of mole% MgCO₃ in the overgrowth, 2) the effect of PCO₂, 3) the effect of precipitation rate, 4) the effect of temperature and 5) the effect of pH on Mg-isotope fractionation. If necessary, a more sophisticated experimental approach to precipitate a chemically homogeneous solid from a solution held under steady-state conditions (i.e., chemo-stat experiments) will be employed. In future years, the effect of additional cations (e.g., Fe, Mn) on Mg-isotope fractionation will be explored.