Inhibition of Calcite Growth: Combined Effects of Mg2+ and SO42-

Magnesium and sulfate are each known to affect calcite growth and dissolution, but little is known about their combined effects on calcite growth rates. We grew calcite using the constant composition approach at ambient conditions, monitoring inhibition in solutions of Mg2+ and SO42- individually and together. The growth rate for pure calcite averaged 4.35 X 1.0(-6) mol 1;m(-2) s(-1) but decreased to 0.34, 0.16, and 0.08 X 10(-6) mol m(-2) s(-1) in solutions with 40 mM of SO42-, 13.3 mM of Mg2+, and 12.7 mM of MgSO4. We characterized the crystal form with scanning electron microscopy and atomic force microscopy. The {10 (1) over bar0} crystal, surface developed as the foreign ion concentration increased in the order SO42- < Mg2+ < MgSO4. Powder X-ray diffraction and X-ray photoelectron spectroscopy showed Mg incorporation of as much as 9.2 mol %. Mg2+ inhibits calcite growth more effectively when SO42- is also present, which we interpret to be the result of MgSO4 ion pair formation. Sulfate promotes Mg2+ dehydration, thereby allowing calcite uptake at lower temperatures. These results improve general understanding about the controls on biomineralisation and imply a need for re-examining the validity of the Mg/Ca thermometer, which uses the Mg composition in foraminifer for interpreting ancient seawater temperatures.