Calcium-isotope fractionation between solution and solids with six, seven or eight oxygens bound to Ca(II)

The aqueous-mineral Δ44/40Casolid-solution fractionation factors were measured for equilibrating solutions and a set of inorganic solids that trap solvated calcium in six, seven and eight coordination as an intact solvated ion in the lattice. The measured values were then compared to Δ44/40Casolid-solution values predicted from electronic-structure calculations and vibrational frequencies of molecular models of the solvated ions and their solid hydrates. Experimentally, the Δ44/40Casolid-solution values differed by ∼2‰ for crystals of six- and seven-hydrated calcium, and by about ∼2‰ between crystals having seven- and eight-hydrated calcium. In comparison, the calculations predict Δ44/40Casolid-solution values between the sixfold- and eightfold-coordinated aquo ions of 5.4‰. Calculations predict 2.6‰ fractionation between the sixfold- and sevenfold-coordinated aquo ions. Stirred and unstirred experiments gave similar results in most, but not all cases. In general, measured isotopic fractionations compare well with the predictions from quantum mechanics and vibrational analysis. Isotopic fractionation is directly correlated with coordination number. We consider this to be a well-poised test of predictions.