Calculated Oxygen-Isotope Fractionations among Brucite, Portlandite, and Water

2019-05-22T00:00:00Z (GMT) by Christopher A. Colla William H. Casey
The oxygen-isotope fractionations between brucite and water, portlandite and water, and brucite and portlandite have been calculated over the temperature range of 0 to 450 °C using quantum-chemical methods and several basis sets and functionals. The calculations also employ embedded clusters that are chosen using the Pauling-bond-strength-conserving termination method that maintains a neutral cluster with fractional charges assigned to terminal atoms. These calculations improve upon the previous semiempirical methods for predicting mineral–mineral fractionations. These semiempirical methods fail to accurately predict the relative enrichment and depletion of oxygen isotopes for the brucite–portlandite pair. The quantum calculations presented here also fail to predict at the absolute values for enrichment of oxygen isotopes between minerals and water, and a simple correction must be employed to achieve agreement with experiments if water is in the reaction. No such correction is needed to predict fractionation between minerals. The trends derived from the calculations are robust to changes in basis sets and functionals.