sp9b00020_si_002.pdf (65.87 kB)
Calculated Oxygen-Isotope Fractionations among Brucite, Portlandite, and Water
journal contribution
posted on 2019-05-22, 00:00 authored by Christopher A. Colla, William H. CaseyThe 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.