Density functional theory calculations of equilibrium oxygen isotope fractionation between borate minerals and aqueous fluids

Borax, ulexite, and colemanite minerals are by far the most important economic source of boron and occur almost exclusively in nonmarine evaporite deposits. While much is known about the geological setting in which they are found, surprisingly little is known about the chemical and physical properties of the brines from which they are formed. Oxygen isotope studies have the potential to reveal important new information regarding borate formation, but unlike most other common oxygen-bearing salts precipitated from brines, there are no experimental data regarding the oxygen isotope fractionation factors between borates and brines. As a first attempt to address this gap in our understanding we have determined Δ18Oborate-water values between 0 and 100 °C using density functional theory calculations (DFT). These results predicted Δ18Oborate-water values of 12.87, 22.32, and 17.5‰ at 25 °C for borax, colemanite and ulexite, respectively.