Oxygen isotopes in synthetic goethite and a model for the apparent pH dependence of goethite–water 18O/16O fractionation

Goethite synthesis experiments indicate that, in addition to temperature, pH can affect the measured value of the 18O/16O fractionation factor between goethite and water (αG–W). A simple model was developed which expresses αG–W in terms of kinetic parameters associated with the growth of goethite from aqueous solution. The model predicts that, at a particular temperature, the range of pH over which αG–W changes as pH changes is expected to be comparatively small (∼3 pH “units”) relative to the range of pH values over which goethite can crystallize (pH from ∼1 to 14). Outside the range of sensitivity to pH, αG–W is predicted to be effectively constant (for constant temperature) at either a low-pH αG–W value or a high-pH αG–W value. It also indicates that the values of αG–W at high pH will be disequilibrium values. Values of αG–W for goethite crystallized at low pH may approach, but probably do not attain, equilibrium values. For goethite synthesized at values of pH from ∼1 to 2, data from two different laboratories define the following equation for the temperature dependence of 1000 ln αG–W (T in degrees Kelvin)equation(IV)1000lnαG–W=1.66×106T2-12.6Over the range of temperatures from 0 to 120°C, values of 1000 ln αG–W from Eq. (IV) differ by ⩽0.1‰from those of a published equation [Yapp C.J., 1990. Oxygen isotopes in iron (III) oxides. 1. Mineral–water fractionation factors. Chem. Geol.85, 329–335]. Therefore, interpretations of data from natural goethites using the older equation are not changed by use of Eq. (IV). Data from a synthetic goethite suggest that the temperature dependence of 1000 ln αG–W at low pH as expressed in Eq. (IV) may be valid for values of pH up to at least 6. This result and the model prediction of an insensitivity of αG–W to pH over a larger range of pH values could explain the observation that Eq. (IV) yields values of αG–W which mimic most 18O/16O fractionations measured to date in natural goethites.