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 O-18/O-16 fractionation factor between goethite and water (alpha(G-W)). A simple model was developed which expresses alpha(G-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 aG-W changes as pH changes is expected to be comparatively small (similar to 3 pH units) relative to the range of pH values over which goethite can crystallize (pH from similar to 1 to 14). Outside the range of sensitivity to pH, alpha(G-w) is predicted to be effectively constant (for constant temperature) at either a low-pH alpha(G-W) value or a high-pH aG-w value. It also indicates that the values of alpha(G-W) at high pH will be disequilibrium values. Values of ocG-w for goethite crystallized at low pH may approach, but probably do not attain, equilibrium values. For goethite synthesized at values of pH from similar to 1 to 2, data from two different laboratories define the following equation for the temperature dependence of 1000 In alpha(G-W) (Tin degrees Kelvin) [GRAPHICS] Over the range of temperatures from 0 to 120 degrees C, values of 1000ln alpha(G-W) from Eq. (IV) differ by <= 0.1 parts per thousand,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 In alpha(G-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 alpha(G-W) to pH over a larger range of pH values could explain the observation that Eq. (IV) yields values Of aG-W which mimic most O-18/O-16 fractionations measured to date in natural goethites. (c) 2006 Elsevier Inc. All rights reserved.