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

Abstract

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 ( α G–W). A simple model was developed which expresses α 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 α G–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, α G–W is predicted to be effectively constant (for constant temperature) at either a low-pH α G–W value or a high-pH α G–W value. It also indicates that the values of α G–W at high pH will be disequilibrium values. Values of α G–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 α G–W ( T in degrees Kelvin) (IV) 1000 ln α G – W = 1.66 × 10 6 T 2 - 12.6 Over the range of temperatures from 0 to 120°C, values of 1000 ln α G–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 α 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 α G–W to pH over a larger range of pH values could explain the observation that Eq. (IV) yields values of α G–W which mimic most 18O/ 16O fractionations measured to date in natural goethites.