Oxygen isotopes in synthetic goethite and a model for the apparent pH dependence of goethite-water O/O 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 (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)

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