Computer Science
Scientific paper
Mar 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007gecoa..71.1115y&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 71, Issue 5, p. 1115-1129.
Computer Science
2
Scientific paper
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)1000lnα=1.66×10T-12.6 Over 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.
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