EXPERIMENTAL TESTS OF THE EFFECTS OF AL SUBSTITUTION ON THE GOETHITE-WATER D/H FRACTIONATION FACTOR

Interpretation of the hydrogen and oxygen isotopic composition of goethite in terms of ancient water sources and temperatures depends upon knowledge of fractionation factors between goethite and water. Al is a common substituent in natural goethites (Schulze, 1984), and a solid solution mixing model has been proposed to assess the effect of the substitution on goethite-water isotopic fractionation (Yapp, 1993). Diaspore (a-AlOOH) is the isomorph of goethite, but its low temperature fractionation factors are not known experimentally. Thus, the mixing model applied the mineral-water fractionation factors of goethite (a-FeOOH) and boehmite (g-AlOOH) as endmember values. The model has been applied with apparent success in studies of natural goethites, but the model predictions had not been tested experimentally.

Goethite synthesis experiments were performed using two general approaches: (1) oxidation of aqueous Fe (II) to Fe (III) followed by hydrolysis to goethite; and (2) precipitation from iron (III) solutions (Schwertmann and Cornell, 2000). Experimental temperatures were either 25°C or 46°C. Experiments were performed at two different values of the final pH (6 and 14). XRD spectra indicate that goethite is the exclusive product of all the discussed synthesis experiments with the exception of two. These two exceptions also contain minor lepidocrocite. Al substitution in the goethite ranges from 0 to 12 mole %. SEM images of the synthetic goethite indicate that crystal sizes are less than 500nm for all samples. Experimental results for D/H are:

Method	pH	T (°C)	Al (%)	Predicted dD range	Measured dD range
Fe(II) to Fe (III)	6	46	0 to 12	12 permil	10(±2) permil
Fe (III) solution	14	46	0 to 5	5 permil	5 permil
Fe (III) solution	14	25	0 to 8	8 permil	9 permil

The measured effects of Al substitution on the goethite-water D/H fractionation factors are consistent with model predictions over the ranges of temperature, pH, and Al tested thus far. These results support use of the existing Al-derived correction of measured dD values to obtain the dD value of the endmember FeOOH component in natural goethites. The predicted effect of Al substitution on goethite-water oxygen isotopic fractionation is to be tested in future experimental studies.