Experimental tests of the effects of Al substitution on the goethite–water D/H fractionation factor

Abstract

Twelve goethite samples with different degrees of substitution of Al for Fe were synthesized at 22–48 °C and pH values of 1.5–14 under closed system conditions and used to study the effects of Al substitution on the hydrogen isotopic fractionation between goethite and its ambient water. The syntheses followed two pathways: (1) Fe 3+ hydrolysis in high pH aqueous solutions; (2) oxidation of Fe 2+ to Fe 3+ in mid to low pH solutions. XRD and SEM analyses indicated that, irrespective of temperature and pH, goethite was the predominant product of the syntheses in all of the experiments (with degrees of Al substitution as high as ∼13 mol %). “High temperature nonstoichiometric” (HTN) water is present in all of the samples and rapidly exchanges D/H with ambient vapor at room temperature. Uncertainties in the value of the apparent D/H fractionation factor ( α e–v) between HTN water and ambient exchange water at 22 °C lead to significant uncertainties in determinations of the δD values of structural hydrogen (δD s) in goethites which contain high proportions of HTN water. As determined for the samples of this study, α e–v has a nominal value of 0.942 (±0.02). δD s values determined using an α e–v value of 0.942 indicate that Al substitution increases the δD value of structural hydrogen in goethite by about 1.4 (±0.4)‰ for each increase in Al of 1 mol %. This dependence on Al is of the same sign as, but somewhat larger in magnitude than, the effect of Al predicted by a published model (∼0.7‰ per mol % Al). The overall uncertainties in the current results suggest that an increase of ∼1‰ per mol % Al, as adopted by previous studies, may be a reasonable estimate with which to adjust δ D s values of natural goethites to those of the pure FeOOH endmember and could be valid for degrees of Al substitution of up to at least 15 mol %. These synthesis experiments also yield a hydrogen isotopic fractionation factor ( D α G–W) between pure goethite (α-FeOOH) and liquid water of 0.900 (±0.006), which is analytically indistinguishable from the published value of 0.905 (±0.004). Thus, use of an D α G–W value of 0.905 in applications to the FeOOH component of natural goethites is supported by the current study.