FE ISOTOPE FRACTIONATION AND KINETICS OF FE RELEASE DURING DISSOLUTION OF TWO VERY DIFFERENT AMPHIBOLES

Fifteen years ago, Don Rimstidt and colleagues conducted leaching experiments on crocidolite (blue asbestos) to investigate the kinetics of Fe release in the presence and absence of various Fe chelators (Werner et al., 1995, Amer. Min. 80:1093), in an effort to quantify how long Fe might be released from this carcinogenic mineral in the lungs.

Here we present results of new leaching experiments on crocidolite and hornblende in which we compare and contrast the behavior of these two very different amphiboles in terms of kinetics of Fe release and the Fe isotope compositions of the leachate solutions. Our experiments ranged from 4 to 70 days in duration and included either no Fe-chelating ligand or one of the following: acetic acid, oxalic acid, citric acid, or desferrioxamine B (DFOB).

We find that the two amphiboles yielded very different results. Fe concentrations in leachates of crocidolite were very low in our experiments (<5 micromolar), except for those containing DFOB, which had [Fe] approximately equal to [DFOB] at all time points (~20 micromolar). [Fe] did not vary with experimental duration. Where sufficient Fe was present for analysis, Fe isotope ratios were enriched in ⁵⁶Fe relative to the bulk mineral by up to 0.6 ‰ (Delta^56/54Fe). In contrast to the crocidolite leachates, Fe concentrations in the hornblende leachates rose steadily for about three weeks to a maximum of ~20 micromolar and then declined for all ligands, likely due to adsorption to the mineral surfaces or precipitation of ferric oxyhydroxides. Fe concentrations correlated with Fe-binding affinity of the ligands, but the differences in concentrations were slight. Fe isotopes in these solutions were all between -0.2 and +0.2 ‰ relative to bulk hornblende, indicating little fractionation during the leaching process.

Our results indicate that the dissolution mechanisms of these two amphiboles may be quite different and that the role of Fe-binding ligands during leaching is not the same for every mineral or every ligand. Further work to elucidate these mechanisms may be valuable both for understanding Fe release from carcinogenic minerals in lungs and Fe release from crustal rocks in the weathering environment.