Schwertmannite and the chemical modeling of iron in acid sulfate waters

Analyses of ochreous sediments and associated solutions from twenty-eight mine drainage sites showed that precipitates formed at pH 6.5 or higher were composed of ferrihydrite (nominally Fe5HO8 . 4H(2)O) or a mixture of ferrihydrite and goethite (alpha-FeOOH), whereas those precipitated from waters having pH values in the range of 2.8 to 4.5 were predominantly schwertmannite (ideally Fe8O8(OH)(6)SO4) with trace to minor amounts of goethite. Solutions of intermediate pH values produced mixtures of ferrihydrite and schwertmannite. Only one sample, formed at pH 2.6, contained a significant amount of jarosite (H, K, Na)Fe-3(OH)(6)(SO4)(2). A solubility window of log IAP(Sh) = 18.0 +/- 2.5 was calculated for schwertmannite from selected mine drainage solutions with pH values in the range of 2.8 to 3.2. The relationship between pH and log alpha(Fe)(3+) over the full range of drainage waters was consistent with published results from other sources, and the combined mineralogy-chemistry data were used to compute a new pe-pH diagram for the system Fe-S-K-O-H that included a field of metastability for schwertmannite. The metastable nature of schweamannite was confirmed in a long-term (1739 d) aqueous equilibrium study wherein a pure, synthetic specimen was completely transformed to goethite over a period of 543 days. The pH and computed activity of Fe3+ in the final equilibrium solutions yielded a log K-Gt = 1.40 +/- 0.01 for goethite. Additional field data supporting a paragenetic relationship between jarosite, schwertmannite, ferrihydrite, and goethite were obtained from a naturally acid alpine stream. Similar results were predicted from the water chemistry using a nonequilibrium reaction path model that included appropriate solubility data for the mineral phases of interest.