Schwertmannite and the chemical modeling of iron in acid sulfate waters
- Ohio State Univ., Columbus, OH (United States)
- Technische Universitaet Muenchen, Freising-Weihenstephan (Germany)
- Institut fuer Hydrologie, Oberschleibheim (Germany)
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 Fe{sub 5}HO{sub 8}{center_dot}4H{sub 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 Fe{sub 8}O{sub 8}(OH){sub 6}SO{sub 4}) 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{sub 3}(OH){sub 6}(SO{sub 4}){sub 2}. A solubility window of log IAP{sub Sh} = 18.0 {plus_minus} 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}{sub Fe}{sup 3+} over the full range of drainage waters was consistent with published results form 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 schwertmannite 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 Fe{sup 3+} in the final equilibrium solutions yielded a log K{sub Gt} = 1.40 {plus_minus} 0.01 for goethite. Additional field data supporting a paragenetic relationship between jarosite, schwertmannite, ferrihydrite, and goethite were obtained form a naturally acid alpine stream. Similar results were predicted form the water chemistry using a nonequilibrium reaction path model that included appropriate solubility data for the mineral phases of interest. 33 refs., 10 figs., 1 tab.
- OSTI ID:
- 287412
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 12 Vol. 60; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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