Other
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2111b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 12, pp.2111-2121
Other
33
Scientific paper
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 5 HO 8 · 4H 2 O) or a mixture of ferrihydrite and goethite ( -FeOOH), whereas those precipitated from waters having pH values in the range of 2.8 to 4.5 were predominantly schwertmannite (ideally Fe 8 O 8 (OH) 6 SO 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 3 (OH) 6 (SO 4 ) 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 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 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 3+ 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.
Bigham Jerry M.
Schwertmann U.
Traina Samuel J.
Winland R. L.
Wolf Manfred
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