Published online by Cambridge University Press: 05 July 2018
Ferric precipitates in the water column at the San Telmo acidic mine pit lake in the Iberian Pyrite Belt, southwest Spain, have been studied by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction, X-ray fluorescence, inductively coupled plasma massspectrometry and other complementary techniques. These Fe(III) precipitates were recovered from sediment traps which were left at different depths (25, 35, 40 and 100 m) in the lake for several months. Seasonal variations in the water chemistry were recorded to link the mineralogical findingsto vertical and temporal changes in aqueous composition. The results indicate that schwertmannite is the first Fe(III) mineral to crystallize after the oxidation of Fe(II), in agreement with previous studies. Schwertmannite is kinetically favoured in comparison to other Fe(III) minerals, andit buffers the pH at 2.6–3.0. It is metastable, and alters to a (H3O+)- and (K+)-bearing jarosite (containing 58 mol.% H3O+ and 42 mol.% K+ on average) at lower pH (e.g. at pH 2.2–2.5 in the summer season), eitherin the water column (during settling) and/or in the benthic sediments, in a time period of weeks to months. The extent of hydronium substitution at the alkali site in the jarosite reflects the higher activity of free aqueous protons in solution (10–2.2 to 10–3.0)in comparison to the activities of K+ (10–4.5) and Na+ (10–3.2). Microscopic examination of mixed schwertmannite–jarosite precipitates found in the water column suggest that some textural and compositional features of metastableschwertmannite (e.g. the internal 'pincushion' arrangement and incorporation of trace amounts of Mg, Al, As and Pb) are conserved in the jarosite during the early stages of the mineralogical transformation, but many of these relics are lost in the later stages of crystal growth. Despite thehydronium-rich nature of the jarosite solid solution, this material is also an important sink for K+, which decreases in concentration with decreasing pH unlike most of the other major cations in the water column (notably Na+, Mg2+, Ca2+, Al3+,Fe3+, Cu2+, Zn2+). In addition to the release of Fe3+ to the aqueous solution, the conversion of schwertmannite to (H3O+, K+)-bearing jarosite consumes protons and thus may represent an additional pH control at SanTelmo and other acidic mine pit lakes of the area.