Abstract

The concentration of iron in Icelandic geothermal waters lies in the range of about 0.004–0.3 ppm. Saline waters are highest in iron. At temperatures above about 150°C, the dominant iron species in the water is Fe(OH)4− but Fe2+ and FeOH+ dominate at lower temperatures. Below 180°C the waters equilibrate with pyrrhotite and marcasite. At higher temperatures pyrite and anhydrite equilibrium is attained, but the waters become pyrrhotite-undersaturated. There are indications that the waters also equilibrate with various iron hydroxides depending on temperature. At the lowest temperatures, speciation calculations indicate equilibrium with amorphous ferric hydroxide which is replaced at successively higher temperatures with lepidocrocite, maghemite, geothite and hematite. With the exception of lepidocrocite, these minerals have been identified in hydrothermally altered rocks in Icelandic geothermal systems. Marcasite had not hitherto been reported, but a special search for it during the present study has revealed its presence. The ratio of (Fe2+)12/H+ and the concentration of H2S show a very good correlation with the temperature of the geothermal waters. This results from simultaneous equilibria with the above alteration minerals.

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