THE SOLUBILITY OF NATURAL AND SYNTHETIC JAROSITES AT 50°C

Jarosite is a hydrous sulfate mineral (KFe₃(SO₄)₂(OH)₆) found in many geological environments, and is of interest in many geological, environmental, and industrial fields. Natural jarosites are most noted for their occurrence in acid-sulfate rich environments such as acid mine drainage and acid-sulfate soils. Recently, jarosite has been identified in Terra Meridiani outcrops on Mars, raising interest in planetary geology and astrobiology. Several studies have addressed the thermodynamic properties of jarosite, however, more accurate thermodynamic data are needed to evaluate the conditions under which jarosites form and remain stable. This presentation will summarize our solubility experiments comprising synthetic and natural jarosites, performed at 50ºC. Jarosite was synthesized following the procedure outlined by Baron et al. (GCA, v.60, 1996), whereas natural samples were from mining districts in the vicinity of Chihuahua, Mexico and provided by Dr. V.W. Lueth (New Mexico Bureau of Geology and Mineral Resources). The solubility experiments were completed in Tetramethyl Ammonium-chloride (TMaCl) at ionic strengths of 0.1 and 0.3 m, over a pH range of 1.5 to 3.5. TMaCl was selected as the electrolyte media because the large cation could not substitute into the jarosite structure. Samples were withdrawn from the experimental solutions at regular intervals and analyzed for Fe, K, and SO₄ and pH. Over 400 days pH decreased, while the K and SO₄ concentrations increased in solution. Variable Fe concentration trends were also observed. For example, within the first 120 days iron concentrations increased rapidly for all the pH 2.5 to 3.5 samples, whereas iron decreased in the lower pH samples. Furthermore, for most natural samples iron concentrations increased during the early part of the experiment, then fluctuated towards the end of the study. Clearly, dissolution of jarosite is incongruent with various Fe-SO₄-oxy(hydr)oxide phases precipitating; these phases may then transform to more stable phases. From these results thermodynamic and/or kinetic data will be calculated for the solubility of jarosite. Research funded by: NSF grant # EAR-0230172