The Effects of Solution Chemistry on the Solubility of Synthetic Jarosite at 50°C: Implications for Terrestrial and Martian Environments

Jarosite, a hydrous sulfate mineral (KFe₃(SO₄)₂(OH)₆), is widespread on Earth. The mineral forms mainly in acid-sulfate rich environments; particularly associated with acid mine drainage (AMD) systems. The precipitation of jarosite in AMD systems and metallurgical processes is of interest due to the potential for jarosite to scavenge heavy metals. More recently, jarosite has been identified at the Terra Meridiani locality on Mars, by the Opportunity Exploration Rover.

            The stability of jarosite in terrestrial environments is fairly well established. In general, stability is limited to oxidizing and acidic systems. Moreover, literature data shows that hydronium jarosite ((H₃O)Fe₃(SO₄)₂(OH)₆) is significantly more soluble than end-member potassium jarosite. However, only a few jarosite dissolution studies have been completed. These studies were completed in pure water, for limited time periods. Thermodynamic data for the solubility of jarosite are, therefore, somewhat limited; making it difficult to evaluate the stability of the mineral under potential Martian conditions.

            This presentation will summarize our solubility experiments performed at 50°C, using synthetic jarosite that was reacted for over 900 days. Jarosite was synthesized following the procedure outlined by Baron et al. (GCA, v.60, 1996); then characterized extensively. The solubility experiments were completed in tetramethyl ammonium-chloride (TMA-Cl) and TMA-nitrate media, at ionic strengths of 0.1 and 0.3 m, over a pH range of 1.5 to 3.5. TMA was selected as the cation-electrolyte ion because of its large size, thus preventing substitution into the jarosite structure. Two anion electrolyte ions were selected to examine the effect of possible ion-pairing with Fe³⁺ on the solubility. Samples were withdrawn from the experimental solutions at regular intervals and analyzed for Fe, K, and SO₄, and pH was recorded at 50°C. Both ionic strength and solution pH had a significant effect on the solubility. Moreover, at low pH an iron-oxy(hydr)oxide phase precipitated