EFFECTS OF ACTIVITY OF WATER ON THE DISSOLUTION RATES OF K-JAROSITE

Jarosite, ((K, H, Na)Fe₃(SO₄)₃(OH)₆), is a ferric sulfate salt that forms both on Earth and on Mars. The presence of jarosite provides evidence of liquid water and can supply valuable information on the geochemistry of formation fluids and environments. Models for Martian jarosite formation are largely based off of terrestrial analogs and vary widely, including acid saline lake environments, where jarosite is known to form on Earth. In order to better understand the conditions necessary for jarosite formation and dissolution, the reactivity of jarosite in solutions with varying ionic strength, and hence activity of water, are reported here.

Mars brines associated with jarosite-bearing sediments have been estimated to contain Fe₂(SO₄)₃, MgSO₄, AlCl₃, Na₂SO₄, CaCl₂, MnSO₄ and KH₂PO₄ at low pH, based on APEX spectra of outcrops at Meridiani Planum, Mars. Dissolution rates of K-jarosite were measured in 3 different fluids (ultra pure water, NaCl-saturated brine, and CaCl₂ –saturated brine) covering a range of water activity (αH₂O of 1.0, 0.75, and 0.35 respectively) in batch reactor experiments. 0.1g of K-jarosite was mixed with 100mL of each fluid and10 mL samples were removed at predetermined intervals over several hours and passed through a 0.2 μm filter. The aqueous samples were then diluted 1:8 and K⁺ concentrations analyzed using matrix-matched Flame Atomic Absorption Spectrophotometry.

Preliminary data show the rate of jarosite dissolution is affected by the αH₂O. The average dissolution rate for the pure water experiment was approximately 4.21x10^-9 mol/m²/s. The K-jarosite dissolution experiments in NaCl brine (αH₂O = 0.75) produced an average rate of 9.27x10^-11 mol/m²/s, while CaCl₂ had the slowest of the three dissolution rates with an average rate of about 5.51x10^-11 mol/m²/s. These results show that the dissolution rates of K-jarosite decrease as the αH₂O decreases. Therefore, jarosite lifetimes are extended in high salinity systems, allowing for longer periods of aqueous alteration at Meridiani Planum.