RATES, TEXTURES, AND NANO-MINERALOGY OF JAROSITE DISSOLUTION

Jarosite is an important metastable ferric sulfate in acid mine drainage systems, incorporating and later releasing heavy metals. Jarosite has also been observed in outcrops throughout Meridiani Planum by the MER Opportunity, leading to interpretations of widespread, though ephemeral, acidic fluids. Rates of jarosite dissolution have been measured under AMD and Mars-relevant conditions to determine the rate of iron and toxic metal release as well as constrain the lifetime of jarosite, and hence the maximum duration of water in jarosite-bearing sediments on Mars. Batch reaction dissolution experiments using synthetic K-endmember jarosite were conducted at 277-323 K in ultrapure water and oxalic acid. Initial rates range from log k= -7.1 at 323 K to -8.6 at 278 K, yielding an activation energy of 34.6 kJ/mol. Low-temperature (<300 K) experiments show a gradual decrease in rate over time; high temperature experiments exhibit a nearly linear increase in K⁺ concentration in the first few minutes, followed by a second phase of slower dissolution. TEM was used to analyze the texture and mineralogy of the reaction products. After several days at 273 K, the originally smooth jarosite spherules develop a surface texture of secondary precipitates indicative of incongruent dissolution with significant microporosity. Samples taken after a few hours of dissolution at 323K similarly show a vesicular texture where the outline of the original spherule is preserved and secondary precipitates have replaced much of the jarosite. Electron diffraction analysis suggests the reaction products at both temperature conditions are composed of nanocrystalline (1-50 nm) ferric hydroxide, hematite, and schwertmannite. The evolution of jarosite surface area during dissolution does not follow a simple relationship with time, and secondary precipitates contained within the original grain boundary likely will retain significant adsorbed or coprecipitated metals. Additional 323 K experiments in 0.01 and 0.001M oxalic acid yield rates of log k=-6.5 and -6.8, respectively. These higher rates suggest iron oxide reaction products prevent further dissolution of jarosite. The implications of incongruent dissolution and factors relating to ferric hydroxysulfate reaction products continue to be investigated.