Jarosite dissolution rates and nanoscale mineralogy

Jarosite, (K,Na,H)Fe 3(SO4)2(OH)6, commonly occurs as an ephemeral phase in acidic, oxidizing, sulfate and iron-rich environments on Earth. The discovery of jarosite within deposits on Mars provides evidence for oxidizing, acidic fluids in these regions during the time of jarosite deposition. In addition, the preservation of jarosite over billions of years on Mars places constraints on the post-jarosite geochemical environment, including the duration of aqueous diagenesis. Observations of hematite spherules within the same outcrops may further constrain the chemistry of these diagenetic fluids. In this study, the effects of pH and temperature on jarosite initial dissolution rates and iron oxide reaction products were determined in the laboratory at initial pH 1–10 and temperatures from 277 to 323 K. Multivariate linear regression of the rate data shows that the dissolution rate varies with pH and temperature, resulting in E a = /C24 79 kJ/mol. At pH < 3.5, the rate increases with increasing aHþ, while at pH > 3.5 increasing aOH/C0 results in increased rates yielding a rate equation of rðmol m/C0 2 s/C0 1Þ¼ 10/C0 6:487a0:899 Hþ þ 10/C0 10:964a0:392 OH/C0 at 296 K. Transmission electron microscopy (TEM) reveals patchy jarosite dissolution textures, suggesting that these geochemical processes are crucial for understanding the environmental contexts of jarosite both on Earth and Mars.