Hi Per, All constituents would go into solution, and what happens with them depends on what other constituents are in solution. It forms in highly acidic solution with high content of iron (ferric and and ferrous) and sulphate. Ferrous may be substituted with magnesium if the solution is high in magnesium. If it is in dissolving in reducing environments you may form H2S with many intermediate steps before the end product. Ferric to ferrous reduction could also take place. Both microbial mediated. You may form some other sulphate minerals as well. Again all depending upon the solution chemistry and the physiochemical conditions. Charlie Alpers (USGS) wrote a book (editor and author on some chapters) on jarosite minerals for the American Mineralogical Association short course handbook series. This may be a good source for more information. Ingar

Thank you Ingar. I agree with your general statements, and the low pH from copiapite dissolution certainly is partly from sulfuric acid, possibly in part derived also from SO2 or even H2S, depending on redox conditions. All copiapites carry an appreciable amount of Fe(III), so if precipitated as Fe(III)compund that will represent an additional source of H+ : Much more might be said (role of intermediate steps etc.). I will again look up in Alpers and others. Best regards Per

Hi Again, at pH 1-2 you would not get iron hydroxide precipitation unless the concentration is very high. However, the ferric iron will be efficient in sulphide oxidation forming ferrous iron. If the dissolution is becoming diluted e.g. rainwater you get the iron hydroxide (e.g. amorphous ferrihydrite) precipitating and buffering the pH. I am most of the time in Oslo these days so we can discussed the issues over a coffee one day. Regards

Ingar

Yes, indeed, Fe3+ is stable at such low pH, etc. Yet, I have seen rusty reaction products in a simple mainly static test I have done on pure copiapite (pH stabilised at pH = 2.75 and Eh (ORP) = +499 mV (oxidative, with double capped bottles). Coffee seems necessary!

In the case of mineral dissolution, you should consider the kinetics of the dissolution in addition to stoichiometry and thermodynamics of the reaction.