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Combined experimental, modeling, and analytical results indicate that the rapid acidification of dilute waters in contact with nominally Fe 2+ -sulfate minerals (FeSO 4 ·nH 2 O) is caused by Fe 3+  hydrolysis, which occurs when low levels (&lt;1 mol%) of a contaminant Fe 3+ -sulfate phase are dissolved along with the FeSO 4 ·nH 2 O. This rapid acidification has previously been attributed to hydrolysis by Fe 2+ . However, dissolution experiments performed using ZnSO 4 ·nH 2 O, in which the Zn 2+  cation has a higher hydrolysis constant (log K = -8.96) than Fe 2+  (log K = -9.5), failed to produce significant changes in solution pH. We present the results of geochemical modeling simulations confirming that FeSO 4 ·nH 2 O dissolution alone cannot explain the experimentally observed change in pH from 5.65 to 3.50. Nor can the experimental observations be explained by oxidation of Fe 2+  to Fe 3+  in solution. Instead, our experimental results can be best explained by modeling the incorporation of &lt;1 mol% Fe 3+  contamination from any number of Fe 3+  or mixed valence Fe-sulfate phases, including anhydrous Fe 3+  2  (SO 4 ) 3 , coquimbite, kornelite, römerite, bilinite, copiapite, or ferricopiapite, all of which are reasonable candidate phases for oxidative breakdown products of FeSO 4 ·nH 2 O. Laboratory Mössbauer spectra are consistent with up to 0.6 mol% of the total Fe in the sample to be present as Fe 3+ . Although the doublet has parameters that are not diagnostic of any specific Fe 3+ -sulfate, they do help constrain its identification. These results demonstrate that minor contamination of labile Fe 2+  sulfates by Fe 3+  can have dramatic effects on solution chemistry that should be considered when studying reactions relevant to acid mine drainage waste sites and other localities where Fe-sulfate minerals occur, such as the surface of Mars.

Acid production by FeSO 4 ·nH 2 O dissolution and implications for terrestrial and martian aquatic systems