SCHWERTMANNITE SOLUBILITY

Schwertmannite, Fe₈O₈(OH)_8-2x(SO₄)_x, is a common mineral in acid sulfate environments. Reported equilibrium constants (K_s) for its solubility reaction,

Fe₈O₈(OH)_8-2x(SO₄)_x + (24-2x)H⁺ = 8Fe³⁺ + xSO₄^2- + (16-2x)H₂O (where ~0.5 < x < ~2.0),

span a wide range of values and are often used without regard to schwertmannite stoichiometry. In order to determine how K_s varies with schwertmannite stoichiometry, we compiled 23 analyses (9 new and 14 from the literature) of schwertmennite and coexisting aqueous solution from natural settings and used the PHREEQC model with the MINTEQ database to calculate log K_s for each solution composition. These calculated log K_s values vary systematically with schwertmannite sulfate contents (x). Linear regression of log K_s versus x gives the relationship: log K_s = 40.40 – 15.10x (R² = 0.68).

For these natural solutions, log (a_Fe(III)/a_H+³) changes systematically with pH. Plotting the solution compositions on a log (a_Fe(III)/a_H+³)–log (a_H+²a_SO4) diagram shows that the low pH the solutions are nearly in equilibrium with goethite and sulfate rich (x ≈ 2) schwertmannite and the high pH the solutions are nearly in equilibrium with ferrihydrite and sulfate poor (x ≈ 1) schwertmannite.