THE INFLUENCE OF PH ON THE SYNTHESIS OF MIXED FE-MN OXIDE MINERALS

Mn-substituted iron oxides were synthesized at pH 4, 6, 8, and 10 from Fe-Mn systems with Mn mole fractions (Mn/(Mn + Fe)) of 0, 0.2, 0.4, 0.6, 0.8, and 1.0, and kept at 50°C for 40 days. The Mn mole fraction in goethite was <0.07 at pH 4 but increased to ~0.47 at pH 6. Goethite and/or hematite formed in Fe and Fe + Mn syntheses at pH 4 and pH 6 at Mn mole fractions ≤0.8, and at Mn mole fractions ≤0.2 at pH 8 and pH 10. Hausmannite and jacobsite formed at pH 8 and pH 10 at Mn mole fractions ≥0.4. In the pure Mn syntheses, manganite (γ-MnOOH) formed at pH 4 and pH 6, whereas hausmannite (Mn3O4) formed at pH 8 and pH 10. As the Mn substitution increased, the unit-cell dimensions of goethite shifted toward those of groutite, and the mean crystallite dimensions of goethite decreased. -Authors