Crystallization of Goethite and Hematite at 70°C

Abstract The concentration and acidity levels of iron(III) solutions are found to be interrelated in their effects on the formation of goethite and hematite at 70°C. A higher iron(III) concentration favors the formation of hematite, whereas a lower concentration favors goethite. For a given iron(III) concentration, the addition of acid favors the formation of hematite. A hypothesis is proposed to generalize the effects of these two factors. Hematite is slightly less soluble than goethite at 70°C. Hematite forms only in solutions that are understaurated with respect to goethite but supersaturated with respect to hematite. In solutions that are supersaturated with respect to both goethite and hematite, goethite will crystallize first. The continuing hydrolysis of the excess Fe 3+ results in a reduction in solution (Fe 3+ )(OH ‐ ) 3 product. When the solution becomes undersaturated with respect to goethite but supersaturated with respect to hematite, the latter begins to form. The goethite already formed changes to hematite at an extremely slow rate or not at all. Thus the relative abundance of goethite and hematite is mainly governed by the initial solution (Fe 3+ )(OH ‐ ) 3 relative to the solubility products of goethite and hematite.