After 8.4 yr at 25 °C under water, ferrihydrite was almost completely converted into a mixture of goethite and hematite. In contrast, aging in the presence of various clay minerals and gibbsite retarded (kaolinite, illite, smectite, gibbsite) or even blocked (soil smectite, allophane) transformation. The conversion rate was inversely related to the final Si concentration in solution. In addition, the presence of clay minerals resulted in higher hematite to goethite ratios, lower mean crystallite dimensions for both oxides, and the incorporation of Al into the goethite and hematite structures. Similar effects have been observed for coexisting goethite and hematite in soils.
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U. Schwertmann (1988) studied this question.