Inhibition of sintering by Si during the conversion of Si-rich ferrihydrite to hematite

The distribution and chemical state of Si in a synthetic 2-line ferrihydrite with a Si/(Si + Fe) molar ratio of 0.11 was studied. Heat treatment under oxidizing conditions shows that Si-rich ferrihydrite is stable to 400 degrees C. The transformation to hematite and the formation of a polymerized amorphous-silica phase occur at 850 degrees C. At this temperature, the specific surface area decreases greatly and the average pore diameter increases, which is indicative of sintering. Heating under severe reducing conditions causes a segregation of Si from Fe and results in a mixture of elemental Fe and SiO2. Surface and structural data suggest that Si is located near the particle surface where it Limits the rearrangement of Fe octahedra to form hematite.