The effect of clay minerals on the formation of goethite and hematite fromferrihydrite after 16 years' ageing at 25 degrees C and pH 4-7

Tn soils and other weathering environments, hematite and goethite are usual ly formed in the presence of clay minerals. Two-line ferrihydrite was aged in the presence of six different clay minerals in aqueous suspension at 25 degrees C and pH 4, 5, 6 and 7 for 16 years. The transformation into hermat ite/goethite mixtures was complete in the systems with gibbsite, kaolinite, illite and smectite, but incomplete in those with soil smectite and alloph ane, Soil smectite and allophane, as well as increasing pH, favoured hemati te over goethite. The grainy and multi-domainic hematite crystals in the al lophane system contained Si and Al (EDX) and showed a Si-O vibration at sim ilar to 920 cm(-1) (FTIR). The unit-cell contraction due to structural Al w as counteracted by structural OH, which was removed at 200 degrees C. Heati ng also increased the magnetic hyperfine field at RT by 1.2-1.6 T. Hematite crystals with irregular shape and grainy interior have also been observed in soils.