Formation of cubic phases on heating ferrihydrite

Abstract

AbstractHeating experiments were carried out with ferrihydrite, in the presence of organics, to gain more insight into the intermediate formation of a ferrimagnetic cubic phase often observed in soils as the result of fire. Without an organic reductant, no intermediate cubic phase was formed when ferrihydrite was transformed into hematite, regardless of whether the ferrihydrite DTA curve showed one or two exothermic peaks. Addition of glucose or charcoal as reductant caused the initial formation of a cubic magnetite and/or magnetite/maghemite phase in samples heated above 300°C in both air and N2 atmospheres. Ferrihydrite reduction, as determined by increasing cubic unit-cell edge lengths between 0.832 and 0.840 nm, increased with reductant concentration and heating time, providing excess reductant remained. Following consumption of the reductant, FeII may be partially or completely reoxidized, forming maghemite and then hematite. The occurrence of maghemite, but not of magnetite, in soils where forest fires have occurred suggests that sufficient oxygen was available, while temperatures remained elevated, to oxidize magnetite to maghemite.