MOSSBAUER SPECTROSCOPIC EVIDENCE FOR CITRATE-BICARBONATE-DITHIONITE EXTRACTION OF MAGHEMITE FROM SOILS

In a previous paper, we used powder X-ray diffraction and changes in m agnetic susceptibility to argue the importance of pedogenic maghemite to soils and the efficacy of the chemical extractant citrate-bicarbona te-dithionite (CBD) to preferentially remove pedogenic maghemite from soil samples while not removing coarse-grained magnetite. Although X-r ay diffraction provides strong support for this contention, Mossbauer spectroscopy is the method of choice for determining the oxidation sta te of iron in minerals and for inferring mineralogy of the iron oxide phases. Our objective in this work was to seek confirming evidence of the importance of maghemite as a pedogenic mineral and the usefulness of the CBD procedure in separating pedogenic maghemite from lithogenic magnetite. We present Mossbauer data on magnetic fractions from pre- and post-CBD treated soil samples. Six of the 10 samples had only magh emite as the sextet component and after CBD treatment, four lost betwe en 96 and 100% of the magnetic susceptibility. Two samples were interp reted as highly oxidized magnetite or a mixture of magnetite and maghe mite. We cannot distinguish between these with Mossbauer spectroscopy. In the remaining two samples, iron existed as hematite, ilmenite, mag netite and minor (<10%) amounts of maghemite. Our results provide addi tional support for pedogenic maghemite in soils and for the preferenti al removal of maghemite by the CBD procedure.