FERRIHYDRITE, LEPIDOCROCITE, AND GOETHITE IN COATINGS FROM EAST TEXASVERTIC SOILS

Iron oxide coatings from ped surfaces and pores of three rice paddy so ils and one non-paddy soil near Beaumont, TX, provided samples for stu dying Fe oxide mineralogy and the relationship of extractable Fe to ex tractable Al, Si, and P. Analyses of Fe oxide coatings, concentrated b y a combination of scraping, sonication, centrifugation, and magnetic separation, showed that these Fe oxide coatings cement clay minerals a nd quartz particles together. Iron oxide coatings in all soils contain ed lepidocrocite and smaller amounts of goethite. An improved differen tial x-ray diffraction (DXRD) method, in situ DXRD on a nonreflecting quartz plate, allowed the identification of ferrihydrite, which is oth erwise difficult to identify because of its poor crystallinity and low concentration. Most of the ferrihydrite was dissolved by 10 min in pH 3 ammonium oxalate in the dark (AOD). Sequential 10-, 50-, and 180-mi n AOD treatments extracted lepidocrocite of progressively increasing p article size. The 180-min AOD treatment extracted minimal amounts of g oethite. The DXRD and chemical data indicate that the paddy soils have more ferrihydrite in relation to total Fe oxides than the non-paddy s oil. The P/Fe ratio decreased with each AOD treatment step for all soi ls. This study demonstrates that physical separation and in situ AOD t reatment can be used with the DXRD method to identify ferrihydrite, le pidocrocite, and goethite in soil clays containing extractable Fe conc entrations as low as 10 to 44 g/kg. We concluded that a short incremen tal AOD treatment (approximate to 10-30 min) is a better approach to D XRD of ferrihydrite in <2-mu m fractions of seasonally reduced soils t han the much longer treatment recommended in the past for soils.