Óxidos de ferro das frações areia e silte de um Nitossolo desenvolvido de basalto

The iron oxide mineralogy of sand, silt and clay fractions was studied in five soil materials. Samples were collected from a pedon of a Red Nitossolo (Brazilian Soil Classification; related to an Alfisol in US Soil Taxonomy) developing on a tholeiitic basalt, near the town Tupaciguara (18 o 35 ' 33 '' S; 48 o 42 ' 18 '' W) in the region of Triângulo Mineiro, state of Minas Gerais, Brazil. Portions of sand and silt were analyzed by magnetic separation and selective chemical treatments with NaOH 5 mol L-1, and with a dithionite-citrate-bicarbonate mixture to identify the main iron oxides and their mineralogical associations. Powder X-ray diffraction and Mossbauer spectroscopy analyses of these granulometric fractions revealed that maghemite (ideal formula, gFe2O3) was the only detectable magnetic mineral; no evidence for the existence of magnetite was found in the coarse fractions of this soil. Based on the Mossbauer data, the possibility of co-existence of more than one crystallochemical maghemite type in the sand fraction may be presumed, but no further analysis with applied magnetic field was performed to confirm this hypothesis. From the electronic microprobe, the magnetic phase with highest Al content presented a chemical composition that corresponds to the averaged formula Fe2,36(2)3+ Al0,24(2)3+ Ti0,06(3)4+ A0,341(1) O4 (A = cation vacancy). The ferric ilmenite found by the Mossbauer data in the magnetic part of the silt fraction is a solid solution (xFe2+Ti4+O3(1-x)Fe2 3+O3; x » 0,83). Some difficulties of interpretation and alternatives to results of the Mossbauer analysis are discussed for cases, as the present, where soil samples contain mineral mixtures of hematite and maghemite.