Recebido em 1/10/03; aceito em 13/7/04; publicado na web em 12/11/04 MAGNETIC IRON OXIDES FROM A TUFFITE FROM ALTO PARANAÍBA, STATE OF MINAS GERAIS, BRAZIL. The Mössbauer analysis along with the structural Rietveld refinement based on powder X-ray data for the magnetic fraction (saturation magnetization, σ = 19 J T -1 kg -1) separated from a tuffite material from Alto Paranaíba, state of Minas Gerais, Brazil, reveal that a (Ti, Mg)-rich maghemite (deduced σ = 17 J T -1 kg -1 ) and, for the first time observed in this lithodomain, magnesioferrite (characteristic σ = 21 J T -1 kg -1 ) respond for the magnetization of the rock material. Consistent models for the ionic distribution in these iron-rich spinel structures are proposed.

RESUMO

Recebido em 13/11/08; aceito em 3/4/09; publicado na web em 26/8/09 CHEMICAL AND MINERALOGICAL MECHANISMS RELATED TO THE TRANSFORMATION OF MAGNESIOFERRITE, IN A SOIL FORMING ON TUFFITE IN ALTO PARANAÍBA, MG. Magnetic soils forming on tuffite of the region of Alto Paranaíba, Minas Gerais, Brazil, usually contain iron-rich spinels exceptionally rich in magnesium and titanium. In this work, samples of the magnetically separated portion from the sand fraction of a Brunizém (Chernossolo) and from its mother-rock material were analyzed with synchrotron X-ray diffraction and 57 Fe-Mössbauer spectroscopy. Magnesioferite (MgFe 2 O 4 ) and maghemite (its pure nonstoichiometric spinel structure, Fe 8/3 ⊕ 1/3 O 4 , where ⊕ = cation vacancy, corresponds to γFe 2 O 3 ) were the magnetic iron oxides so identified. Basing on these data, a consistent chemical-mineralogical model is proposed for the main transformation steps involving these iron oxides in the pedosystem, starting on magnesioferrite to finally render hematite (aFe 2 O 3 ), passing through maghemite as an intermediate specie.