The main objective of this work was to characterize the magnetic minerals and to identify their pedogenic transformation on a steatite-forming soil of Minas Gerais, Brazil. The iron-rich spinel phase was characterized by chemical analysis, powder X-ray diffraction, Mössbauer spectroscopy, with and without an externally applied magnetic field of 6 tesla, and saturation magnetization measurements. Nearly stoichiometric and well-crystallized magnetite was the only magnetic mineral actually detected. The cubic unit cell parameter of the fresh rock magnetite was found to be a o = 0.8407(5) nm. Hematite (hexagonal cell; a = 0.5036(3) nm, c = 1.375(4) nm) was detected in the altered rock and in the sand-soil and silt-soil fractions. Magnetite is assumed to transform into hematite during pedogenesis through progressive oxidation of structural Fe2+ to Fe3+. In partially oxidized magnetites, a relatively small proportion of Fe3+ was interpreted as being uncoupled from the Fe2+-Fe3+ charge transfer system, in octahedral sites of the spinel structure. Compositional formulae of magnetite with different degrees of non-stoichiometry are proposed. Ilmenite was found in minor proportions in the magnetically extracted portions from both rock and soil samples.
Iron-rich spinel; iron oxide; soapstone; magnetic soil; Mössbauer spectroscopy