Kinetic study of the thermal transformation of limonite to hematite by X-ray diffraction, μ-Raman and Mossbauer spectroscopy

A kinetic study about the phase limonite (FeO(OH)-nH(2)O) was performed through X-ray diffraction, mu-Raman spectroscopy and Mossbauer spectroscopy. The oxide powder sample was extracted from Taraco district, Huancane province of Puno (Peru). X-ray diffraction identified the phase goethite as the main mineralogical component, and then the sample was subjected to in-situ heat treatment in the temperature range: 100 to 500 degrees C in oxidizing (air) and inert (nitrogen) atmospheres. The goethite phase remains stable in this range: room temperature to 200 degrees C. Between 200 degrees C to 250 degrees C there is a phase transition: alpha-Fe3+ O(OH) -> alpha-Fe2O3, i.e., from goethite to hematite phase, taking as evidence the evolution of the diffraction profiles. At 200 degrees C spectra shows the start of broadened magnetic component and it was adjusted through of a magnetic distribution giving a mean field of 38.6T and a relative area of 52.9%, which is a characteristic of goethite. Also, it is noticed the presence of a small amount of hematite with a mean field of 49.0T linked with a superparamagnetic broadened doublet of relative area of 47.1% where the domains of the particles have sizes smaller than 100 angstrom and it is evidence the superparamagnetic limit; i.e., the superparamagnetic effect tends toward a distribution of magnetic fields. Moreover, the Raman spectra of the in-situ thermal treatment, support the transition at 290 degrees C through the transformation of characteristic bands of goethite to hematite phase at the frequency range from 200 to 1,800 cm(-1).