Study of the in-situ thermal transformations of Limonite used as pigment coming from Peru

It has been performed a kinetic study of the thermal transformation of Limonite [FeO(OH).nH2O] by thermal gravimetric analysis (TGA), thermal X-ray Diffraction (XRD) and μ-Raman spectroscopy. The mineral was extracted from the Taraco District, Huancane Province, Puno region (Peru). Powdered XRD identify goethite phase as the main mineralogical component besides quartz. It was subjected to in-situ thermal treatment in the range of temperatures 100 to 500 degree centigrade in air and an inert atmosphere (nitrogen). The goethite phase remains stable from room temperature until 200 degree centigrade. From 250 degree centigrade there is a phase transformation α-FeO(OH) →α-Fe2O3 with 8 wt % weight loss and a chromatic change, i.e., the change of the hydroxylated goethite phase (yellow) to oxided hematite phase (red), as evidenced by the TGA and evolution of the XRD profiles. Moreover, the μ-Raman spectra of the in-situ thermal treatment corroborate that there is a transition at 290 degree centigrade by the transformation of the characteristic Raman bands of the goethite to hematite phase, in the frequency range from 200 to 1800cm-1. (Author)