Photoacoustic, EPR and electrical conductivity investigations of three synthetic mineral pigments: Hematite, goethite and magnetite

Three iron oxide pigments: Hematite, goethite and magnetite, have been synthesized from the copperas by-product produced as a waste material in large quantities in the titanium dioxide plant in Chemical Works "Police," Poland. These pigments have been characterized by X-ray diffractometry and X-ray fluorescence spectroscopy and it has been found that they are mono-phase systems with only a small addition of other spurious phases. The electrical resistivity measurements reveals that the ion oxide samples are semiconductor materials. Conductivity of magnetite at room temperature is over six orders of magnitude greater than that of the other two materials. Photoacoustic (PA) spectra of hematite and goethite in the visual region of the electromagnetic radiation shows a single intense absorption peak centred at 558 and 480nm, respectively, while for the magnetite a continuous spectrum of increasing amplitude with increasing wavelength of the measurement is obtained. EPR spectra for the iron(III) complexes have been registered at room temperature which indicate hematite and goethite have only one similar EPR line centred at g ∼ 2 with different linewidths, while magnetite has two very intense overlapping and broad EPR lines centred at low and high magnetic resonance fields. Additional EPR line of Mn²⁺ ions has been also recorded for the hematite sample. Application of these pigments is discussed in terms of economic and environmental advantages.