Red and yellow ochres from the archaeological site Pedra do Cantagalo I, in Piripiri, Piauí, Brazil

Abstract

The archaeological site Pedra do Cantagalo I is a sandstone shelter displaying rupestrian inscriptions. It is located in the rural area of the municipality of Piripiri, in the northern region of the Piauí state, Brazil. The site was found as being originally decorated with more than 1,900 prehistoric rupestrian paintings, along with engravings, lithics, ceramic fragments and mineral pigments forming reddish and yellowish ochres. Materials of these ochres, collected from recent excavations in this archaeological site, were analyzed by energy dispersive X-ray fluorescence (EDXRF); backscattering and transmission ⁵⁷Fe-Mössbauer spectroscopy at 298 K and 25 K and powder X-ray diffraction (XRD), in an effort to assess the chemical and mineralogical characteristics of these sources of pigments that composed the ancient paint materials of the site. The iron contents (expressed as Fe ₂ O ₃) for the reddish ochres were found to range from ∼60 to ∼68 mass%; for the yellowish ochres the corresponding content was ∼34 mass%, as determined by EDXRF. From the Mössbauer spectra for these red ochre samples, hematite (αFe ₂ O ₃) or a mixture of hematite and goethite (αFeOOH) were identified. Actually, the spectra at room temperature for the yellow ochres are rather complex, as it is usual for most soil clay materials. At least part of the intense central doublet was assumed to be from superparamagnetic iron oxides in very small particles. The Mössbauer patterns at 25 K allowed confirming this assumption, as the superparamagnetic relaxation effects were virtually suppressed; the spectral contributions due to goethite could be thus more easily separated.