Chemical and mineralogical characteristics of the pigments of archaeological rupestrian paintings from the Salao dos Indios site, in Piaui, Brazil

The archaeological site Salao dos Indios is located in the municipality of Castelo do Piaui, Brazil. The site is geographically part of a larger concentration of several archaeological sites of similar rupestrian graphical characteristics, including non-figurative paintings represented in a variety of colors. The work essentially devoted to identifying the mineralogical features of the pigments from this Salao dos Indios archaeological site is reported, basing on results from microscopic examinations and from physical and chemical analyses by electronically-and X ray-induced energy dispersive spectroscopies, Fourier transform infrared spectroscopy, powder X-ray diffraction and Fe-57 Mossbauer spectroscopy. The way by which the prehistorical painting pigments were prepared and applied over the rock wall was auxiliary investigated by examining samples under binocular lenses. Results reveal that the pigments were primarily prepared in the liquid or paste form, using iron-rich clayey materials. The Mossbauer data obtained for three samples of reddish rupestrian paintings showed that the dominant iron oxide in the pigment is hematite (alpha Fe2O3). The range of red hues of these rupestrian paintings can be explained by the different proportions of hematite that make up the layers of these prehistoric paints, with different mean particle sizes, reflecting fractions of particles undergoing different degrees of collective magnetic excitations as it can be drawn from the Mossbauer subspectra. Other iron-bearing species rendering spectral doublets were also observed, certainly due to superparamagnetic iron (oxyhydr) oxides (likely including some goethite) in very small particles or paramagnetic iron in the crystalline structure of phyllosilicates.