Iron-bearing minerals of a rupestrian painting from the Manantial Solís site, Cardiel Lake, Patagonia, Argentina

Abstract

The archaeological site Manantial Solís is located on shores of the Cardiel Lake, Santa Cruz Province, Patagonia, Argentina. The main characteristic of this site is the occurrence of 251 rupestrian paintings, namely abstract figures, anthropomorphic and zoomorphic motifs, painted in different hues of red, but also in yellow, orange, pink, violet and white; these paintings are often formed by overlapping pictorial motifs. The basaltic rock-wall supporting these graphisms has been naturally degraded and, as a consequence, covered with saline efflorescence. The chemical and mineralogical analyses of a micro-sample from a red rupestrian painting were made in the laboratory by (i) energy dispersive X-ray fluorescence; (ii) CHN elemental analysis and (iii) ⁵⁷Fe transmission Mössbauer spectroscopy at 298 K and 110 K, in an effort to identify the main iron-bearing minerals composing the painting pigments. The stratigraphy of the paint layer was investigated on a polished cross-section. From the Mössbauer spectra of this red painting, two sextets assignable to hematite (α Fe₂O₃) were identified. The Fe³⁺ central doublet is assumed to be due to superparamagnetic phases, more likely as iron (oxyhydr)oxides (likely including some goethite, α FeOOH) in very small particles and paramagnetic iron in the crystalline structure of aluminosilicates. The Fe²⁺ doublet was interpreted as being due to the basalt rock support of this prehistoric painting.