XRD study of the goethite-hematite transformation: Application to the identification of heated prehistoric pigments

Abstract

When heated, yellow goethite dehydrates and transforms to red hematite. Both iron oxides were used by the Palaeolithic artists as pigments, one question being whether those people took advantage of the phase transformation. To answer this question, the dehydration of synthetic goethite was studied by XRD coupled to Rietveld refinement. It was shown that no hydroxylated hematite is formed during the early stages of dehydration, the presence of hydroxyl ions in materials treated at high temperatures being explained by trapped water inside porous microstructure (TEM). Archaeological samples from the south of France were investigated. Some of them exhibit distinctive features of heating which, supports the idea that Palaeolithic people used both natural and ex-goethite hematite.