Mineralogical transformations in sandstone: a fingerprint for prehistorical heating of Palaeolithic hearth stones

At the prehistoric site of Rosalinde in Ruien (Belgium) sandstone blocks of local origin have been found in a configuration that was tentatively interpreted by the archaeologists as a hearth. These sandstones are identified as ferruginous sandstone and glauconiferous sandstone, and each block shows a discoloured rim which is not typically observed on these lithotypes. In order to confirm their use as hearth stones, the mineralogy in the discoloured rim was studied and compared to the bulk mineralogical composition by means of optical petrography, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and dithionite-citrate bicarbonate extraction (DCB). In the ferruginous sandstone, XRD and DSC illustrate the transformation of amorphous iron oxides and goethite into hematite. In the glauconiferous sandstone, however, the amount of iron oxide phases is too low for detection with XRD and DSC. In this case, the dark brown discolouration of the glauconite grains, together with the higher free iron oxide content in the rim than in the bulk as indicated by DCB analysis, are most suggestive of a mineralogical transformation due to heating. The heating temperature is estimated between 300 and 600 degrees C. Despite the absence of charcoal, the combined evidence obtained on the two lithotypes suggests the use of this specific stone arrangement as a hearth at the prehistoric site of Ruien.