Assessing the authenticity of a sample taken from the Altar Stone at Stonehenge in 1844 using portable XRF and automated SEM-EDS

Megalithic Stone 80 at Stonehenge, the so-called Altar Stone, is traditionally considered to be part of the bluestone assemblage, a diverse range of lithologies exotic to the Wiltshire Landscape. However, the Altar Stone, a grey-green micaceous sandstone, is anomalous when compared with the other (predominantly igneous) bluestones, in terms of its lithology, size and weight, and certainly in terms of its provenance. Recent investigations into the character of the Altar Stone have focussed on excavated fragments now attributed to be derived from the Altar Stone, as well as non-destructive portable XRF (pXRF) analysis on the Altar Stone itself (re-analysed as part of this investigation). In this study we have investigated a sample from the collections of Salisbury Museum, 2010K 240 (also referred to as Wilts 277), which bears a label recording that it was collected from the underside of the Altar Stone in 1844. We examined the sample petrographically and also by using pXRF and automated SEM-EDS techniques. Like the excavated fragments, this sample from the Altar Stone shows a distinctive mineralogy characterised by the presence of baryte and kaolinite along with abundant calcite cement. The presence of baryte leads to relatively high Ba being recorded during pXRF analysis (0.13 wt%). Combined, these results validate the history recorded on the specimen label and, as far as we know, makes this the only specimen taken purposely from that megalith. As such sample 2010K 240 provides a ‘go-to’ proxy for future studies of the Altar Stone as well as validating those samples recently assigned to the Altar Stone. In addition, this study demonstrates the vital importance of historic collection specimens and their preservation, conservation and documentation, as well as the role pXRF can play in the analysis of sensitive cultural artefacts and monuments that cannot be analysed using invasive or destructive techniques.