A new approach to determine the geological provenance of variscite artifacts using the P/Al atomic ratios

Abstract

The procurement and exchange of variscite was an important part of Iberian 4th-to-2nd millennia political economy. For decades, archaeologists have sought to chemically characterize variscite deposits. However, these studies have met with limited success due to intrinsic limitations of trace element analyses of compositionally complex minerals such as aluminophosphates. Previous works by the author bring about a new approach to variscite provenance based on P/Al atomic ratio (Odriozola et al., J Archaeol Sci 37(12):3146–3157, 2010b). The goal of this research is to develop a solid technique to track archaeological variscite artifacts procurement areas using energy-dispersive X-ray fluorescence spectrometry (EDX), supported by X-ray diffraction (XRD) mineralogical identification and refined by magic angle spinning-nuclear magnetic resonance (MAS-NMR) structural characterization. The MAS-NMR analysis of the local structure of aluminum and phosphorus in natural aluminophosphates, show that sources and beads have two crystallographic sites for phosphorus and one for aluminum supporting that some Iberian green aluminophosphates may be considered anionic framework aluminophosphate crystals where the presence of phosphate (Q4) and hydroxyl groups linked to phosphorus (Q3) as in H2PO4 − anions make P/Al atomic ratio vary from unity. Therefore, the P/Al atomic ratio can be used to characterize variscite deposits, thus allowing us to link variscite artifacts to prehistoric mines. The method is tested for eight variscite mines successfully in defining provenance regions; and 19 artifacts analyzed from eight sites in Iberia fell into several distribution patterns of characterized source regions.