Towards a deeper understanding of the third millennium BC Iranian metallurgy: Use of synchrotron light for characterizing arsenic-bearing minerals in metal objects from Espidej

Abstract

New excavations at Espidej at the Kerman Province of the Halil Basin corridor in Iran offer a unique opportunity to reconsider the third millennium BC (i.e. Bronze Age) metallurgical practices related particularly to arsenical copper (Cu-As) alloying and to explore arsenic-bearing raw materials. This paper presents results of optical microscopy and environmental scanning electron microscopy (ESEM) on a selected group of copper-based artefacts from Espidej. Additionally, we have benefited from synchrotron light for further investigations on a dagger sample. The scientific examinations on metal corpora adds new information regarding the microchemistry and production techniques of metals of the south-eastern cultural zone of Iran. Synchrotron micro X-ray diffraction (SR-mu XRD) data of the sample demonstrates traces of arsenic-bearing minerals in the corrosion products indicative of types of ores used in alloying processes. Preliminary research on copper ores indicates possible extraction of local ore deposits that were outcropped along the south-east Makran orogeny zone of Iran. This area is part of hydrothermal mineralization zone consisting of arsenopyrite (FeAsS), sinnerite (Cu6As4S9), bornite (Cu5FeS4) and algodonite (Cu6As). Noticeable arsenic-bearing phases within the metallic core of the sample were frequently characterized as sinnerite and algodonite.