Green, grey and black: A comparative study of Sierra de las Navajas (Mexico) and Lipari (Italy) obsidians

Sierra de Las Navajas (State of Hidalgo, Mexico) and Lipari (Aeolian Islands, Italy) were among the most important sources for obsidian trade in Mesoamerica and in the Mediterranean during the Stone Age. In this paper obsidians from these two localities were compared in terms of their aspect, chemical composition, microcrystallinity and microvesiculation. In Sierra de las Navajas, the typical deep green obsidian with a golden hue has been analyzed together with a less common dark grey, porphyritic variety. Lipari obsidian is commonly black, but a light grey variety also occurs. Obsidian of both varieties was analyzed for this paper. Microvesicularity was investigated through Scanning Electron Microscope (SEM) observation, microporosimetry and, for Sierra de las Navajas green obsidian, through a preliminary X-ray computed microtomographic study. Crystallinity and micro- or nano-crystallinity were investigated through X-Ray Powder Diffraction and EDS (Energy Dispersion System) microanalyses. Finally, the chemical composition in terms of major and trace elements, including rare earth elements, was determined using X-Ray Fluorescence (XRF) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The comparison of samples of different colors suggests that the characteristic green color of Sierra de las Navajas obsidians could be related to their relatively high iron content, and to the occurrence of many elongated and iso-oriented vesicles which may also be responsible for the obsidian's golden hue. Low iron and an absence of vesicles give Lipari obsidian its "normal" black color. The light grey obsidian from Lipari probably owes its color and imperfect conchoidal fracture to numerous bubbles of less than 1 μm in size and to nano-crystallinity.Sierra de las Navajas obsidians show a significant chemical variability in terms of trace elements, that can be explained by common evolutionary processes in the magma chamber. However, this variability is also internal to a single volcanic complex and this makes the trace element contents unsuitable to differentiating between the different sub-sources of the same area. On Lipari, our data do not allow us to distinguish between the two sub-sources of Vallone del Gabellotto and Canneto Dentro on the basis of major and trace elements. On the whole, our study suggests that caution should be used for both Lipari and Sierra de las Navajas when identifying obsidian sub-sources on the basis of trace element contents.