Abstract
I n this study we applied petrochemical methods (SEM-EDS; FT-IR) in order to characterize a group of obsidian flakes collected at Ustica island (Sicily). Despite the absence of obsidian geological outcrops, a lot of obsidian fragments still emerging from the lands of Ustica testify that the island was a major import center of obsidian during the prehistory. On this island, there are some prehistoric settlements, dated from the Neolithic to the Middle Bronze Age (6000- 1200 BC), in which the use of obsidian continued until the beginning of metals age. Our study includes: i) Macroscopic and microscopic optical observations, which allowed selecting 18 obsidian flakes (starting from 50 obsidian flakes) on the base of their morphological characteristics. ii) Density measurements (hydrostatic balance). iii) Scanning electron microscope determination (SEM-EDS) of major elements of the obsidian glasses and minerals. Results of our analyses were compared with 12 geological samples collected in obsidian sources from Monte Arci (Sardinia), Palmarola, Lipari and Pantelleria, i.e. the four most exploited obsidian sources of the ancient world in the Western-Central Mediterranean. This study confirms the presence of the Lipari and Pantelleria sources (Sicily) in our obsidian set. iv) We also determined (by FT-IR) the hydration degree of some obsidian flakes in order to detect a possible hydration gradient between the rim and the core of the flake sample. The width of the hydration rim, if any, can be used for an approximate evaluation of the age of the tool.
Highlights
Obsidians are volcanic glass, generally black to grey or green in color, originated by rapid cooling of viscous lava, a process that inhibits or minimizes crystal growth
Besides tracking back obsidian tool fragments to their original sources, in this paper we present results of a Fourier Trasformate-Infrared Spectroscopy (FT-IR) study on the hydration of edges of obsidian cutting tools
We examined a group of 10 geological samples collected in situ by the authors in geological obsidian sources: (i) Pantelleria (1 sample from Salto la Vecchia), (ii) Sardinia (3 samples from western side of Monte Arci, near Marrubiu), (iii) Lipari (3 samples from Pomiciazzo, Forgia Vecchia, Rocche Rosse), (iv) Palmarola (3 samples from La Radica- Central-east coast)
Summary
Generally black to grey or green in color, originated by rapid cooling of viscous lava, a process that inhibits or minimizes crystal growth. Their silica content is high (SiO2 = 65% to 80%, trachytic to rhyolitic compositions), and commonly H2O poor (< 1.0 %). 50, were recovered in the proximity of Ustica’s prehistoric settlements and were analyzed for their petrochemical characterization (SEM-EDS), in comparison with obsidians directly sampled in the four major outcrops of western Mediterranean exploited during the prehistory. SEM-EDS analysis of the major elements has been performed both on the vitreous mass of each obsidian sample and on the microphenocrysts present in it, i.e.: pyroxenes, biotites, feldspar, etc. As some authors have shown, the integration of all these analytical data offers more reliable criteria for characterizing the original sources [Acquafredda and Paglionico, 2004]
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