Analysis of building stone materials used in Flavian Amphitheatre in Pozzuoli (Italy): types and provenance

This work is the result of the interaction between archeological and geological investigations on the Flavian Amphitheatre, one of the most important archeological sites in Campania region, situated in an area of great geological and archeological interest: the Campi Flegrei. The first aim of this research was to analyze stone material whenever employed for structural use, focusing on the type of materials and their provenance. After several surveys and photographic campaigns, considering the overall complexity of the monument and the presence of different geomaterials, our work has been focused on subterranuem corbels and on pillars of external arches that show an severe state of decay. Thanks to the permission allowed by Special Superintendence of Archeological Heritage of Naples and Pompeii it has been possibile to take out small non-invasive, but representative, samples of geomaterials in order to reach our characterization scopes and to provide useful information on probable future restoration activities. Mineralogical-petrographic characterization of samples taken by corbels and pillars have been performed mainly by X-ray powder diffraction (XRPD) and polarizing microscope analysis on thin sections (OM, optical microscopy). The macroscopic analysis suggested, as regards materials used for the abovementioned structures, an interleaving of Piperno (eutaxitic texture characterized by long dark grey lenses – fiamme – and alveolization, a peculiar alteration style of this specific rock) and lavas from the Campi Flegrei district (porphyritic texture, along with spalling and fracturing as weathering pathologies). This alternation was also confirmed by historical literature data (Maiuri A., 1955). On the contrary, detailed OM investigation on thin sections revealed mineralogical and textural characteristics typical just of Phlegrean lavas. From a comparison with available geo-petrographycal literature data (Melluso et al., 2012), samples taken from pillars revealed a similarity with lavas from the Monte Olibano dome, due to the precence of plagioclase and olivine xenocrysts, glomeroporphyritic clusters of clinopyroxene-plagiocase, or plagioclase and sanidine. On the other hand, the weakly porphyritic texture, and the presence of sodalite and sanidine phenocrysts in the samples from corbels suggested their provenence from one of the other rare lava outcrop in the Campi Flegrei: the Punta Marmolite dome. These findings (mineralogical and petrographic analysis vs. archeological descriptions) are clearly an example of what could be the results of a complementary study performed by different expertise, that could lead to more correct inputs in the framework of future restoration intervention, whenever any of these expertise provide their essential contribution in the field of diagnostics and conservation. Maiuri A. 1955. Studi e ricerche sull’anfiteatro Flavio puteolano, Memorie dell’accademia archeologia, lettere e belle arti- serie 3, Napoli. Melluso L., de Gennaro R., Fedele L, Franciosi L. Morra V. 2012. Evidence of crystallization in residual, Cl-F-rich, agpaitic trachyphonolitic magmas and primitive Mg-rich basalt-trachyphonolite interaction in the lava domes of the Phlegrean Fields (Italy); Geo. Mag 149, 532-550.