Representative archaeological finds of pietra ollare from Comacchio (Italy): identifying provenance and high-T mineral breakdown reactions hindering lithotype classification

Abstract

Three different lithotype groups have been recognized in representative fragments of pietra ollare artefacts found in the mediaeval archaeological excavations of Comacchio (Italy). Petrographic, mineralogical and chemical compositions clearly show that Valchiavenna and Valmalenco, in the Central Alps, represent the provenance areas of two of the pietra ollare groups, i.e. grey carbonate talc schist (also known as soapstone) and green chlorite schist, respectively. The third representative pietra ollare group consists of whitish-yellowish rocks inferred to be Alpine talc schists that have suffered some artificial pyrometamorphic transformation hindering their classification in the framework of pietra ollare lithotype categories and source localities reported in literature. A distinguishing feature of these high temperature-fired (HT-fired) talc schist samples is the occurrence of talc and chlorite dehydroxylation and subsolidus reaction products, mainly protoenstatite/enstatite + forsterite, together with relatively low loss on ignition values. These talc schists might have been exposed to temperatures >900 °C (with the resulting talc dehydroxylation producing a void-rich texture), but never reaching 1200 °C, which is the sintering temperature for soapstones. The whitish-yellowish artefacts were definitely not used as containers for cooking food at temperatures below or around 500–600 °C (as instead shown by the preservation of the original physical features of the grey pietra ollare lithotypes), but in some kind of higher-T pyrotechnological processes.