The rhyolites of Tresnuraghes (North-West Sardinia) and their kaolin mineralization: microprobe study of alteration processes at microscopic scale.

At Tresnuraghes (NW Sardinia), a rhyolitic volcanic formation of oligo-miocenic age outcrops, and is affected by kaolinization phenomena. EPMA and SEM studies were performed to characterize textures and chemical composition of rocks and kaolinitic alteration. Analytic results show that: - color changes of the matrix correspond to compositional and structural variations; - the spherulitic structures ("piromeridi") range from microscopic size to 5-6 mn in diameter; - the alteration starts in the core of the structures showing a centrifugal trend; - the phenocrysts are generally plagioclases;