Unusual Perforations in Phlogopite Crystals from Caldara di Manziana (Italy) Caused by Sulphuric Acid Generated by Microbial Oxidation of H2S Emanations

Phlogopite flakes strewn on the soil of Caldara di Manziana (Italy) display multiple minute perforations. The site is a caldera linked to recent volcanism (90 ka 0.8 Ma) with present emanations CO2 (~150 t d−1) and H2S (~2.55 d−1). Stereomicroscopy SEM–EDX observation phlogopite crystals shows holes depressions <200 µm 2 mm across. They are circular, pseudo-hexagonal, or irregular. Within depressions, there deposits alteration products consistent sulphuric acid attack, showing loss Mg K. Some thin homogeneous; others thick, irregular, chemically heterogeneous, including plates, flakes, tubes Fe-beidellite Fe-bearing halloysite, silica, Fe oxides, gypsum. Areas surface also altered. Sulphuric produced from gas by mediation sulphur-oxidizing bacteria. Pseudo-hexagonal perforations interpreted result slow attack dissolution controlled crystal symmetry. developed surrounding films pseudo-hexagonal shape, as jarosite crystallizing radially outwards depressions. This style possibly promoted local high humidity precipitation that generate long-lived water droplets mineral surfaces where active for prolonged times.