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