Phlogopite crystals from Caldara di Manziana (Italy) caused by sulphuric acid generated by microbial oxidation of H2S

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Marco E. Ciriotti
Messaggi: 31675
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Phlogopite crystals from Caldara di Manziana (Italy) caused by sulphuric acid generated by microbial oxidation of H2S

Messaggio da Marco E. Ciriotti » gio 20 mag, 2021 18:39

Referenza:
▪ Pinzari, F. & Cuadros, J. (2021): Unusual Perforations in Phlogopite Crystals from Caldara di Manziana (Italy) Caused by Sulphuric Acid Generated by Microbial Oxidation of H2S Emanations. Minerals, 11, 547.

Abstract:
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 sulphur-oxidizing 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.
Marco E. Ciriotti

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