This is the first detailed account of the copper sulfate posnjakite (Cu4(SO4)(OH)6·H2O) coating cm-long filaments of a microbial consortium of four cyanobacteria and Herminiimonas arsenicoxydans. It was first observed on immersed plant leaves and stalks in a quarry sump of the abandoned Yanqul gold mine in the northern region of Oman; rock surfaces in the immediate vicinity show no immediate evidence of posnjakite. However, a thin unstructured layer without filaments but also containing the brightly coloured turquoise posnjakite covers ferruginous muds in the sump. Although copper is a potent bactericide, the microbes seem to survive even at the extreme heavy metal concentrations that commonly develop in the sump during the dry season (Cu2+. ≈. 2300. ppm; Zn2+ = 750. ppm; Fe2+. ≈. 120. ppm; Ni2+ = 37. ppm; Crtotal = 2.5. ppm; Cl- = 8250. ppm; and SO42- = 12,250. ppm; pH ∼2.6), thus leading to the precipitation of posnjakite over a large range of physicochemical conditions. Upon exposure to the prevailing arid climate, dehydration and carbonation quickly replace posnjakite with brochantite (Cu4(SO4)(OH)6) and malachite (Cu2(CO3)(OH)2). To characterise and understand the geochemical conditions in which posnjakite precipitates from undersaturated fluids (according to our thermodynamic modelling of the dominant elements), waters from rainy and dry periods were analysed together with various precipitates and compared with the observed field occurrences. The findings imply that posnjakite should not form in the examined environment through purely inorganic mechanisms and its origin must, therefore, be linked to the encountered microbial activities.
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