Microbially influenced formation of anhydrite at low temperature

Calcium sulfate minerals are abundant in nature – on Earth and on Mars – and important in several fields of material sciences. With respect to gypsum and bassanite, anhydrite represents the anhydrous crystalline phase in the CaSO4–H2O system. Despite years of research, the formation of anhydrite in the laboratory at low temperature remains challenging and, in the geological record, this mineral is mostly interpreted as a secondary phase that form through metamorphic dehydration of gypsum. Here, we present the results of laboratory precipitation experiments showing that anhydrite can form at 35 °C from evaporated seawater through a microbially influenced mineralization process. The experiments were conducted in the presence of extracellular polymeric substances (EPS) produced by bacterial strains isolated from a modern evaporitic environment, the Dohat Faishakh sabkha in Qatar. Without organic molecules, only gypsum formed in parallel control experiments. This finding provides a possible explanation for the origin of several natural occurrences of anhydrite that cannot be satisfactorily explained by existing models and reveals a new precipitation pathway that may have industrial applications. [Display omitted] •The formation of gypsum and anhydrite is commonly linked to low (< 60°C) and high temperatures processes, respectively.•Microbially-produced EPS can cause anhydrite precipitation from evaporated seawater at 35°C.•This discovery has implications for interpreting the paleoenvironment of sulfate minerals on both Earth and Mars.