Rosickyite is a rare form of sulfur (γ-sulfur; monoclinic symmetry) that is not thermodynamically predicted to be stable at Earth's surface temperatures; instead, it reverts to the more common α-sulfur form (orthorhombic symmetry). Here we show, for the first time, that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of Death Valley, California. We hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum (CaSO4·2H2O) to sulfide and reoxidation of the sulfide to elemental sulfur (rosickyite) within a stable oxygen-sulfide gradient maintained by the organisms. Furthermore, we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community. Knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on Earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes.

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