Trona (a hydrated sodium carbonate–bicarbonate salt) from the Eocene Wilkins Peak Member of the Green River Formation, Wyoming, contains micrometer-scale fibers and spherical bodies of presumed microbial origin. Upon dissolution in distilled water an insoluble residue (IR) is produced. A conspicuous feature of a portion of this IR is neutral buoyancy, which separates it from the denser IR fraction. When the micrometer-sized spherical bodies are seen, they are often found associated with the fibrous material. This association suggests the hypothesis that the neutrally buoyant fibrous material, which ‘floats’ just above the denser IR, is a desiccated biofilm.

To test this hypothesis, the fibrous IR was characterized using optical petrography, scanning electron microscopy, energy dispersive X-ray analysis (EDX), and X-ray diffraction crystallography (XRD). Petrographic analysis reveals abundant crystals, colorless in plane light, of sub-micrometer diameter and mm-scale lengths with a fibrous habit. These crystals exhibit parallel extinction and are length slow. EDX reveals a composition of Si, Al, O, Mg, Na and Ca. XRD indicates that these fibers are zeolite crystals, hydrous alumino-silicates of Na, Ca, and K.

Although exact species identification in zeolites is difficult owing to the similarity of their physical properties, the data suggest that these fibers are erionite, a mineral with a single six–fold ring of silica tetrahedra lying perpendicular to a triad axis. This arrangement produces an open cage-like structure, which, along with the presence of loosely bound water molecules, results in enhanced ion exchange and molecular retention. This increased chemical activity may facilitate the growth of microorganisms.