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Organics and travertine precipitation


, : Organics and travertine precipitation. Abstracts with Programs - Geological Society of America 31(7): 323

The role of organic material and bacteria in precipitation of travertine was studied in samples from Le Zitelle hot springs near Viterbo, Italy. Samples were collected at regular intervals downstream from the spring orifice. Water conditions vary from 62.0 degrees C/pH 6.19 at the orifice to 51.2 degrees C/pH 7.06. Precipitation of calcite and aragonite where acidic pH should preclude it suggests that bacteria and organic matter effect travertine precipitation. Furthermore, mineralogy and morphology of travertine deposits vary with the bacterial inhabitants of the stream. However, precipitation rates increase downstream progressively, suggesting that inorganic geochemical constraints also influence precipitation. Samples were processed specifically to fix bacteria for study under high-magnification scanning electron microscopy (SEM). Polysaccharide mucus in the form of sheets, strings, and globs is abundant in most samples, particularly those taken near the spring orifice where salmon-colored sulfur-bacterial mats thrive. Full-sized bacteria are present, but not common in our samples. Small (100 nm diameter), spherical features, which may be nannobacteria, were found in two samples. Less definitive, nannobacteria-like shapes are common in globs of mucus. The geochemical complexity of this system is obvious under SEM. Multiple generations of calcite are present. In most samples, the first cement to precipitate was calcite with a low length to width ratio. This early calcite is covered with nanometer-scale (25-70 nm) spherical textures. Later calcites encrust S-bacterial threads and take on a morphology reminiscent of a gothic arch. A free-floating generation of calcite with a high length to width ratio is the last crystal formed in most samples. Concurrent with calcite formation, aragonite precipitates as spheres, splays, and dumbbells that have nanometer-scale spherical textures in the center and grade into elongate, needle-shaped crystals with smooth surfaces.

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