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Abstract

Travertines of central Italy have been deposited from warm, fresh-water springs very rich in H,S. Probably as a result of this, the carbonate sediments are largely constructed by bacteria instead of algae. Even the chemically-precipitated forms of calcite show a variety of bizarre characteristics, some of them probably due to the interaction of S with the calcite crystal lattice. Among these are thick crusts of radial fibrous calcite crystals up to a meter long, that show probable daily lamination due to the diurnal activity of photosynthetic bacteria and have depositional rates of as much as one meter per year. Fine ray-crystal fans only one mm or so thick are made of helically-twisting radial ribbons of calcite and also show daily growth bands. Bacterial clumps are enclosed within an elliptical ball of calcite, and the bacteria plate out upon crystal planes; the ball is then surrounded by a tattered hollow carapace of very thin-walled calcite forming a single crystal outline. Fibrous nests of square-ended calcite rods with inclined extinction (“lublinite”) occur among the bacterial calcite clumps. Pore-filling calcite cement in the rocks is also weird, sometimes showing forms like superimposed, curving gothic arches, or multiple spikes. Some calcite crystals also show features resembling screw-dislocations. Palisade calcite cement shows a nearly basal parting.

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