Bizarre Forms of Depositional and Diagenetic Calcite in Hot-Spring Travertines, Central Italy
Published:January 01, 1985
Robert L. Folk, Henry S. Chafetz, Pamela A. Tiezzp, 1985. "Bizarre Forms of Depositional and Diagenetic Calcite in Hot-Spring Travertines, Central Italy", Carbonate Cements: Based on a Symposium Sponsored by the Society of Economic Paleontologists and Mineralogists, Nahum Schneidermann, Paul M. Harris
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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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Carbonate Cements: Based on a Symposium Sponsored by the Society of Economic Paleontologists and Mineralogists
In the annals of carbonate sedimentology, few fields have undergone more discussion than the area of carbonate cements. Why this interest? Studies of carbonate cements provide visual gratification to carbonate petrographers and mineralogists. Economic geologists exploring for minerals and hydrocarbons worry about their effect on porosity occlusion or preservation. Inorganic geochemists are now able to precipitate carbonate cements under controlled conditions, and organic geochemists can observe their interactions with living or fossil organic matter. Students of modern carbonate environments of deposition can observe almost instantaneous cementation processes in a diverse group of environments ranging from fresh water to the deep sea floor. The SEPM Special Symposium, held at the 1983 AAPG/SEPM Annual Convention in Dallas, Texas, was designed to bring together specialists working on modern marine and fresh-water cements, on their ancient analogs and their proposed relationship to burial conditions.