Bizarre forms of Depositional and Diagenetic Calcite in Hot-Spring Travertines, Central Italy

Travertines of central Italy have been deposited froni warm: fresh-water springs very rich in H2S. 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, solue of them probably due to the interaction of S \\,it11 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 yea,: Fine ray-crystal fans only one mni 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 the11 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 thc bactel-ial 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 crystala also show features resembling screw-dislocations. Palisade calcite cement sliows a ncarly basal pal-ting. Travertine from Central Italy has been used as prime building and ornarnental stone since the late second century B.C. (Blake, 1947, 1959), included in such monuments as the "Temple of Sibylla" (at Tivoli itself), the Coliseum, colonnade around Piazza San Pietro, the famous fountains of Roma, as well as the modern University, apartment houscs, and curbstones. During the first century B.C. its use became standard for Roman bridges and other heavy structures and at that time Vitruvius wrote that it was almost an ideal building stone because of its ability to bear heavy loads and resist weathering, yet it could be cut like wood with a toothed saw and the porosity facilitated binding for coats of stucco (Briggs. 1956, p. 27). Most of the quarries were near Tivoli, and in fact the nanlc "travertine" comes frorn Latin "Lapis Tiburtinus" (stone of Tibur), the original name for Tivoli in use prior to the eighth century (Hare, 1884). Today it is probably the most widely exported and conlmercially valuable building stone in the world, being shipped worldwitle froni dozens of very active quarries. The romantic cliarn-r of the grottos and waterfalls in the Tivoli region has attracted geologists for almost two centuries, having been discussed by such famous early giants as Lyell (1829), who cited earlier st~~dies by von Buch and Brongniart. d'Halloy (1812) concluded correctly that the plain of Tivoli had once been a lake, subsequently filled by deposition of travertine from hot springs except for a few tiny surviving ponds such as the Acquac Albulae north of Bagni. Our study (Chafetz and Folk, 1984) has concen