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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 67 (1983)

Issue: 3. (March)

First Page: 461

Last Page: 461

Title: Diagenetic Calcite Varieties from Travertines of Central Italy: ABSTRACT

Author(s): Robert L. Folk, Henry S. Chafetz

Article Type: Meeting abstract

Abstract:

Quaternary travertines in Latium and Tuscany, Italy, were deposited by warm springs containing abundant CO2 and H2S. The major rock-formers were bacteria that precipitated calcite to form shrubs, pisolites, mats, foamrock, and clastic silt-size sediment. The calcite cement in these rocks shows extraordinary morphology. The elementary building block is a subspherical clump of bacteria 10 to 40 ┬Ám in diameter, which becomes surrounded by a single crystal of calcite shaped like a pecan shell. SEM shows that these crystals are riddled with cavities (voids representing bacteria) and contain internal moats. Later the bacteria-rich crystals became coated with clear, chemically-precipitated inorganic calcite cement; but even these crystals are strange, s they have curving edges and consist of a series of scales parallel to the rhomb faces, like superimposed Gothic arches. Other cement crystals bear a forest of sharp spikes parallel to the C axis like a fakir's bed. The final sparry calcite that fills large pore spaces occurs as large bladed crystals with apparent basal parting planes.

Primarily inorganic sediment includes ray-crystals and pisolites. Ray-crystals can be up to 1 m (39 in.) long and in some places show daily, bacteria-rich growth bands indicating deposition rates of as much as 1 m (39 in.) per year. These formed as travertine dams, on sloping surfaces, and at ancient spring orifices. Crystals can be shaped like cedar-tree needles or club-shaped stromatolites, show undulose extinction, and in the SEM consist of helically twisted calcite ribbons. Inorganic pisolites formed in hot-spring mouths, and are made of smoothly concentric rings of tightly bundled radial rods, like fasces.

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