Travertine
(c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Travertine is a kind of limestone deposited by springs. Groundwater traveling through limestone beds dissolves calcium carbonate, an environmentally sensitive process that depends on a delicate balance between temperature, water chemistry and carbon dioxide levels in the air. As the mineral-saturated water encounters surface conditions, this dissolved matter precipitates in thin layers of calcite or aragonite, two crystallographically different forms of calcium carbonate (CaCO3). With time, the minerals build up into deposits of travertine. It is an odd geological resource that can be harvested and renewed.
The region around Rome produces large travertine deposits that have been exploited for thousands of years. The stone is generally solid but has pore spaces and fossils that give the stone character. The name travertine comes from the ancient deposits on the Tibur River, hence lapis tiburtino. See a small example from California in the Subduction Rocks Tour.
"Travertine" is sometimes used to mean cavestone, the calcium carbonate rock that makes up stalactites and other cave formations.
See more sedimentary rocks in the Sedimentary Rocks Gallery.
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