 thumb|upright|calcium-carbonate-encrusted, yet growing moss, early stage of porous travertine formation. travertine is a form of limestone deposited by mineral springs, especially hot springs. travertine often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. it is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. in the latter, it can form stalactites, stalagmites, and other speleothems. it is frequently used in italy and elsewhere as a building material. travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in ground and surface waters, and/or geothermally heated hot-springs.dictionary of geological terms, 1962. a dolphin reference booka glossary of karst terminology, 1970. geol. surv. water-supply paper 1899-k, u. s. gov. print. off., washington. similar (but softer and extremely porous) deposits formed from ambient-temperature water are known as tufa. ==features== travertine forms from geothermal springs and is often linked to siliceous systems that form siliceous sinter. macrophytes, bryophytes, algae, cyanobacteria, and other organisms often colonise the surface of travertine and are preserved, giving travertine its distinctive porosity. some springs have temperatures high enough to exclude macrophytes and bryophytes from the deposits. as a consequence, deposits are, in general, less porous than tufa. thermophilic microbes are important in these environments and stromatolitic fabrics are common. when it is apparent that deposits are devoid of any biological component, they are often referred to as calcareous sinter. ==geochemistry== modern travertine is formed from geothermally heated supersaturated alkaline waters, with raised pco2 (see partial pressure). on emergence, waters degas co2 due to the lower atmospheric pco2, resulting in an increase in ph. since carbonate solubility decreases with increased ph,bialkowski, s.e. 2004. use of acid distributions in solubility problems.  precipitation is induced. precipitation may be enhanced by factors leading to a reduction in pco2, for example increased air-water interactions at waterfalls may be important,zhang, d., y. zhang, a. zhu and x. cheng, 2001, physical mechanisms of river waterfall tufa (travertine) formation, journal of sedimentary research, 71, pp. 205-216 as may photosynthesis.riding, r., 2000, microbial carbonates: the geological record of calcified bacterial-algal mats and biofilms, sedimentology, 47, pp. 179-214 pr
Calcium_minerals Limestone Sedimentary_rocks Springs_(hydrology)