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Travertine
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 Book〕〔A 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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