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Travertine ( TRAV-ər-teen) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and 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. Similar (but softer and extremely porous) deposits formed from ambient-temperature water are known as tufa. Definition Travertine is a sedimentary rock formed by the chemical precipitation of calcium carbonate minerals from fresh water, typically in springs, rivers, and lakes; that is, from surface and ground waters. In the broadest sense, travertine includes deposits in both hot and cold springs, including the porous, spongy rock known as tufa, and also the cave features known as speleothems (which include stalactites and stalagmites). Calcrete, which is calcium minerals deposited as a horizon in the soil profile, is not considered a form of travertine. Travertine is often defined in a more narrow sense as dense rock, sometimes massive but more commonly banded or with a fibrous internal structure, deposited in hot springs. In this more narrow sense, travertine is distinct from speleothems and tuff. Travertine is sometimes defined by its mode of origin, as rock formed by inorganic precipitation of calcium carbonate minerals onto a surface following exchange of carbon dioxide between the atmosphere and groundwater. Calcrete, lake marls, and lake reefs are excluded from this definition, but both speleothems and tuff are included. Fresh travertines vary widely in their porosity, from about 10% to 70%. Ancient ones may have porosities as low as 2% due to crystallization of secondary calcite in the original pore spaces, while some of the fresh aragonite travertine at Mammoth Hot Springs in Wyoming has a porosity greater than 80%. A porosity of about 50% is typical for cold spring travertine while hot spring travertines have a mean porosity of about 26%. Speleothems have low porosities of less than 15%. Landforms Travertine forms distinctive landforms: Spring mounds are domes of travertine ranging in height from less than a meter to over 100 metres (330 ft) surrounding a spring orifice. Because the spring orifice is above ground level, the formation of terrestrial mounds requires either an artesian spring or a geyser. Travertine mounds also are found under water, often in saline lakes. Fissure ridges form from spring discharge along joints or faults. These can be over 15 metres (49 ft) in height and 0.5 kilometres (0.31 mi) in length. These generally show signs of progressive widening of the fissure, balanced by deposition of travertine on the fissure wall. Cascade deposits are formed by a series of waterfalls. Dam deposits are similar to cascades but have localized vertical buildup of travertine that creates a pond or lake behind the travertine buildup. Travertine forms various kinds of fluvial and lacustrine deposits. Paludal (marsh) deposits are shallow accumulations in poorly-drained areas. Speleothems are the characteristic "formations" of caves. Etymology The word 'travertine' is derived from the Italian travertino, a derivation of the Latin tiburtinus meaning 'of Tibur', now known as Tivoli, near Rome, Italy. Geochemistry The formation of travertine begins when groundwater (H2O) containing an elevated concentration of dissolved carbon dioxide (CO2) comes in contact with limestone or other rock containing calcium carbonate (CaCO3). The dissolved carbon dioxide acts as a weak acid, carbonic acid, which dissolves some of the limestone as soluble calcium bicarbonate (Ca+2 + 2HCO−3): CaCO3 + H2O + CO2 ⇌ Ca2+ + 2HCO−3 This is a reversible reaction, meaning that as the concentration of dissolved calcium bicarbonate builds up, the calcium bicarbonate begins to revert to calcium carbonate, water, and carbon dioxide. So long as there is nowhere for the carbon dioxide to go, chemical equilibrium is reached where dissolution of calcium carbonate is balanced by precipitation of calcium carbonate. If the groundwater moves into an environment with a lower concentration of carbon dioxide (as measured by its partial pressure, pCO2), some of the carbon dioxide will escape into the environment, disturbing the equilibrium and allowing net precipitation of calcium carbonate to take place: Ca2+ + 2HCO−3 → CaCO3 + H2O + CO2 The calcium carbonate most readily precipitates onto solid surfaces bathed by the groundwater, eventually building up thick deposits of travertine. Because of the role of CO2 in dissolving and transporting calcium carbonate, it is sometimes described as the carrier CO2 or simply as the carrier. The most important sources of elevated carbon dioxide concentration in groundwater are soil and volcanic activity. Water passing through soil picks up carbon dioxide from plant roots and decaying organic matter. This CO2 is described as meteoric carrier, and the travertine formed by this mechanism as meteogene travertine. This is the principal mechanism for formation of speleothems. Groundwater with an enhanced concentration of CO2 absorbed from soil infiltrates underlying limestone, dissolving some of the limestone. When this groundwater then emerges into a cave with a lower concentration of CO2, some of the CO2 escapes, allowing calcium carbonate to precipitate and build up stalactites, stalagmites, and other speleothems. Volcanic activity is the source of carbon dioxide in groundwater that emerges from hot springs. When the water reaches the mouth of the spring, it rapidly loses carbon dioxide to the open air and precipitates calcium carbonate around the spring mouth. Travertine formed this way is described as thermogene travertine. This can form spectacular deposits of travertine, such as those of Pamukkale or Mammoth Hot Springs. The carbon dioxide may come from sources deep in the Earth, such as metamorphism of deeply buried rock. The carbon dioxide is carried to the surface by magma and is a major component of volcanic gases. Carbon dioxide may also be generated by magma bodies heating solid rock near the surface, through thermal decomposition of organic matter, or by reactions of quartz or other silica minerals with carbonate minerals. Precipitation may be enhanced by factors leading to a reduction in pCO2, for example increased air-water interactions at waterfalls may be important, as may photosynthesis. Rarely, travertine may form from highly alkaline water containing dissolved calcium hydroxide (Ca+2 + 2OH−) produced during serpentinization of ultramafic rock. When this alkaline water reaches the surface, it absorbs carbon dioxide from the air to precipitate calcium carbonate: Ca2+ + 2OH− + CO2 → CaCO3 + H2O While water carbonated by volcanic activity is .... Discover the J L N Lewitin popular books. Find the top 100 most popular J L N Lewitin books.