Travertine ( TRAV-ər-teen) is a form of fresh water 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. Travertine 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.
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.
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 tufa. 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 tufa 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. Dunns River Falls is one of the very few travertine falls that empty into the ocean. 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.
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.
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