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. 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, 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, as may photosynthesis. Precipitation may also be enhanced by evaporation in some springs.

Both calcite and aragonite are found in hot spring travertines; aragonite is preferentially precipitated when temperatures are hot, while calcite dominates when temperatures are cooler. When pure and fine, travertine is white, but often it is brown to yellow due to impurities.

Travertine may precipitate out directly onto rock and other inert materials as in Pamukkale or Yellowstone for example.

Occurrence
In Italy, well-known travertine quarries exist in Tivoli and Guidonia Montecelio, where we can find the most important quarries since Ancient Roman times like the old quarry of Bernini in Guidonia. The latter has a major historic value, because it was one of the quarries that Gian Lorenzo Bernini selected material from to build the famous Colonnade of St. Peter's Square in Rome (colonnato di Piazza S.Pietro) in 1656-1667. Michaelangelo also chose travertine as the material for the external ribs of the dome of St Peter's Basilica. Travertine derives its name from the former town, known as Tibur in ancient Roman times. The ancient name for the stone was lapis tiburtinus, meaning tibur stone, which was gradually corrupted to travertine. Detailed studies of the Tivoli and Guidonia travertine deposits revealed diurnal and annual rhythmic banding and laminae, which have potential use in geochronology.

Cascades of natural lakes formed behind travertine dams can be seen in Pamukkale, Turkey, which is a UNESCO World Heritage Site. Other places with such cascades include Huanglong in Sichuan Province of China (another UNESCO World Heritage Site), the Mammoth Hot Springs in the USA, Egerszalók in Hungary, Mahallat, Abbass Abad, Atash Kooh, and Badab-e Surt in Iran, Band-i-Amir in Afghanistan, Lagunas de Ruidera, Spain, and Semuc Champey, Guatemala.

In Central Europe's last post-glacial palaeoclimatic optimum (Atlantic Period, 8000-5000 B.C.), huge deposits of tufa formed from karst springs. Important geotopes are found at the Swabian Alb, mainly in valleys at the foremost northwest ridge of the cuesta; in many valleys of the eroded periphery of the karstic Franconian Jura; at the northern Alpine foothills; and the northern Karst Alps. On a smaller scale, these karst processes are still working. Travertine has been an important building material since the Middle Ages.

Travertine has formed sixteen huge, natural dams in a valley in Croatia known as Plitvice Lakes National Park. Clinging to moss and rocks in the water, the travertine has built up over several millennia to form waterfalls up to 70 m in height.

In the U.S., the most well-known place for travertine formation is Yellowstone National Park, where the geothermal areas are rich in travertine deposits. Oklahoma has two parks dedicated to this natural wonder. Turner Falls, the tallest waterfall in Oklahoma, is a 77 ft cascade of spring water flowing over a travertine cave. Honey Creek feeds this waterfall and creates miles of travertine shelves both up and downstream. Many small waterfalls upstream in the dense woods repeat the travertine-formation effect. The city of Davis now owns thousands of acres of this land and has made it a tourist attraction. Another travertine resource is in Sulphur, Oklahoma, 10 mi east of Turner Falls. Travertine Creek flows through a spring-water nature preserve within the boundaries of the Chickasaw National Recreation Area.

In Texas, the city of Austin and its surrounding "Hill Country" to the south is built on limestone. The area has many travertine formations, such as those found at Gorman Falls within Colorado Bend State Park, the nature preserve known as Hamilton Pool, the West Cave Preserve, and Krause Springs in Spicewood.

In North East Sulawesi, Indonesia is the Wawolesea Karst. A notable feature of this area is a pond several meters from the beach, formed by a salty, hot water fountain extant since the Neogene period.

In Arizona, on the south side of the Grand Canyon there is the Havasupi Reservation. Flowing through it is Havasu Creek, which has extensive travertine deposits. Three major waterfalls: Navajo Falls, Havasu Falls, and Mooney Falls are all located downstream from the town of Supai. There are numerous smaller cataracts formed by travertine dams. These features are accessible by foot (10 mile hike), horseback (arranged with native people), or helicopter (arranged with private companies).

Uses


Travertine is often used as a building material. The Romans mined deposits of travertine for building temples, aqueducts, monuments, bath complexes, and amphitheaters such as the Colosseum , the largest building in the world constructed mostly of travertine.

Other notable buildings using travertine extensively include the Sacré-Cœur Basilica in Paris and the 20th-century Getty Center in Los Angeles, California, and Shell-Haus in Berlin. The travertine used in the Getty Center and Shell-Haus constructions was imported from Tivoli and Guidonia.

Travertine is one of several natural stones that are used for paving patios and garden paths. It is sometimes known as travertine limestone or travertine marble; these are the same stone, although travertine is classified properly as a type of limestone, not marble. The stone is characterised by pitted holes and troughs in its surface. Although these troughs occur naturally, they suggest signs of considerable wear and tear over time. Some installers use a grout to fill the holes, whereas others leave them open &mdash; travertine can be purchased "filled" or "unfilled." It can also be polished to a smooth, shiny finish, and comes in a variety of colors from grey to coral-red. Travertine is most commonly available in tile sizes for floor installations.

Travertine is one of the most frequently used stones in modern architecture. It is commonly used for façades, wall cladding, and flooring. The lobby walls of the modernist Willis Tower (1970) (formerly Sears Tower) in Chicago are made of travertine. Architect Welton Becket frequently incorporated travertine into many of his projects. The first floor of the Becket-designed UCLA Medical Center has thick travertine walls. Architect Ludwig Mies van der Rohe used travertine in several of his major works, including the Toronto-Dominion Centre, S.R. Crown Hall and the Farnsworth House.

The relative softness of the stone, combined with its holes and troughs, make travertine flooring difficult to finish and maintain. Aggressive grinding — sometimes called honing — can reveal previously hidden air pockets that significantly change the look of the floor.

Supply
A decade ago, Italy had a near-monopoly on the world travertine market; now significant supplies are quarried in mainly Turkey, Iran, Mexico and Peru.

Two or three small travertine producers operate in the western United States. U.S. demand for travertine is about 0.85 million tons per year, almost all of it imported.