terrace formations at
Badab-e Surt,
Iran Types of
sedimentary rock – usually
limestone (
calcium carbonate) – are sometimes formed by the
evaporation, or rapid
precipitation, of minerals from spring water as it emerges, especially at the mouths of hot mineral springs. In cold mineral springs, the rapid precipitation of minerals results from the reduction of acidity when the gas bubbles out. (These mineral deposits can also be found in dried lakebeds.) Spectacular formations, including terraces,
stalactites,
stalagmites and 'frozen waterfalls' can result (see, for example,
Mammoth Hot Springs). One light-colored porous
calcite of this type is known as
travertine and has been used extensively in
Italy and elsewhere as building material. Travertine can have a white, tan, or cream-colored appearance and often has a fibrous or concentric 'grain'. Another type of spring water deposit, containing
siliceous as well as
calcareous minerals, is known as
tufa. Tufa is similar to travertine but is even softer and more porous.
Chalybeate springs may deposit iron compounds such as
limonite. Some such deposits were large enough to be
mined as iron ore. ==See also==