Travertine is a type of freshwater limestone composed of carbonate minerals precipitated at room temperature.
Travertine is composed primarily of calcium carbonate, is commonly white to cream in color, and is characterized by high porosity.
Similar freshwater limestones to travertine include travertine and speleothem, but they differ in their environment, formation process, and lithologic characteristics.
Travertines are found all over the world, with more than 300 documented occurrences.
The English name for tufa, “tufa,” has often caused confusion due to its lack of clear definition.
The term “tufa” has been used since Roman times to refer to friable, white sedimentary layers, which at the time included both calcareous sediments and tuffaceous volcanic rocks.
However, the meaning of “tufa” was later clarified when volcanic rocks were classified separately as tuff.
However, travertine was still referred to as “calcareous tufa” regardless of its age and was sometimes classified as a type of travertine.
In the 1990s, it was proposed that travertine is a separate rock with distinct adult and characteristic properties from travertine and the confusion over terminology has since been largely cleared up, with travertine being used primarily for freshwater limestones that are non-hydrothermal, porous, and rich in plant remains.
Travertines are composed primarily of calcite, but some are also composed of aragonite.
When purely composed of calcium carbonate, travertine is usually white to cream in color, but as impurities increase, it becomes tan or brown in color.
Travertine characteristically contains the remains of large aquatic plants, microscopic organisms, invertebrates, and bacteria.
Travertine has historically been thought of as poorly cemented, but this is being challenged by the increasing number of reports of well-cemented travertine.
The development of travertine is initiated by localized carbonate precipitation in a variety of freshwater environments, including streams, springs, ponds, and lakes.
Subsequent carbonate precipitation and cementation gradually binds the surrounding sediments, resulting in the development of a phytoherm, which in turn leads to the formation of calcification dams of varying sizes, sometimes forming cascading waterfalls.
Calcification can also occur in the form of columns in saline lakes, which are referred to as tufa columns.
It has been thought that tufa columns contain little aquatic plant remnants and are formed by inorganic precipitation of CO2 induced by the seepage of CO2-rich water into alkaline lakes.
However, based on the observation that traces of algal remains have been observed in some calcification columns and that the algal residues that contributed to the formation of calcification can be removed by attachment, there is a view that calcification columns are a type of generalized calcification formed by the microbial activity of algae.
In the past, it was thought that the deposition of calcification occurred primarily by physicochemical processes in areas where there is a rise in pH due to the degassing of carbon dioxide, such as at the mouths of springs, streams, and waterfalls.
However, in recent years, it has been argued that microbiologically induced precipitation through the metabolism of algae, bacteria, and lichens is equally important or even more important than physicochemical precipitation, and is gradually becoming accepted as the orthodoxy.