travertine, tufa

travertine, tufa When water becomes supersaturated in dissolved calcium carbonate, solid calcium carbonate can grow by precipitation from the water. This solid build-up is a form of ‘chemical limestone’ (as opposed to biogenic or clastic limestone), usually consisting of the calcite variety (or polymorph) of calcium carbonate, but sometimes as the alternate form aragonite.

Rock formed in this way is usually referred to as travertine or tufa. The terms are sometimes used interchangeably, but it is more useful to distinguish dense varieties as travertine and porous or spongy varieties as tufa. Calcium carbonate precipitated near the water-table within soils or gravels that eventually acts to cement the formerly loose deposit together is of comparable origin but is not usually referred to as either travertine or tufa, the term calcrete being preferred to describe the cemented deposit as a whole.

The most common ways for travertine to form are by evaporation of fresh water, and around hot springs (as, for example, at Mammoth Hot Springs in Yellowstone Park, Wyoming). Perhaps the best-known locations of travertine formation are caves in limestone regions. Where water rich in dissolved calcium carbonate drips slowly from the roof a downward-hanging travertine stalactite grows, and where drops persistently hit the floor an upward-pointing stalagmite grows. Cave walls down which water habitually runs may become covered with sheets of travertine flowstone.

Travertine is often attractively banded and takes a good polish. It has therefore long been a favoured ornamental stone. For example, travertine quarried near Tivoli was used as the facing stone for the Colosseum in Rome.

Tufa usually gets its porous structure because of the decay of algae or other plant material around which the material was initially precipitated. It is sometimes called calcareous sinter, by analogy with siliceous sinter, which is a comparable deposit formed in a similar way by precipitation of opaline silica. Most of the world's so-called petrifying springs deposit a sintery encrustation on objects immersed in them. In other situations, algae play a direct role in encouraging precipitation to form algal mats or mounds (stromatolites), many of which are made of tufa bound by algal filaments.

Tufa's porosity makes it a soft and easily cut variety of stone and it has found many uses. Notable among these is its use to line the Aqua Appia underground aqueduct, 16 km long, built in 312 bc to supply water to the city of Rome.

The chemical reaction by which travertine and tufa form may be summarized as:Ca²⁺ (in solution) + 2(HCO₃²⁻) (in solution) →CaCO₃ (solid) + H₂O + CO₂ (in solution).

The bicarbonate ion, (HCO₃²⁻), is soluble in water, but if either water (H₂O) or carbon dioxide (CO₂) is removed the effect will be to drag the reaction to the right and cause calcium carbonate to be precipitated. Carbon dioxide can be removed by plant metabolism or driven out of solution by heat (it is less soluble in warm water than in cold water), and these conditions give rise to the formation of tufa at hot springs and in stromatolites. Water can be lost by evaporation. This produces stalactites and related deposits in underground caves. It can even cause a canopy-like feature to grow over a waterfall, which may completely bridge the river to create a natural tunnel referred to as a tufa cave.

David A. Rothery