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The Travertine

The carsica phenomenon:

This is a slow process due to the circulating waters both on the surface and in the subsoil, which are at first enriched of carbon dioxide, attack the carbonates rocks which combining with the calcium carbonate give life to calcium bicarbonate.
The calcium bicarbonate is a very soluble salt in molecular balance, that when one of the environmental conditions change (pressure, temperature), it decomposes into: calcium carbonate (CaCO3), carbon dioxide (CO2) and water (H2O).
So the carsica phenomenon begins:

The calcium carbonate precipitates giving life to rocky terrain that grow with variable speeds, up to few millimetres per year.

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The travertine ( chalky and sedimentary rock of chemical origin):

In general, we can say that the travertine deposits form when waters rich in carbonates give carbon dioxide to the atmosphere and to vegetal photosynthetic organisms.

 

The balance of this chemical reaction (CARSICA PHENOMENON) involves the precipitation of calcium carbonate.
The precipitation gives rise to clusters that grow and make compact, giving rise to complex buildings which can reach thicknesses higher than 100 meters and extensions of several kilometres (in short geologically times).

 

 

Under the genetic profile, we can distinguish two types of travertine:
1. Travertine coming from warm waters (from the subsoil);

 

Travertine of thermal type (warm water) form in the proximity of springs. The cooling of such waters and the decrease of pressure determine the chemical balance of calcium bicarbonate and then the phenomenon of the precipitation of calcium carbonate, the evaporation of carbon dioxide and of the water. (Karsification)

 

 

Obviously, the sediments, which looks like chalky mud, after the precipitation undergo a process of "diagenesis" that is of transformation of the sediment into rock. During the diagenesis process also begins the imprisonment of vegetation (it plays a very modest role about the precipitation), which decomposing leaves some gaps that help to give rise to the characteristic porosity of travertine. But most of the gaps being in the travertine are due to the sulphurous gases which go up in the surface from the subsoil forming bubbles in the chalky mud still not cemented.
This slow but continuous work is sometimes broken by natural events of exception type, which could be the overflow of rivers and floods. Therefore it is not difficult to find overlapping travertine banks, separated by thin layers of clay, the so-called "ground ashes."

 

The compactness of travertine is very variable from zone to zone, this depends on the pressure of the layers of overhanging sediments on those below, furthermore, the travertine being very permeable, a conspicuous amount of water rich in carbonate can reach the basal layers, depositing other carbonate calcium obstructing/reducing the porosity, giving rise to a very compact travertine (recrystallization process).

 

The colour of the travertine is not determined by the calcium carbonate, which is white coloured (uncontaminated state), but as stated above, other factors become part of a formation cycle, and depending on the contaminant agents of the place, and on natural events of exceptional type it assumes various colours. They can change from milk-white to straw yellow, to light brown, to yellow, up to gray and red. Not necessarily these polluting agents are dangerous, sometimes they improve some mechanical features, as to be comparable to granite.
This type of travertine is very common in Central Italy thanks to the widespread presence of thermal waters associated with a recent volcanism.

 

 

The travertine formed from room temperature water are generally associated with springs of waters rich in salts.
When the underground water come up on the surface and for the above-mentioned carsica phenomenon occurs the precipitation of calcium carbonate. Obviously, the loss of carbon dioxide is higher near the spring, next to jumps, rapids, waterfalls, however, where the agitation of the waters is favoured. A further contribution to the loss of carbon dioxide is given by the photosynthetic organisms which come into contact with this type of environment. The role of vegetation for this type of formation of travertine is very important. Through the filtering function it holds the calcium carbonate molecules with the roughness of the vegetable fibres favouring the deposition and the formation of travertine clusters. The phenomenon involves the whole environment affected by the waters flow, the deposits cover the vegetation, the rock, the animal carcasses, and also the waste left by mankind in recent times.

 

 

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