Hello. In hot springs, aragonite should be the main component. Travertine or its different varieties also commonly occur around cold springs. The decisive factor is not so much the temperature, but the partial pressure of CO2 in the groundwater. CO2 dissolves to form H2CO3*, or HCO3-, which extracts Ca ions from the environment. When the CO2 pressure drops due to degassing on the surface in the springs, the water becomes heavily saturated with calcite or aragonite.

So calcium comes from, or it is transported as Ca(HCO3)+. There is a lot of literature on this. Every better hydrochemistry textbook has a chapter on carbonate equilibrium.

Thank you Dr. Dalibor Matýsek! I was really confused before getting your answer.

The calcium in travertine primarily originates from the dissolution of calcium carbonate-rich water sources, such as underground aquifers or hot springs.

You've brought up some interesting points about the formation of travertine and the role of temperature and carbon dioxide (CO2) in the solubility of calcite. To understand why travertine forms near hot springs despite the dynamics of calcite solubility and CO2 degassing, let’s delve into the geochemical processes at play:

1- Calcite Solubility and Temperature: Calcite, primarily composed of calcium carbonate (CaCO3), indeed has a solubility that generally decreases as temperature increases. In simpler terms, warmer water can hold less calcium carbonate dissolved in it compared to cooler water.

2- CO2 and Water Chemistry: CO2 plays a crucial role in the solubility of calcium carbonate. In water, CO2 reacts to form carbonic acid (H2CO3), which subsequently dissociates to bicarbonate (HCO3-) and carbonate ions (CO3^2-), increasing the water’s capacity to dissolve calcium carbonate. The overall reaction can be summarized as:

CO2+H2O↔H2CO3↔H++HCO3−↔2H++CO32−

3- Deep-Hot Aquifer Conditions: In deep, hot aquifers, water is often under high pressure, which increases its capacity to hold CO2. The high levels of dissolved CO2 lead to higher amounts of dissolved carbonate species (bicarbonate and carbonate), thus allowing more calcium carbonate to dissolve in the water due to the formation of more bicarbonate.

4- Emergence at the Surface and CO2 Degassing: When this hot water rises and emerges at the surface as a hot spring, it experiences a drop in pressure and a decrease in temperature. The drop in pressure causes CO2 to degas (escape from the water), which reduces the concentration of carbonic acid and bicarbonate in the water. This shift drives the reaction backwards towards the formation of solid calcium carbonate:

Ca2++CO32−→CaCO3(s)

5- Deposition of Travertine: As the CO2 degasses from the emerging hot water, the decreased levels of bicarbonate mean that the water can no longer hold as much calcium carbonate in solution. Consequently, calcium carbonate precipitates out, forming travertine. This deposition typically happens around the hot spring where the water flow is turbulent, enhancing CO2 release, or where water spills over surfaces, increasing the water-air interface and facilitating further CO2 degassing.

6- Environmental Factors: Other environmental factors such as the presence of algae, microbes, and varying flow rates can also influence the rate and manner of travertine deposition. Biological activity can alter local pH levels and CO2 concentrations, further affecting calcite deposition.

The simplest and shortest answer to the source of Ca is groundwater fluids which can be from hydrothermal fluids to formational interparticle waters of different origins. And as these fluids with high pressure from subsurface environments eventually find a way to top (surface), due to decreasing pressure, they lose dissolved CO2 and thermodynamically it allows calcite to be formed as travertine and tufa.