Travertine

Synonym

Calcareous sinter

Required Geological Setting

Island arc volcanoes

Occurrence

Massive aggregates of calcite and/or aragonite deposited from hot spring water. This material is often reffered to as calcareous sinter along with tufa. Travertine indicates calcareous sinter of hydrothermal CO₂ origin deposited from hot spring water, whereas Tufa does calcareous sinter of soil CO₂ origin deposited from meteoric water. Travertine is formed near hot water vents as towers, domes, mounds, rimpools, or rimstones. Travertine often has growth ring structure in it. The deposition speed is estimated as a few hundreds mm/year. The deposition process of travertine is considered as follows:

Contacting air, CO₂ degasses from the hot spring water. The following reaction progresses as a result of lowered dissolved carbon dioxide concentration in the spring water.

HCO^3- + H⁺ = H₂CO₃ = CO₂ (aq) + H₂O

The pH of spring water elevated and it becomes alkaline, then carbonate ions formed by this reaction.

HCO^3- + OH^- = CO₃^2- + H₂O

Combining carbonate ions with Ca ions in the spring water, calcite or aragonite deposits.

CO₃^2- + Ca²⁺ = CaCO₃ (Calcite)

Mineral Assemblages

Calcite, Aragonite