Tufa

Synonym

Calcareous sinter

Required Geological Setting

Neogene and Quaternary sediments, Paleogene and older sediments, Accretionary complexes

Occurrence

Massive aggregates of calcite and/or aragonite formed on the surface of limestone. This material is often reffered to as calcareous sinter along with travertine. Tufa indicates calcareous sinter of soil CO₂ origin deposited from meteoric water, whereas travertine does calcareous sinter of hydrothermal CO₂ origin deposited from hot spring water. Tufa formed in limestone caves is called stalactite. Tufa is formed around cold spring water vents near limestone blocks as towers, domes, mounds, rimpools, or rimstones. The deposition speed is estimated as a few mm/year in the mid-latitude region such as Japan, and a few tens mm/year in the subtropical region such as Okinawa islands. It is considered that cyanobacteria are concerned in deposition process of tufa and have an influcence on deposition speed. The dissolution process of limestone into meteoric water is considered as follows:

Carbon dioxide is dissolved in the meteoric water passing through soil. Carbon dioxide in soil is mainly originated from exhalation of plant roots and decomposition of organic matter by microbes.

Contacting limestone, CaCO₃ is dissolved in the carbon dioxide-rich meteoritc water.

CaCO₃ + H₂O + CO₂ (Soil) = Ca²⁺ + 2HCO₃^-

This reaction is most active on the limestone surface contacting soil. The longer contacting time, the more the reaction progresses until the meteoric water is saturated with CaCO₃.

The deposition process of tufa from the meteoric water rich in carbon dioxide and Ca ion is considered as follows:

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

HCO^3- + H⁺ = H₂CO₃ = CO₂ (aq) + H₂O

The pH of the meteoric 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 meteoric water, calcite or aragonite deposits.

CO₃^2- + Ca²⁺ = CaCO₃ (Calcite)

Mineral Assemblages

Calcite, Aragonite