Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination, often forming crusts or sedimentary rock layers.

Calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

sinter should not be confused with

siliceous sinter Geyserite, or siliceous sinter, is a form of opaline silica that is often found as crusts or layers around hot springs and geysers. Botryoidal geyserite is known as fiorite. Geyserite is porous due to the silica enclosing many small cavities. Si ...

, which the term sinter more frequently refers to. It has been suggested that the term "sinter" should be restricted to siliceous spring deposits and be dropped for calcareous deposits entirely.

Features

Calcareous sinter is characterised by laminations of prismatic crystals growing perpendicular to the substrate; laminations are separated by thin layers of microcrystalline carbonate. Calcareous sinter is porous due to the calcareous crystals enclosing many small cavities. Macrophytes are absent, consequently porosity is very low. Exclusion of species is due either to high temperature (

travertine Travertine ( ) is a form of terrestrial limestone deposited around mineral springs, especially hot springs. It often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a pro ...

), high pH/

ionic strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as ...

(

tufa Tufa is a variety of limestone formed when carbonate minerals precipitate out of water in unheated rivers or lakes. Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits, which are known as travertine. ...

) or absence of light (

speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on ...

s). Pedley (1990) suggests the term be abandoned in favour of tufa for ambient temperature deposits. This avoids any potential confusion with siliceous sinter and prevents deposits formed in different environmental conditions (hot spring deposits, cold spring deposits and speleothems are all lumped together under the term sinter) from being amalgamated into one group.

Geochemistry

Deposits are formed from either

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

or aragonite. Precipitation is brought about by degassing of CO₂, which decreases the solubility of calcite/aragonite. (See tufa/geochemistry)

Notable deposits

Pamukkale Pamukkale, meaning "cotton castle" in Turkish, is a natural site in Denizli Province in southwestern Turkey. The area is famous for a carbonate mineral left by the flowing of thermal spring water. It is located in Turkey's Inner Aegean region, i ...

, Turkey – terraces of travertine. * Mono Lake, California - towers of tufa.

Gallery

Image:Kalksinter.JPG, Pure calc sinter, typical for aqueducts and all kinds of water facilities. Image:Eifelwasserleitung05.jpg, Roman aqueduct in Germania Inferior. Eifel Aqueduct, clogged by fine sinter. Image:Travertine-mound_Bridgeport_CA.jpg, Deposits of several thousand years of calc sinter (

) in the Bridgeport, California Hot Springs

Medieval extraction and use

The build-up of calc-sinter material in the Eifel Aqueduct was commercially exploited in the 11th and 12th centuries. With deposits up to thick, the material was cut into vertical columns of polished brown rock with impressive layered patterns, which made it much in demand by cathedral builders across large parts of central Europe and beyond. In England it was used to provide polychromy, contrasting with the pale limestone favoured by

Norman Norman or Normans may refer to: Ethnic and cultural identity * The Normans, a people partly descended from Norse Vikings who settled in the territory of Normandy in France in the 10th and 11th centuries ** People or things connected with the Norm ...

English Cathedrals. The stone was for many years known as 'Onyx Marble' despite being very obviously neither onyx nor

marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or Dolomite (mineral), dolomite. Marble is typically not Foliation (geology), foliated (layered), although there are exceptions. In geology, the ...

. Those studying the stonework at

Canterbury Cathedral Canterbury Cathedral in Canterbury, Kent, is one of the oldest and most famous Christian structures in England. It forms part of a World Heritage Site. It is the cathedral of the Archbishop of Canterbury, currently Justin Welby, leader of the ...

were unaware of its origins in the aqueduct until 2011. Such large-scale use as the cloisters around a cathedral quadrangle needed many hundreds of columns, which must have been supplied by a well-organised extraction and transport operation. The Eifel deposits, have also been identified at Rochester and in the now lost Romanesque cloister at

Norwich Norwich () is a cathedral city and district of Norfolk, England, of which it is the county town. Norwich is by the River Wensum, about north-east of London, north of Ipswich and east of Peterborough. As the seat of the See of Norwich, with ...

as well as the Infirmary Cloister, Chapter House windows, Anselm Chapel door and the Treasury gateway at Canterbury.

References