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Flowstones are sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave.Hill, C A, and Forti, P, (1997). Cave Minerals of the World, (2nd edition). Huntsville, p.70 They are typically found in "solution caves", in limestone, where they are the most common speleothem. However, they may form in any type of cave where water enters that has picked up dissolved minerals. Flowstones are formed via the degassing of vadose percolation waters.
Flowstone may also form on manmade structures as a result of calcium hydroxide being leached from concrete, lime or mortar. These secondary deposits created outside the cave environment, which mimic the shapes and forms of speleothems, are classified as "" and are associated with concrete degradation.Smith, G.K., (2016). “Calcite Straw Stalactites Growing From Concrete Structures”, Cave and Karst Science, Vol.43, No.1, P.4-10, (April 2016), British Cave Research Association, ISSN 1356-191X.
There are two common forms of flowstones, tufa and travertine. Tufa is usually formed via the precipitation of calcium carbonate, and is spongy or porous in nature. Travertine is a calcium carbonate deposit often formed in creeks or rivers; its nature is laminated, and it includes such structures as and .
The deposits may grade into thin sheets called "draperies" or "curtains" where they descend from overhanging portions of the wall. Some draperies are translucent, and some have brown and beige layers that look much like bacon (often termed "cave bacon").
Though flowstones are among the largest of , they can still be damaged by a single touch. The sebum causes the flowing water to avoid the area, which then dries out. Flowstones are also good identifiers of periods of past droughts, since they need some form of water to develop; the lack of that water for long periods of time can leave traces in the rock record via the absence or presence of flowstones, and their detailed structure.
A detail often missed is a pattern common in flowstones and especially in draperies that looks like the teeth of a saw. This is caused by the way calcite crystals grow, which is with the long axis of the crystal perpendicular to the lower edge of the draperie.
Concrete derived secondary deposits are classified as "". These calcium carbonate deposits mimic the forms and shapes of , created in caves. e.g. , , flowstone etc. It is most likely that calthemite flowstone is precipitated from leachate solution as calcite, "in preference to the other, less stable polymorphs, aragonite and vaterite."
Other trace elements such as iron from rusting reinforcing or copper oxide from pipework may be transported by the leachate and deposited at the same time as the CaCO3. This may cause the calthemites to take on colours of the leached oxides.White W.B., (1997), “Color of Speleothems”, Cave Minerals of the World, (2nd Edition) Hill C. and Forti P. Huntsville, 239-244
There are a number of US caves called "Onyx Cave" because of the presence in them of such deposits.
Concrete derived flowstone
Flowstone derived from concrete, lime or mortar, can form on manmade structures, much more rapidly than in the natural cave environment due to the different chemistry involved.Hill, C A, and Forti, P, (1997). Cave Minerals of the World, (2nd edition). Huntsville, pp. 217 and 225
On concrete structures, these secondary deposits are the result of concrete degradation,Macleod, G, Hall, A J and Fallick, A E, (1990). "An applied mineralogical investigation of concrete degradation in a major concrete road bridge". Mineralogical Magazine, Vol.54, 637–644Smith G.K., (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science, Vol.43, No.1, P.4-10, (April 2016), British Cave Research Association, ISSN 1356-191X when calcium ions have been leached from the concrete in solution and redeposited on the structure's surface to form flowstone, stalactites and stalagmites. Carbon dioxide (CO2) is absorbed into the hyperalkaline leachate solution as it emerges from the concrete. This facilitates the chemical reactions which deposits calcium carbonate (CaCO3) on vertical or sloping surfaces, in the form of flowstone.
Uses
Cave onyx is any of various kinds of flowstone considered desirable for ornamental architectural purposes. "Cave onyx" was a common term in certain areas of the United States—particularly the Tennessee-Alabama-Georgia area and the Ozarks—during the 19th and early 20th centuries, being applied to calcite speleothems that were banded in a way suggestive of true onyx.
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