Calcite rafts

Last updated
BSE-SEM image at 89 times magnification showing topsides of the calcite rafts (flat surfaces) and bottom crystals growing underneath Calcite lace SEM.jpg
BSE-SEM image at 89 times magnification showing topsides of the calcite rafts (flat surfaces) and bottom crystals growing underneath

Calcite crystals form on the surface of quiescent bodies of water, even when the bulk water is not supersaturated with respect to calcium carbonate. The crystals grow, attach to one other and appear to be floating rafts of a white, opaque material. The floating materials have been referred to as calcite rafts or "leopard spots".

Calcite carbonate mineral

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite".

Calcium carbonate Chemical compound

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous.

Contents

Chemistry

Calcite rafts in Kartchner Caverns, Arizona Calcite rafts Kartchner Caverns.JPG
Calcite rafts in Kartchner Caverns, Arizona

Calcium carbonate is known to precipitate as calcite crystals in water supersaturated with calcium and carbonate ions. Under quiescent conditions, calcite crystals can form on a water surface when calcium carbonate supersaturation conditions do not exist in the bulk water. Water evaporates from the surface and carbon dioxide degasses from the surface layer to create a thin layer of water with high pH and concentrations of calcium and carbonate ions far above the saturation concentration for calcium carbonate. Calcite crystals precipitate in this highly localized environment and attach to one another to form what appear to be rafts of a white material. [1]

Carbonate Salt or ester of carbonic acid

In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−
3. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.

An ion is an atom or molecule that has a net electrical charge. Since the charge of the electron is equal and opposite to that of the proton, the net charge of an ion is non-zero due to its total number of electrons being unequal to its total number of protons. A cation is a positively charged ion, with fewer electrons than protons, while an anion is negatively charged, with more electrons than protons. Because of their opposite electric charges, cations and anions attract each other and readily form ionic compounds.

Supersaturation State of a solution that contains more solute than can be dissolved at equilibrium

Supersaturation is a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound.

Scanning electron micrographs of calcite rafts show interconnected calcite crystals formed around holes on the raft surface. The holes may be caused by air bubbles or other foreign matter on the water surface. [2] Micrographs of calcite rafts show lace-like structure. The surface tension of the water keeps the interconnected calcite crystals, which individually have a specific gravity of 2.7, floating on the water surface.

Specific gravity Relative density compared to water

Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance for liquids is nearly always water at its densest ; for gases it is air at room temperature. Nonetheless, the temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa).

Cave and river system formation

Carpinteria Reservoir with rigid aluminium cover Carpenteria reservoir.jpg
Carpinteria Reservoir with rigid aluminium cover
Calcite rafts on the surface of water in Carpinteria Reservoir Leopard spot1.JPG
Calcite rafts on the surface of water in Carpinteria Reservoir
Detail of calcite rafts on the surface of water in Carpinteria Reservoir Leopard spot2.JPG
Detail of calcite rafts on the surface of water in Carpinteria Reservoir

Calcite rafts are most commonly formed in limestone cave systems. Limestone caves provide a favorable environment due to little air movement and water containing significant concentrations of calcium and carbonate ions. Evidence of calcite rafts has been found in limestone caves all over the world. [2] [3] [4] [5]

One example of calcite raft formation in a spring-fed river system has been reported. [1]

Spring (hydrology) A point at which water emerges from an aquifer to the surface

A spring is a point at which water flows from an aquifer to the Earth's surface. It is a component of the hydrosphere.

Drinking water reservoir

In 2005, the Carpinteria Valley Water District in Carpinteria, California, raised water quality concerns when "leopard spots" approximately 5 to 10 cm. in diameter appeared on the water surface under a newly constructed aluminum reservoir cover. The floating material had not been observed when the reservoir (13 million gallons) was open to the atmosphere. The concern raised was that a potentially toxic metallic precipitate was forming on the water surface from condensate dripping from the metal cover. [6] [7]

Carpinteria, California City in California, United States

Carpinteria is a small oceanside city located in southeastern Santa Barbara County, California, east of Santa Barbara and northwest of Ventura. The population was 13,040 at the 2010 census.

Water quality chemical, physical, biological, and radiological characteristics of water

Water quality refers to the chemical, physical, biological, and radiological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to assess water quality relate to health of ecosystems, safety of human contact, and drinking water.

Water analyses found that the water in the reservoir was saturated with respect to calcium carbonate but no calcite crystals were formed in the bulk solution. X-ray diffraction analysis showed that the floating solid material was greater than 97 percent calcite. Scanning electron micrographs confirmed that the shape of the crystalline material was rhombohedral, which is consistent with calcite crystal formation. [6] [7]

While the floating material was not toxic, it was recommended that movement of the water surface be induced so that quiescent conditions would be avoided which would eliminate the primary condition for calcite raft formation. [6] [7]

Concrete leachate drops

Lattice work or calcite rafts on a calthemite (soda) straw stalactite drop Lattice work of calcite rafts on slow dripping calthemite straw drop.jpg
Lattice work or calcite rafts on a calthemite (soda) straw stalactite drop

Micro calcite rafts have been observed on (soda) straw stalactites solution drops suspended beneath concrete structures. These secondary deposits which form outside the cave environment, are known as calthemites. [8] They are derived from concrete, lime or mortar, and mimic the shapes and forms of speleothems created in caves. [9] [8]

The micro rafts which form on the surface of hyperalkaline leachate solution drips are typically about 0.5 mm in size when visible to the naked eye, and appear on the drip's surface after it has been suspended for greater than ≈5 minutes. [8] The chemical reaction which creates the rafts, involves carbon dioxide (CO2) being absorbed (diffusing) into solution from the atmosphere and calcium carbonate (CaCO3) precipitates as rafts or deposited as a stalagmite, stalactite or flowstone. [9] This chemistry is very different to that which creates speleothems in caves.

Internal water pulses from the straw (into the drop) and air movement around the suspended solution drop, can cause the rafts to spin swiftly around the drop surface. [10] [11] [8] If there is almost no air movement around the suspended drop, then after approximately 12 minutes or more, the micro rafts may join up and form a latticework, which covers the entire drop surface. [8] If the solution drop hangs too long on the straw (≈ >30 minutes), it may completely calcify over and block the calthemite straw tip. [8]

Related Research Articles

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolomite, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.

Stalactite elongated mineral formation which hangs down from a cave ceiling

A stalactite is a type of formation that hangs from the ceiling of caves, hot springs, or manmade structures such as bridges and mines. Any material that is soluble, can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, minerals, mud, peat, pitch, sand, sinter, and amberat. A stalactite is not necessarily a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.

Aragonite carbonate mineral

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, CaCO3 (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

Stalagmite elongated mineral formation which forms on a cave floor

A stalagmite is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites may be composed of lava, minerals, mud, peat, pitch, sand, sinter and amberat.

Travertine A form of limestone deposited by mineral springs

Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites, stalagmites, and other speleothems. It is frequently used in Italy and elsewhere as a building material.

Speleothem A structure formed in a cave by the deposition of minerals from water

Speleothems, commonly known as cave formations, are secondary mineral deposits formed in a cave. Speleothems typically form in limestone or dolomite solutional caves. The term "speleothem" as first introduced by Moore (1952), is derived from the Greek words spēlaion "cave" + théma "deposit". The definition of "speleothem" in most publications, specifically excludes secondary mineral deposits in mines, tunnels and on man-made structures. Hill and Forti more concisely defined "secondary minerals" which create speleothems in caves as;

A "secondary" mineral is one which is derived by a physicochemical reaction from a primary mineral in bedrock or detritus, and/or deposited because of a unique set of conditions in a cave; i.e., the cave environment has influenced the mineral's deposition.

Soda straw

A soda straw is a speleothem in the form of a hollow mineral cylindrical tube. They are also known as tubular stalactites. Soda straws grow in places where water leaches slowly through cracks in rock, such as on the roofs of caves. Soda straws in caves rarely grow more than a few millimetres per year and may average one tenth of a millimetre per year. A soda straw can turn into a stalactite if the hole at the bottom is blocked, or if the water begins flowing on the outside surface of the hollow tube.

Luray Caverns landform

Luray Caverns, originally called Luray Cave, is a cave just west of Luray, Virginia, United States, which has drawn many visitors since its discovery in 1878. The cavern system is generously adorned with speleothems such as columns, mud flows, stalactites, stalagmites, flowstone, and mirrored pools. The caverns are perhaps best known for the Great Stalacpipe Organ, a lithophone made from solenoid-fired strikers that tap stalactites of various sizes to produce tones similar to those of xylophones, tuning forks, or bells.

Dolomite (rock) Sedimentary carbonate rock that contains a high percentage of the mineral dolomite

Dolomite (also known as dolostone, dolomite rock or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO3)2. In old USGS publications, it was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones. Dolomite has a stoichiometric ratio of nearly equal amounts of magnesium and calcium. Most dolomites formed as a magnesium replacement of limestone or lime mud before lithification. Dolomite is resistant to erosion and can either contain bedded layers or be unbedded. It is less soluble than limestone in weakly acidic groundwater, but it can still develop solution features (karst) over time. Dolomite can act as an oil and natural gas reservoir.

Flowstone

Flowstones are composed of sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave. 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.

Gruta Rei do Mato cave in Brazil

The Gruta Rei do Mato (MG-3653) is a cave located at the edge of the BR-040 highway, close to the off ramp to the city of Sete Lagoas, in Minas Gerais, Brazil. From Sete Lagoas the cave can easily be reached by bus, taxi or car. Sete Lagoas is 70 km (43.5 mi) from Belo Horizonte, the capital of the state.

Efflorescence migration of a salt to the surface of a porous material

In chemistry, efflorescence is the migration of a salt to the surface of a porous material, where it forms a coating. The essential process involves the dissolving of an internally held salt in water, or occasionally in another solvent. The water, with the salt now held in solution, migrates to the surface, then evaporates, leaving a coating of the salt.

Cave popcorn common cave formation

Cave popcorn, or coralloids, are small nodes of calcite, aragonite or gypsum that form on surfaces in caves, especially limestone caves. They are a common type of speleothem.

Cave of the Mounds cave in United States of America

Cave of the Mounds, a natural limestone cave located near Blue Mounds, Wisconsin, United States, is named for two nearby hills called the Blue Mounds. It is located in the southern slope of the east hill. The cave's beauty comes from its many varieties of mineral formations called speleothems. The Chicago Academy of Sciences considers the Cave of the Mounds to be "the significant cave of the upper Midwest" because of its beauty, and it is promoted as the "jewel box" of major American caves. In 1987, the United States Department of the Interior and the National Park Service designated the cave as a National Natural Landmark.

Rimstone

Rimstone, also called gours, is a type of speleothem in the form of a stone dam. Rimstone is made up of calcite and other minerals that build up in cave pools. The formation created, which looks like stairs, often extends into flowstone above or below the original rimstone. Often, rimstone is covered with small, micro-gours on horizontal surfaces. Rimstone basins may form terraces that extend over hundreds of feet, with single basins known up to 200 feet long from Tham Xe Biang Fai in Laos

Speleogenesis is the origin and development of caves, the primary process that determines essential features of the hydrogeology of karst and guides its evolution. It often deals with the development of caves through limestone, caused by the presence of water with carbon dioxide dissolved within it, producing carbonic acid which permits the dissociation of the calcium carbonate in the limestone.

Solutional cave cave formed in soluble rock such as limestone, chalk, dolomite, marble, salt beds or gypsum

A solutional cave or karst cave is a cave usually formed in the soluble rock limestone. It is the most frequently occurring type of cave. It can also form in other rocks, including chalk, dolomite, marble, salt beds, and gypsum.

Concrete degradation Concrete technology

Concrete degradation may have various causes. Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage. This process adversely affects concrete exposed to these damaging stimuli.

Calthemite

Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment. Calthemites grow on or under, man-made structures and mimic the shapes and forms of cave speleothems, such as stalactites, stalagmites, flowstone etc. Calthemite is derived from the Latin calx "lime" + Latin < Greek théma, "deposit" meaning ‘something laid down’, and the Latin –ita < Greek -itēs – used as a suffix indicating a mineral or rock. The term "speleothem", due to its definition can only be used to describe secondary deposits in caves and does not include secondary deposits outside the cave environment.

References

  1. 1 2 Taylor, P.M., Drysdale, R.N. and Carthew, K.D. (2004). “The formation and environmental significance of calcite rafts in tropical tufa-depositing rivers of northern Australia.” Sedimentology. 51:5 October. 1089.
  2. 1 2 Taylor, P.M. and Chafetz, H.S. (2004). “Floating Rafts of Calcite Crystals in Cave Pools, Central Texas, USA.” Jour. Sedimentary Res. 74:3 328–41.
  3. ↑ Davis, Donald G. (2000). “Extraordinary Features of Lechuguilla Cave, Guadalupe Mountains, New Mexico.” Jour. Cave and Karst Studies. 62:2 147–57.
  4. ↑ van Hengstum, P.J., et al. (2011). “Sea Level Controls Sedimentation and Environments in Coastal Caves and Sinkholes.” Marine Geology. 286: 35–50.
  5. ↑ Juhasz, E., Korpas, L. and Balog, A. (1995). “200-Million Years of Karst History, Dachsein Limeson, Hungary.” Sedimentology. 42:3 473–89.
  6. 1 2 3 McGuire, M.J., Blute, N.K., Hamilton, C., and Brabander, D.J. (2005). “Why Are There Leopard Spots Floating on My Covered Reservoir.” presented at the Fall Conference, California Nevada Section, American Water Works Association, Reno, NV, October 10–14.
  7. 1 2 3 McGuire, M.J., Blute, N.K., Hamilton, C., and Brabander, D.J. (2006). “Formation of Floating Calcite Rafts in a Drinking Water Reservoir.” Proceedings Water Quality Technology Conference. American Water Works Association, Denver Colorado, November 6.
  8. 1 2 3 4 5 6 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.
  9. 1 2 Hill, C A, and Forti, P, (1997). Cave Minerals of the World, 2nd editions. [Huntsville, Alabama: National Speleological Society Inc.]
  10. ↑ Allison, V C, (1923). "The growth of stalagmites and stalactites". Journal of Geology, Vol.31, 106–125.
  11. ↑ Ver Steeg, K, (1932). "An unusual occurrence of stalactites and stalagmites". The Ohio Journal of Science, Vol.32(2), 69–83.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.