Calcite

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".

Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300°C,[5][6] and vaterite is even less stable.

Calcite
Calcite-20188
General
CategoryCarbonate minerals
Formula
(repeating unit)
CaCO3
Strunz classification5.AB.05
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3c
Unit cella = 4.9896(2) Å,
c = 17.0610(11) Å; Z = 6
Identification
ColorColorless or white, also gray, yellow, green,
Crystal habitCrystalline, granular, stalactitic, concretionary, massive, rhombohedral.
TwinningCommon by four twin laws
CleavagePerfect on {1011} three directions with angle of 74° 55'[1]
FractureConchoidal
TenacityBrittle
Mohs scale hardness3 (defining mineral)
LusterVitreous to pearly on cleavage surfaces
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.71
Optical propertiesUniaxial (-)
Refractive indexnω = 1.640–1.660
nε = 1.486
Birefringenceδ = 0.154–0.174
SolubilitySoluble in dilute acids
Other characteristicsMay fluoresce red, blue, yellow, and other colors under either SW and LW UV; phosphorescent
References[2][3][4]
Calcite
Crystal structure of calcite

Etymology

Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx (genitive calcis) with the suffix -ite used to name minerals. It is thus etymologically related to chalk.[7]

When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum; but also for a similar-looking, translucent variety of fine-grained banded deposit of calcite.[8]

Unit cell and Miller indices

In publications, two different sets of Miller indices are used to describe directions in calcite crystals - the hexagonal system with three indices h, k, l and the rhombohedral system with four indices h, k, l, i. To add to the complications, there are also two definitions of unit cell for calcite. One, an older "morphological" unit cell, was inferred by measuring angles between faces of crystals and looking for the smallest numbers that fit. Later, a "structural" unit cell was determined using X-ray crystallography. The morphological unit cell has approximate dimensions a = 10 Å and c = 8.5 Å, while for the structural unit cell they are a = 5 Å and c = 17 Å. For the same orientation, c must be multiplied by 4 to convert from morphological to structural units. As an example, the cleavage is given as "perfect on {1 0 1 1}" in morphological coordinates and "perfect on {1 0 1 4}" in structural units. (In hexagonal indices, these are {1 0 1} and {1 0 4}.) Twinning, cleavage and crystal forms are always given in morphological units.[3][9]

Properties

Form

Over 800 forms of calcite crystals have been identified. Most common are scalenohedra, with faces in the hexagonal {2 1 1} directions (morphological unit cell) or {2 1 4} directions (structural unit cell); and rhombohedral, with faces in the {1 0 1} or {1 0 4} directions (the most common cleavage plane).[9] Habits include acute to obtuse rhombohedra, tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. A fibrous, efflorescent form is known as lublinite.[10] Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain.

Scalenohedral faces are chiral and come in pairs with mirror-image symmetry; their growth can be influenced by interaction with chiral biomolecules such as L- and D-amino acids. Rhombohedral faces are achiral.[9]

Hardness

It has a defining Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, red, orange, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities.

Optical

Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. A transparent variety called Iceland spar is used for optical purposes. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar".

Calcite-refraction-property
Photograph of calcite displaying the characteristic birefringence optical behaviour.

Single calcite crystals display an optical property called birefringence (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the Danish scientist Rasmus Bartholin in 1669. At a wavelength of ~590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively.[11] Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.[12]

Chemical

Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures. When conditions are right for dissolution, the removal of calcite can dramatically increase the porosity and permeability of the rock, and if it continues for a long period of time may result in the formation of caves. On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography.

Use and applications

Tutankhamun's Alabaster Jar
One of several calcite or alabaster perfume jars from the tomb of Tutankhamun, d. 1323 BC

Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster because of the close association. Many other cultures have used the material for similar carved objects and applications.

High-grade optical calcite was used in World War II for gun sights, specifically in bomb sights and anti-aircraft weaponry.[13] Also, experiments have been conducted to use calcite for a cloak of invisibility.[14]

Microbiologically precipitated calcite has a wide range of applications, such as soil remediation, soil stabilization and concrete repair.

Calcite, obtained from an 80 kg sample of Carrara marble,[15] is used as the IAEA-603 isotopic standard in mass spectrometry for the calibration of δ18O and δ13C.[16]

Natural occurrence

Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the shells of dead marine organisms. Approximately 10% of sedimentary rock is limestone. It is the primary mineral in metamorphic marble. It also occurs in deposits from hot springs as a vein mineral; in caverns as stalactites and stalagmites; and in volcanic or mantle-derived rocks such as carbonatites, kimberlites, or rarely in peridotites.

Calcite is often the primary constituent of the shells of marine organisms, e.g., plankton (such as coccoliths and planktic foraminifera), the hard parts of red algae, some sponges, brachiopods, echinoderms, some serpulids, most bryozoa, and parts of the shells of some bivalves (such as oysters and rudists). Calcite is found in spectacular form in the Snowy River Cave of New Mexico as mentioned above, where microorganisms are credited with natural formations. Trilobites, which became extinct a quarter billion years ago, had unique compound eyes that used clear calcite crystals to form the lenses.[17]

The largest documented single crystal of calcite originated from Iceland, measured 7×7×2 m and 6×6×3 m and weighed about 250 tons.[18]

Formation processes

Calcite formation can proceed via several pathways, from the classical terrace ledge kink model[19] to the crystallization of poorly ordered precursor phases (amorphous calcium carbonate, ACC) via an Ostwald ripening process, or via the agglomeration of nanocrystals.[20]

The crystallization of ACC can occur in two stages: first, the ACC nanoparticles rapidly dehydrate and crystallize to form individual particles of vaterite. Secondly, the vaterite transforms to calcite via a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite.[21] The second stage of the reaction is approximately 10 times slower. However, the crystallization of calcite has been observed to be dependent on the starting pH and presence of Mg in solution.[22] A neutral starting pH during mixing promotes the direct transformation of ACC into calcite. Conversely, when ACC forms in a solution that starts with a basic initial pH, the transformation to calcite occurs via metastable vaterite, which forms via a spherulitic growth mechanism.[23] In a second stage this vaterite transforms to calcite via a surface-controlled dissolution and recrystallization mechanism. Mg has a noteworthy effect on both the stability of ACC and its transformation to crystalline CaCO3, resulting in the formation of calcite directly from ACC, as this ion destabilizes the structure of vaterite.

Calcite may form in the subsurface in response to activity of microorganisms, such as during sulfate-dependent anaerobic oxidation of methane, where methane is oxidized and sulfate is reduced by a consortium of methane oxidizers and sulfate reducers, leading to precipitation of calcite and pyrite from the produced bicarbonate and sulfide. These processes can be traced by the specific carbon isotope composition of the calcites, which are extremely depleted in the 13C isotope, by as much as −125 per mil PDB13C).[24]

In Earth history

Calcite seas existed in Earth history when the primary inorganic precipitate of calcium carbonate in marine waters was low-magnesium calcite (lmc), as opposed to the aragonite and high-magnesium calcite (hmc) precipitated today. Calcite seas alternated with aragonite seas over the Phanerozoic, being most prominent in the Ordovician and Jurassic. Lineages evolved to use whichever morph of calcium carbonate was favourable in the ocean at the time they became mineralised, and retained this mineralogy for the remainder of their evolutionary history.[25] Petrographic evidence for these calcite sea conditions consists of calcitic ooids, lmc cements, hardgrounds, and rapid early seafloor aragonite dissolution.[26] The evolution of marine organisms with calcium carbonate shells may have been affected by the calcite and aragonite sea cycle.[27]

Calcite is one of the minerals that has been shown to catalyze an important biological reaction, the formose reaction, and may have had a role in the origin of life.[9] Interaction of its chiral surfaces (see Form) with aspartic acid molecules results in a slight bias in chirality; this is one possible mechanism for the origin of homochirality in living cells.[28]

Gallery

Calcite-Mottramite-cktsu-45b

Calcite with mottramite.

Erbenochile eye

Trilobite eyes employed calcite.

CalciteEchinosphaerites

Calcite crystals inside a test of the cystoid Echinosphaerites aurantium (Middle Ordovician, northeastern Estonia).

Calcite-Dolomite-Gypsum-159389

Rhombohedrons of calcite that appear almost as books of petals, piled up 3-dimensionally on the matrix.

Calcite-Hematite-Chalcopyrite-176263

Calcite crystal canted at an angle, with little balls of hematite and crystals of chalcopyrite both on its surface and included just inside the surface of the crystal.

GeopetalCarboniferousNV

Thin section Calcite crystals inside a recrystallized bivalve shell in a biopelsparite.

Calcite-Aragonite-Sulphur-69380

Several well formed milky white casts, made up of many small sharp calcite crystals, from the sulfur mines at Agrigento, Sicily.

Calcite-tch21c

Reddish rhombohedral calcite crystals from China. Its red color is due to the presence of iron.

Calcite-75480

Calcite (var.: Cobaltoan calcite).

Calcite-114508

Sand calcites (calcites heavily included with desert sand) in South Dakota

Calcite-Aurichalcite-146924

Calcite from Ojuela Mine, Mapimí, Mapimí Municipality, Durango, Mexico.

See also

References

  1. ^ Dana, James Dwight; Klein, Cornelis and Hurlbut, Cornelius Searle (1985) Manual of Mineralogy, Wiley, p. 329, ISBN 0-471-80580-7
  2. ^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2003). "Calcite" (PDF). Handbook of Mineralogy. V (Borates, Carbonates, Sulfates). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209740.
  3. ^ a b "Calcite". mindat.org. Retrieved 4 May 2018.
  4. ^ Barthelmy, Dave. "Calcite Mineral Data". webmineral.com. Retrieved 6 May 2018.
  5. ^ Yoshioka S.; Kitano Y. (1985). "Transformation of aragonite to calcite through heating". Geochemical Journal. 19: 24–249.
  6. ^ Staudigel P.T.; Swart P.K. (2016). "Isotopic behavior during the aragonite-calcite transition: Implications for sample preparation and proxy interpretation". Chemical Geology. 442: 130–138.
  7. ^ "calcite (n.)". Online Etymology Dictionary. Retrieved 6 May 2018.
  8. ^ More about alabaster and travertine, brief guide explaining the different use of the same terms by geologists, archaeologists and the stone trade. Oxford University Museum of Natural History, 2012 [1]
  9. ^ a b c d Hazen, Robert M. (2004). "Chiral crystal faces of common rock-forming minerals". In Palyi, C.; Zucchi, C.; Caglioti, L. Progress in Biological Chirality. Oxford: Elsevier. pp. 137&ndash, 151.
  10. ^ "Lublinite". mindat.org. Retrieved 6 May 2018.
  11. ^ Elert, Glenn. "Refraction". The Physics Hypertextbook.
  12. ^ Thompson, D. W.; Devries, M. J.; Tiwald, T. E.; Woollam, J. A. (1998). "Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry". Thin Solid Films. 313–314: 341–346. Bibcode:1998TSF...313..341T. doi:10.1016/S0040-6090(97)00843-2.
  13. ^ "Borrego's calcite mine trail holds desert wonders". Retrieved 2011-06-03.
  14. ^ Chen, Xianzhong; Luo, Yu; Zhang, Jingjing; Jiang, Kyle; Pendry, John B.; Zhang, Shuang (2011). "Macroscopic invisibility cloaking of visible light". Nature Communications. 2 (2): 176. arXiv:1012.2783. Bibcode:2011NatCo...2E.176C. doi:10.1038/ncomms1176. PMC 3105339. PMID 21285954.
  15. ^ Department of Nuclear Sciences and Applications, IAEA Environment Laboratories (16 July 2016). "Reference Sheet: Certified Reference Material : IAEA-603 (calcite) – Stable Isotope Reference Material for δ13C and δ18O" (PDF). IAEA. p. 2. Retrieved 28 February 2017.
  16. ^ "IAEA-603 , Calcite". Reference Products for Environment and Trade. International Atomic Energy Agency. Retrieved 27 February 2017.
  17. ^ Angier, Natalie (3 March 2014). "When Trilobites Ruled the World". The New York Times. Retrieved 10 March 2014.
  18. ^ Rickwood, P. C. (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
  19. ^ De Yoreo, J J; Vekilov, P G (2003). "Principles of crystal nucleation and growth". Reviews in Mineralogy and Geochemistry. 54: 57. doi:10.2113/0540057.
  20. ^ De Yoreo, J; Gilbert, PUPA; Sommerdijk, N A J M; Penn, R L; Whitelam, S; Joester, D; Zhang, H; Rimer, J D; Navrotsky, A; Banfield, J F; Wallace, A F; Michel, F M; Meldrum, F C; Cölfen, H; Dove, P M (2015). "Crystallization by particle attachment in synthetic, biogenic, and geologic environments". Science. 349 (6247): aaa6760. doi:10.1126/science.aaa6760. PMID 26228157.
  21. ^ Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. (2011). "The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite". Nanoscale. 3 (1): 265–71. Bibcode:2011Nanos...3..265R. doi:10.1039/C0NR00589D. PMID 21069231.
  22. ^ Rodriguez-Blanco, J. D.; Shaw, S.; Bots, P.; Roncal-Herrero, T.; Benning, L. G. (2012). "The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate". Journal of Alloys and Compounds. 536: S477. doi:10.1016/j.jallcom.2011.11.057.
  23. ^ Bots, P.; Benning, L. G.; Rodriguez-Blanco, J. D.; Roncal-Herrero, T.; Shaw, S. (2012). "Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC)". Crystal Growth & Design. 12 (7): 3806–3814. doi:10.1021/cg300676b.
  24. ^ Drake, H.; Astrom, M.E.; Heim, C.; Broman, C.; Astrom, J.; Whitehouse, M.; Ivarsson, M.; Siljestrom, S.; Sjovall, P. (2015). "Extreme 13C depletion of carbonates formed during oxidation of biogenic methane in fractured granite" (PDF). Nature Communications. 6: 7020. Bibcode:2015NatCo...6E7020D. doi:10.1038/ncomms8020. PMC 4432592. PMID 25948095.
  25. ^ Porter, S. M. (2007). "Seawater Chemistry and Early Carbonate Biomineralization". Science. 316 (5829): 1302. Bibcode:2007Sci...316.1302P. doi:10.1126/science.1137284. PMID 17540895.
  26. ^ Palmer, Timothy; Wilson, Mark (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia. 37 (4): 417–427. doi:10.1080/00241160410002135.
  27. ^ Harper, E.M.; Palmer, T.J.; Alphey, J.R. (1997). "Evolutionary response by bivalves to changing Phanerozoic sea-water chemistry". Geological Magazine. 134 (3): 403–407. Bibcode:1997GeoM..134..403H. doi:10.1017/S0016756897007061.
  28. ^ Meierhenrich, Uwe (2008). Amino acids and the asymmetry of life caught in the act of formation. Berlin: Springer. pp. 76&ndash, 78. ISBN 9783540768869.

Further reading

  • Schmittner, Karl-Erich; and Giresse, Pierre; 1999. "Micro-environmental controls on biomineralization: superficial processes of apatite and calcite precipitation in Quaternary soils", Roussillon, France. Sedimentology 46/3: 463–476.
Alabaster

Alabaster is a mineral or rock that is soft, often used for carving, and is processed for plaster powder. Archaeologists and the stone processing industry use the word differently from geologists. The former use is in a wider sense that includes varieties of two different minerals: the fine-grained massive type of gypsum and the fine-grained banded type of calcite. Geologists define alabaster only as the gypsum type. Chemically, gypsum is a hydrous sulfate of calcium, while calcite is a carbonate of calcium.Both types of alabaster have similar properties. They are usually lightly colored, translucent, and soft stones. They have been used throughout history primarily for carving decorative artifacts.The calcite type is also denominated "onyx-marble", "Egyptian alabaster", and "Oriental alabaster" and is geologically described as either a compact banded travertine or "a stalagmitic limestone marked with patterns of swirling bands of cream and brown". "Onyx-marble" is a traditional, but geologically inaccurate, name because both onyx and marble have geological definitions that are distinct from even the broadest definition of "alabaster".

In general, ancient alabaster is calcite in the wider Middle East, including Egypt and Mesopotamia, while it is gypsum in medieval Europe. Modern alabaster is probably calcite but may be either. Both are easy to work and slightly soluble in water. They have been used for making a variety of indoor artwork and carving, and they will not survive long outdoors.

The two kinds are readily distinguished by their different hardnesses: gypsum alabaster is so soft that a fingernail scratches it (Mohs hardness 1.5 to 2), while calcite cannot be scratched in this way (Mohs hardness 3), although it yields to a knife. Moreover, calcite alabaster, being a carbonate, effervesces when treated with hydrochloric acid, while gypsum alabaster remains almost unaffected when thus treated.

Apache Calcite

Apache Calcite is an open source framework for

building databases and data management systems.

It includes a SQL parser,

an API for building expressions in relational algebra,

and a query planning engine.

As a framework, Calcite does not store its own data or metadata,

but instead allows external data and metadata to be accessed by means of plug-ins.

Several other Apache projects use Calcite.Hive uses Calcite for cost-based query optimization;Drill and Kylin use Calcite for SQL parsing and optimization;

Samza and Storm use Calcite for streaming SQL.

As of August 2016, Apex, Phoenix and Flink have projects under development that use Calcite.

Aragonite

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.

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching stalactitic forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.

Calcareous sponge

The calcareous sponges of class Calcarea are members of the animal phylum Porifera, the cellular sponges. They are characterized by spicules made out of calcium carbonate in the form of calcite or aragonite. While the spicules in most species have three points, in some species they have either two or four points.

Calcite rafts

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".

Calcium carbonate

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.

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3) and dolostone, which is composed of the mineral dolomite (CaMg(CO3)2).

Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.

When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.

Karst topography and caves develop in carbonate rocks because of their solubility in dilute acidic groundwater. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.

Marble is the metamorphic carbonate rock. Rare igneous carbonate rocks exist as intrusive carbonatites and even rarer volcanic carbonate lava.

Cementation (geology)

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms

"bridges" between original sediment grains, thereby binding them together. In this way sand becomes "sandstone", and gravel becomes "conglomerate" or "breccia". Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz or silica phases like cristobalite, iron oxides, and clay minerals, but other mineral cements also occur.

Cementation is continuous in the groundwater zone, so much so that the term "zone of cementation" is sometimes used interchangeably. Cementation occurs in fissures or other openings of existing rocks and is a dynamic process more or less in equilibrium with a dissolution or dissolving process.

Cement found on the sea floor is commonly aragonite and can take different textural forms. These textural forms include pendant cement, meniscus cement, isopachous cement, needle cement, botryoidal cement, blocky cement, syntaxial rim cement, and coarse mosaic cement. The environment in which each of the cements is found depends on the pore space available. Cements that are found in phreatic zones include: isopachous, blocky, and syntaxial rim cements. As for calcite cementation, which occurs in meteoric realms (freshwater sources), the cement is produced by the dissolution of less stable aragonite and high-Mg calcite. (Boggs, 2011)

Classifying rocks while using the Folk classification depends on the matrix, which is either sparry (prominently composed of cement) or micritic (prominently composed of mud).

Chalk

Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).

Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have very few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones. This presumably indicates very stable conditions over tens of millions of years.

Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall, steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

Dogtooth spar

Dogtooth spar is a speleothem found in limestone caves that consists of very large calcite crystals resembling dogs' teeth (hence the name). They form through mineral precipitation of water-borne calcite. Dogtooth spar crystals are not limited to caves, but can grow in any open space including veins, fractures, and geodes.

These sharp tooth-shaped crystals are generally of the magnitude of centimeters long, but anomalous samples decimeters long exist, notably in Sitting Bull Crystal Caverns. A layer of crystalline calcite can be found underneath the surface of crystal points.

The sharply tooth-shaped crystals typically consist of acute scalenohedrons, twelve triangular crystal faces that ideally form scalene triangles. However, modification of these faces is common, and individual crystal faces may have many more than three edges. Calcite crystallizes in the rhombohedral system, and the most common scalenohedron form has the Miller index [2131].

Spar is a general term for transparent to translucent, generally light-colored and vitreous crystalline minerals.

Hard water

Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone and chalk which are largely made up of calcium and magnesium carbonates.

Hard drinking water may have moderate health benefits, but can pose critical problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of foam formation when soap is agitated in water, and by the formation of limescale in kettles and water heaters. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects.

Hexacorallia

Hexacorallia is a subclass of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are colonial and reef-forming, as well as all sea anemones, and zoanthids, arranged within five extant orders. The hexacorallia are distinguished from another subclass of Anthozoa, Octocorallia, in having six or fewer axes of symmetry in their body structure; the tentacles are simple and unbranched and normally number more than eight. These organisms are formed of individual soft polyps which in some species live in colonies and can secrete a calcite skeleton. As with all Cnidarians, these organisms have a complex life cycle including a motile planktonic phase and a later characteristic sessile phase. Hexacorallia also include the significant extinct order of rugose corals.

Limestone

Limestone is a sedimentary rock which 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).

About 10% of sedimentary rocks are limestones. The solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock.

Limestone has numerous uses: as a building material, an essential component of concrete (Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.

Magnesite

Magnesite is a mineral with the chemical formula MgCO3 (magnesium carbonate). Iron, manganese, cobalt and nickel may occur as admixtures, but only in small amounts.

Manganoan calcite

Manganoan calcite or Manganocalcite is a variety of calcite rich in manganese, which gives the mineral a pink color. Its chemical composition is (Ca,Mn)CO3. It was first reported from the Banská Štiavnica Mining District, Slovak Republic, but is widely distributed around the world, notably at Naica, Chihuahua, Mexico, in the Cave of Swords and Bulgaria.

Manganoan calcite is sometimes confused with rhodochrosite. The amount of manganese in manganocalcite varies at different localities, and the mineral forms a solid solution series between calcite and rhodochrosite, with the color becoming redder with a higher proportion of manganese.

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Speleothem

Speleothems ( ; Ancient Greek: "cave deposit"), commonly known as cave formations, are secondary mineral deposits formed in a cave. Speleothems typically form in limestone or dolostone 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.

Sphalerite

Sphalerite ((Zn, Fe)S) is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides along with calcite, dolomite, and fluorite. Miners have also been known to refer to sphalerite as zinc blende, black-jack and ruby jack.

Thermolithobacteria

Thermolithobacteria is a class of rod-shaped Gram-positive bacteria within phylum Firmicutes. Species within this class are thermophilic lithotrophs isolated from sediment in Calcite Springs in Yellowstone National Park. Thermolithobacter ferrireducens strain JW/KA-2(T) metabolism consists of the oxidation of hydrogen gas and reduction of ferric oxide to magnetite. Thermolithobacter carboxydivorans strain R1(T) is hydrogenic and oxidizes carbon monoxide.

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