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A structure formed in a cave by the deposition of minerals from water
Image showing the six most common speleothems with labels. Enlarge to view labels.
Speleothems (/ˈspiːliːəθɛm/; Ancient Greek: "cave deposit"), commonly known as cave formations, are secondary mineral deposits formed in a cave. Speleothems typically form in limestone or dolomitesolutional 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 other man-made structures. Hill and Forti more concisely defined "secondary minerals" which create speleothems in caves:
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.
319 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate in the form of calcite or aragonite, or calcium sulfate in the form of gypsum. Calcareous speleothems form via carbonate dissolution reactions. Rainwater in the soil zone reacts with soil CO2 to create weakly acidic water via the reaction:
Speleothems take various forms, depending on whether the water drips, seeps, condenses, flows, or ponds. Many speleothems are named for their resemblance to man-made or natural objects. Types of speleothems include:
Dripstone is calcium carbonate in the form of stalactites or stalagmites
Stalactites are pointed pendants hanging from the cave ceiling, from which they grow
Soda straws are very thin but long stalactites having an elongated cylindrical shape rather than the usual more conical shape of stalactites
Helictites are stalactites that have a central canal with twig-like or spiral projections that appear to defy gravity
Include forms known as ribbon helictites, saws, rods, butterflies, hands, curly-fries, and "clumps of worms"
Chandeliers are complex clusters of ceiling decorations
Ribbon stalactites, or simply "ribbons", are shaped accordingly
Stalagmites are the "ground-up" counterparts of stalactites, often blunt mounds
Broomstick stalagmites are very tall and spindly
Totem pole stalagmites are also tall and shaped like their namesakes
Fried egg stalagmites are small, typically wider than they are tall
Columns result when stalactites and stalagmites meet or when stalactites reach the floor of the cave
Flowstone is sheet like and found on cave floors and walls
Draperies or curtains are thin, wavy sheets of calcite hanging downward
Bacon is a drapery with variously colored bands within the sheet
Rimstone dams, or gours, occur at stream ripples and form barriers that may contain water
Stone waterfall formations simulate frozen cascades
Dogtooth spar are large calcite crystals often found near seasonal pools
Frostwork is needle-like growths of calcite or aragonite
Cave popcorn, also known as "coralloids" or "cave coral", are small, knobby clusters of calcite
Cave pearls are the result of water dripping from high above, causing small "seed" crystals to turn over so often that they form into near-perfect spheres of calcium carbonate
Snottites are colonies of predominantly sulfur oxidizing bacteria and have the consistency of "snot", or mucus
Calcite rafts are thin accumulations of calcite that appear on the surface of cave pools
Hells Bells, a particular speleothem found in the El Zapote cenote of Yucatan in the form of submerged, bell-like shapes
Speleothems made of sulfates, carbonates, mirabilite or opal occur in some lava tubes. Although sometimes similar in appearance to speleothems in caves formed by dissolution, lava stalactites are formed by the cooling of residual lava within the lava tube.
Speleothems formed from salt, sulfur and other minerals are also known.
Speleothems made of pure calcium carbonate are a translucent white color, but often speleothems are colored by chemicals such as iron oxide, copper or manganese oxide, or may be brown because of mud and silt particulate inclusions.
Many factors impact the shape and color of speleothem formations including the rate and direction of water seepage, the amount of acid in the water, the temperature and humidity content of a cave, air currents, the above ground climate, the amount of annual rainfall and the density of the plant cover. Most cave chemistry revolves around calcium carbonate (CaCO3), the primary mineral in limestone and dolomite. It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide (CO2). It is paradoxical in that its solubility decreases as the temperature increases, unlike the vast majority of dissolved solids. This decrease is due to interactions with the carbon dioxide, whose solubility is diminished by elevated temperatures; as the carbon dioxide is released, the calcium carbonate is precipitated.
Most other solution caves that are not composed of limestone or dolomite are composed of gypsum (calcium sulfate), the solubility of which is positively correlated with temperature.
As climate proxies
Speleothems are studied as climate proxies because their location within cave environments and patterns of growth allow them to be used as archives for several climate variables. The principal proxies measured are oxygen and carbon isotopes and trace cations. These indicators, alone and in conjunction with other climate proxy records, can provide clues to past precipitation, temperature, and vegetation changes over the last ~ 500,000 years.
A particular strength of speleothems in this regard is their unique ability to be accurately dated over much of the late Quaternary period using the uranium-thorium dating technique. Stable isotopes of oxygen (δ18O) and carbon (δ13C) are recorded well in speleothems, giving high-resolution data that can show annual variation in temperature (oxygen isotopes primarily reflect rainfall temperature) and precipitation (carbon isotopes primarily reflect C3/C4 plant composition and plant productivity, but the interpretation is often complicated). By sampling along a dated transect of a speleothem, these isotope values and speleothem growth rates provide a paleoclimate records similar to those from ice cores. Variations in precipitation alter the width of new ring formation, where close ring formation shows little rainfall, and wider spacing for heavier rainfall.
The geometrical way in which stalagmites grow is also used in paleoclimate applications, which varies based on the height the water is falling from, and the rate of flow. Weaker flows and short travel distances form more narrow stalagmites, while heavier flow and a greater fall distance tends to form more broad ones. Additionally, drip rate counting and trace element analysis on the water drops themselves have been shown to record shorter-term variations in the climate at high resolution, such as drought conditions attributed to the El Niño-Southern Oscillation (ENSO) climate events.
A recent technique is the use of CT-scanning on intact specimens to analyze density, where more dense speleothem development indicates higher moisture availability.
Another dating method using electron spin resonance (ESR) — also known as electron paramagnetic resonance (EPR) — is based on the measurement of electron-hole centers accumulated with time in the crystal lattice of CaCO3 exposed to natural radiations. In principle, in the more favorable cases, and assuming some simplifying hypotheses, the age of a speleothem could be derived from the total radiation dose cumulated by the sample and the annual dose rate to which it was exposed. Unfortunately, not all the samples are suited for ESR dating: indeed, the presence of cationic impurities such as Mn2+, Fe2+, or Fe3+, humic acids (organic matter), can mask the signal of interest, or interfere with it. Moreover, the radiation centers must be stable on geologic time, i.e., to have a very large lifetime, to make dating possible. Many other artifacts, such as, e.g., surface defects induced by the grinding of the sample can also preclude a correct dating. Only a few percents of the samples tested are in fact suitable for dating. This makes the technique often disappointing for the experimentalists. One of the main challenge of the technique is the correct identification of the radiation-induced centers and their great variety related to the nature and the variable concentration of the impurities present in the crystal lattice of the sample. ESR dating can be tricky and must be applied with discernment. It can never be used alone: "One date only is No date", or in other words, "multiple lines of evidence and multiple lines of reasoning are necessary in absolute dating". However, "good samples" might be found if all the selection criteria are met.
Calthemites: secondary deposits not in caves
Secondary deposits derived from concrete, lime, mortar or calcareous material as found on man-made structures outside the cave environment or in artificial caves (e.g. mines and tunnels), can mimic the shapes and forms of speleothems, but are classed as calthemites. The occurrence of calthemites is often associated with concrete degradation, but could also be linked to leaching of lime, mortar or other calcareous material (e.g. limestone and dolomite). Despite similar appearances, "calthemites" (created outside the cave environment) are not considered to be "speleothems" (created inside the cave environment), and vice versa, as per their definitions.
^Hendy, C. H (1971-08-01). "The isotopic geochemistry of speleothems—I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeoclimatic indicators". Geochimica et Cosmochimica Acta. 35 (8): 801–824. Bibcode:1971GeCoA..35..801H. doi:10.1016/0016-7037(71)90127-X.
^Richards, David A.; Dorale, Jeffrey A. (2003-01-01). "Uranium-series Chronology and Environmental Applications of Speleothems". Reviews in Mineralogy and Geochemistry. 52 (1): 407–460. doi:10.2113/0520407. ISSN1529-6466.
^Li, Zheng-Hua; Driese, Steven G.; Cheng, Hai (2014-04-01). "A multiple cave deposit assessment of suitability of speleothem isotopes for reconstructing palaeo-vegetation and palaeo-temperature". Sedimentology. 61 (3): 749–766. doi:10.1111/sed.12078. ISSN1365-3091.
^McDonald, Janece; Drysdale, Russell; Hill, David (2004-11-01). "The 2002–2003 El Niño recorded in Australian cave drip waters: Implications for reconstructing rainfall histories using stalagmites". Geophysical Research Letters. 31 (22): L22202. Bibcode:2004GeoRL..3122202M. doi:10.1029/2004gl020859. ISSN1944-8007.
^Walczak, Izabela W.; Baldini, James U. L.; Baldini, Lisa M.; McDermott, Frank; Marsden, Stuart; Standish, Christopher D.; Richards, David A.; Andreo, Bartolomé; Slater, Jonathan (2015-11-01). "Reconstructing high-resolution climate using CT scanning of unsectioned stalagmites: A case study identifying the mid-Holocene onset of the Mediterranean climate in southern Iberia". Quaternary Science Reviews. Novel approaches to and new insights from speleothem-based climate reconstructions. 127: 117–128. Bibcode:2015QSRv..127..117W. doi:10.1016/j.quascirev.2015.06.013.
Speleothem are similar to travertine in the way they form and in their composition but there are differences. 
De Cannière, P.; Debuyst R.; Dejehet F.; Apers D.; Grün R. (1986). "ESR dating: a study of 210Po-coated geological and synthetic samples". Nuclear Tracks and Radiation Measurements. 11 (4–5): 211–220. doi:10.1016/1359-0189(86)90037-3.
Viktor A. Slyotov. Concerning the ontogeny of crystallictite and helictite aggregates of calcite and aragonite from the karst caves of Southern Fergana. «Cave Geology», vol. 2, Number 4, March 1999, p. 197-207 // USA, Petersburg, PA 16669-9211