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.
Chemical originsMore than 300 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of in the form of or , or in the form of . Calcareous speleothems form via carbonate dissolution reactions. Rainwater in the soil zone reacts with soil CO2 to create weakly acidic water via the reaction: :H2O + CO2 → H2CO3 As the lower pH water travels through the calcium carbonate bedrock from the surface to the cave ceiling, it dissolves the bedrock via the reaction: :CaCO3 + H2CO3 → Ca2+ + 2 HCO3− When the solution reaches a cave, degassing due to lower cave pCO2 drives of CaCO3: :Ca2+ + 2 HCO3− → CaCO3 + H2O + CO2 Over time the accumulation of these precipitates form , , and s, which compose the major categories of speleothems. Calthemites which occur on concrete structures, are created by completely different chemistry to speleothems.
Chemistry and physical characteristicsMore than 250 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate (CaCO3) in the form of or . Less commonly, speleothems are composed of , mirabilite, or opal. 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. Speleothems of pure calcium carbonate and calcium sulfate are translucent and colorless. The presence of iron oxide or copper provides a reddish brown color. The presence of manganese oxide can create darker colors such as black or dark brown. Speleothems can also be brown due to the presence of mud and silt. Most cave chemistry revolves around calcium carbonate (CaCO3), the primary mineral in . It is a slightly soluble mineral whose solubility increases with the introduction of carbon dioxide (CO2). 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.
Types and categoriesSpeleothems 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 with 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 ** Stalagnate, Columns result when stalactites and stalagmites meet or when stalactites reach the floor of the cav * 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, Rimstone dams, or gours, occur at stream ripples and form barriers that may contain water ** Stone waterfall formations simulate frozen cascades * Cave crystals ** Dogtooth spar are large calcite crystals often found near seasonal pools ** Frostwork is needle-like growths of calcite or aragonite ** Moonmilk is white and cheese-like ** Anthodites are flower-like clusters of aragonite crystals ** ''Cryogenic'' calcite crystals are loose grains of calcite found on the floors of caves, and are formed by segregation of solutes during the freezing of water. * Speleogens (technically distinct from speleothems) are formations within caves that are created by the removal of bedrock, rather than as secondary deposits. These include: ** Pillars ** Scallops ** Boneyard ** Boxwork * Others ** 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 Redox, 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 (cave formations), Hells Bells, a particular speleothem found in the El Zapote cenote of Yucatan in the form of submerged, bell-like shapes ** Lava tubes contain speleothems composed of sulfates, mirabilite or opal. When the lava cools, precipitation occurs.
As climate proxiesSpeleothems are studied as climate Proxy (climate), proxies because their location within cave environments and patterns of growth allow them to be used as archives for several climate variables. By sampling along a dated transect of a speleothem, speleothems chemical composition and growth rates provide paleoclimate records similar to those from Ice core, ice cores. Variations in precipitation alter the width of new ring formation, where closed ring formation shows little rainfall, and wider spacing shows heavier rainfall. Denser speleothems indicates higher moisture availability. The principal proxies that are recorded in speleothems include stable isotopes of oxygen (Δ18O, δ18O) and carbon (Δ13C, δ13C), giving high-resolution data that can show annual variation in rainfall temperature and precipitation. 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. The geometrical way in which stalagmites grow, which varies based on the height the water is falling from and the rate of flow, is also used in paleoclimate applications. Weaker flows and short travel distances form narrower stalagmites, while heavier flow and a greater fall distance tend to form broader ones. Additionally, drip rate counting and trace element analysis of 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 particular strength of speleothems for paleoclimate proxies is their unique ability to be accurately and precisely dates over much of the late Quaternary by radiocarbon dating and uranium-thorium dating. For accurate dating, the speleothem must have been in a closed system without Recrystallization (geology), recrystallization.
Absolute datingAnother 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 radiation. 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. Not all the samples are suited for ESR dating: indeed, the presence of cationic impurities such as Mn2+, Fe2+, or Fe3+, and Humus, 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 percent of the samples tested are in fact suitable for dating. 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.
Calthemites: secondary deposits not formed in cavesSecondary deposits derived from concrete, Lime (material), lime, Mortar (masonry), mortar or calcareous material as found on man-made structures ''outside the cave environment'' or in artificial caves (e.g. Mining, mines and tunnels), can mimic the shapes and forms of speleothems, but are classed as calthemites.Smith, G.K., (2016). "Calcite Straw Stalactites Growing From Concrete Structures", ''Cave and Karst Science'', Vol. 43, No. 1, pp. 4–10, (April 2016), British Cave Research Association, . The occurrence of calthemites is often associated with 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, 637–644. but could also be linked to Leaching (chemistry), 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.