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Concept
Speleothem
Summary
A speleothem (ˈspiːliəθɛm; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on their depositional history and environment. Their chemical composition, gradual growth, and preservation in caves make them useful paleoclimatic proxies.
More than 300 variations of cave mineral deposits have been identified. The vast majority of speleothems are calcareous, composed of calcium carbonate (CaCO3) minerals (calcite or aragonite). Less commonly, speleothems are made of calcium sulfate (gypsum or mirabilite) or opal. Speleothems of pure calcium carbonate or 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.
Many factors impact the shape and color of speleothems, including the chemical composition of the rock and water, water seepage rate, water flow direction, cave temperature, cave humidity, air currents, aboveground climate, and aboveground plant cover. Weaker flows and short travel distances form narrower stalagmites, while heavier flow and a greater fall distance tend to form broader ones.
Most cave chemistry involves calcium carbonate (CaCO3) containing rocks such as limestone or dolomite, composed of calcite or aragonite minerals. Carbonate minerals are more soluble in the presence of higher carbon dioxide (CO2) and lower temperatures. Calcareous speleothems form via carbonate dissolution reactions whereby rainwater reacts with soil CO2 to create weakly acidic water via the reaction:
H2O + CO2 → H2CO3
As the acidic 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, the lower pCO2 in the cave drives the precipitation of CaCO3 via the reaction:
Ca2+ + 2 HCO3− → CaCO3 + H2O + CO2
Over time, the accumulation of these precipitates form dripstones (stalagmites, stalactites), and flowstones, two of the major types of speleothems.
Official source
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