A stalactite (Greek "stalaktites", (Σταλακτίτης), from the word for "drip" and meaning "that which drips") is a type of
speleothem(secondary mineral) that hangs from the ceiling or wall of limestone caves. It is sometimes referred to as dripstone.
Formation and type
Stalactites are formed by the deposition of
calcium carbonateand other minerals, which is precipitated from mineralized water solutions. Limestoneis calcium carbonate rock which is dissolved by waterthat contains carbon dioxideforming a calcium hydrogencarbonate solution. The chemical formulafor this reaction is:
:: → + ]
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "
soda straw" stalactite. Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite. The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws." Given enough time, these formations can meet and fuse to create "columns".
Stalactites can also form in
lava tubes, although the mechanism of formation is much different.
Stalactites can also form on
concrete, and on plumbing where there is a slow leak and limestone (or other minerals) is in the water supply, although they form much more rapidly there than in the natural cave environment (description and experiments see literature).
The way stalactities form on concrete due to different chemistry then those that form naturally in limestone caves and is the result of the presence of
calcium oxidein concrete. This calcium oxide reacts with with any rainwater that penetrates the concrete and forms a solutionof calcium hydroxide. The chemical formulafor this is:
Over time this calcium hydroxide solution reaches the edge of the concrete and, if the concrete is suspended in the air, for example, in a ceiling or a beam, then this will drip down from the edge. When this happens the solution comes into contact with
airand another chemical reactiontakes place. The solution reacts with carbon dioxidein the air and precipitates calcium carbonate.
* [http://www.goodearthgraphics.com/virtcave/staltite/staltite.html The Virtual Cave's page on stalactites]
*" [http://demonstrations.wolfram.com/Stalactites/ Stalactites] " by Enrique Zeleny,
The Wolfram Demonstrations Project.
Wikimedia Foundation. 2010.