Editing Stalactite (section)

===Limestone stalactites===

The most common stalactites are [[speleothem]]s, which occur in [[limestone]] caves. They form through [[Deposition (geology)|deposition]] of [[calcium carbonate]] and other minerals, which is [[precipitate]]d from mineralized water [[Solution (chemistry)|solution]]s. Limestone is the chief form of calcium carbonate [[Rock (geology)|rock]] which is [[Dissolution (chemistry)|dissolve]]d by [[water]] that contains [[carbon dioxide]], forming a [[calcium bicarbonate]] solution in caverns.<ref>C. Michael Hogan. 2010. [http://www.eoearth.org/article/Calcium?topic=49557 ''Calcium''. eds. A.Jorgensen, C. Cleveland. Encyclopedia of Earth]. National Council for Science and the Environment.</ref>  The [[chemical formula]] for this reaction is:<ref name="Classroom"/>

::{{chem2|CaCO3|}}(s) + {{chem2|H2O|}}(l) + {{chem2|CO2|}}(aq) → {{chem2|Ca(HCO3)2(aq)|}}

This solution travels through the rock until it reaches an edge and if this is on the roof of a [[cave]] it will drip down.  When the solution comes into contact with [[air]] the [[chemical reaction]] that created it is reversed and particles of calcium carbonate are deposited.  The reversed reaction is:<ref name="Classroom"/>

::{{chem2|Ca(HCO3)2(aq)|}} → {{chem2|CaCO3(s)|}} + {{chem2|H2O(l)|}} + {{chem2|CO2(aq)|}}

An average growth rate is {{convert|0.13|mm|abbr=in}} a year.  The quickest growing stalactites are those formed by a constant supply of slow dripping [[water]] rich in calcium carbonate (CaCO<sub>3</sub>) and carbon dioxide (CO<sub>2</sub>), which can grow at {{convert|3|mm|abbr=in}} per year.<ref name="Cave">{{Citation | last1 =Kramer | first1 =Stephen P. | last2 =Day  | first2 =Kenrick L. | publication-date =1994 | title =Caves | page=23| publisher =Carolrhoda Books | isbn =978-0-87614-447-3 | year =1995}}</ref><ref name=Hill&Forti1986>Hill, C A, and Forti, P, (1986, 1997). Cave Minerals of the World, 1st & 2nd editions. [Huntsville, Alabama: National Speleological Society Inc.]</ref> The drip rate must be slow enough to allow the CO<sub>2</sub> to degas from the solution into the cave atmosphere, resulting in deposition of CaCO<sub>3</sub> on the stalactite. Too fast a drip rate and the solution, still carrying most of the CaCO<sub>3</sub>, falls to the cave floor where degassing occurs and CaCO<sub>3</sub> is deposited as a stalagmite.

All limestone stalactites begin with a single mineral-laden drop of water. When the drop falls, it deposits the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (≈4 to 5&nbsp;mm diameter), 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.

Stalactite formation generally begins over a large area, with multiple paths for the mineral rich water to flow.  As minerals are dissolved in one channel slightly more than other competing channels, the dominant channel begins to draw more and more of the available water, which speeds its growth, ultimately resulting in all other channels being choked off.  This is one reason why formations tend to have minimum distances from one another.  The larger the formation, the greater the interformation distance.

==== Pillars ====
[[Image:World's greatest stalagnate in the Nerja's cave, Spain.JPG|thumb|right|upright|Pillars in the [[Caves of Nerja]], Spain]]
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 a [[speleothem]] of calcium carbonate known as a pillar, column, or stalagnate.<ref>{{cite web |url=https://www.showcaves.com/english/explain/Speleothem/Pillar.html |title=Pillars |website=showcaves.com}}</ref>