Editing Stalactite (section)

==Formation and type==
[[File:Tropfsteinanimation.gif|right|thumb|upright|Demonstration of drip stone formation in a lab. The blue color is due to the addition of cupric ions (Cu<sup>2+</sup>) to the mother solution.]]

===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>

===Lava stalactites===

Another type of stalactite is formed in [[lava tubes]] while molten and fluid [[lava]] is still active inside.<ref name="BAIRD">{{cite journal | last = Baird | first = A.K. | title = Basaltic "stalactite" mineralogy and chemistry, Kilauea | year = 1982 | volume = 4 | issue = 4 | pages = 146–147 | publisher=Geological Society of America Bulletin, abstracts with programs }}</ref> The mechanism of formation is the deposition of molten dripping material on the ceilings of caves, however with lava stalactites formation happens very quickly in only a matter of hours, days, or weeks, whereas limestone stalactites may take up to thousands of years. A key difference with lava stalactites is that once the lava has ceased flowing, so too will the stalactites cease to grow. This means that if the stalactite were to be broken it would never grow back.<ref name="LARSON"/>

The generic term ''lavacicle'' has been applied to lava stalactites and stalagmites indiscriminately and evolved from the word icicle.<ref name="LARSON"/>

Like limestone stalactites, they can leave lava drips onto the floor that turn into lava stalagmites and may eventually fuse with the corresponding stalactite to form a column.

==== Shark tooth stalactites ====
[[File:Shark tooth stalactites.jpg|right|thumb|Shark tooth stalactites]]
The shark tooth stalactite is broad and tapering in appearance. It may begin as a small driblet of lava from a semi-solid ceiling, but then grows by accreting layers as successive flows of lava rise and fall in the lava tube, coating and recoating the stalactite with more material. They can vary from a few millimeters to over a meter in length.<ref name="BUNNELL">
{{Cite book 
  | last = Bunnell
  | first = Dave
  | title = Caves of Fire: Inside America's Lava Tubes
  | year = 2008
  | pages = 124
  }}
</ref>

==== Splash stalactites ====
As lava flows through a tube, material will be splashed up on the ceiling and ooze back down, hardening into a stalactite. This type of formation results in an irregularly-shaped stalactite, looking somewhat like stretched taffy{{clarify|date=November 2019}}. Often they may be of a different color than the original lava that formed the cave.<ref name="BUNNELL"/>

==== Tubular lava stalactites ====
When the roof of a lava tube is cooling, a skin forms that traps semi-molten material inside. Trapped gases expansion forces lava to extrude out through small openings that result in hollow, tubular stalactites analogous to the soda straws formed as depositional [[speleothems]] in solution caves. The longest known is almost 2 meters in length. These are common in Hawaiian lava tubes and are often associated with a drip stalagmite that forms below as material is carried through the tubular stalactite and piles up on the floor beneath. Sometimes the tubular form collapses near the distal end, most likely when the pressure of escaping gases decreased and still-molten portions of the stalactites deflated and cooled.
Often these tubular stalactites acquire a twisted, vermiform appearance as bits of lava crystallize and force the flow in different directions. These tubular lava helictites may also be influenced by air currents through a tube and point downwind.<ref name="BUNNELL"/>

===Ice stalactites===
[[File:Ice stalactite.jpg|thumb|Ice stalactites on the gutter of a house]]
[[File:Ice stalactites beach.jpg|thumb|Ice stalactites on a frozen beach in [[Bete Grise, Michigan]]]]
A common stalactite found [[season]]ally or year round in many caves is the ice stalactite, commonly referred to as [[icicle]]s, especially on the surface.<ref name="KEIFFER">{{cite web | last =Keiffer | first=Susan | title =Ice stalactite dynamics | year =2010 | url=http://www.geologyinmotion.com/2010/11/ice-stalactite-dynamics.html | access-date =2013-07-08}}</ref> [[Water]] [[seep]]age from the surface will penetrate into a cave and if [[temperature]]s are below [[freezing]], the water will form stalactites. They can also be formed by the freezing of water [[vapor]].<ref name="LACELLE">{{cite web | last =Lacelle | first=Denis | title =Formation of seasonal ice bodies and associated cryogenic carbonates in Cavene De L'Ours, Que' Bec, Canada: Kinetic isotope effects and pseudo-biogenic crystal structures | year =2009 | url=http://www.caves.org/pub/journal/PDF/v71/cave-71-01-48.pdf | publisher =Journal of Cave and Karst Studies | volume = v. 71, no. 1 | pages = 48–62 | access-date =2013-07-08}}</ref> Similar to lava stalactites, ice stalactites form very quickly within hours or days. Unlike lava stalactites however, they may grow back as long as water and temperatures are suitable.

Ice stalactites can also form under [[sea ice]] when [[salinity|saline]] water is introduced to ocean water. These specific stalactites are referred to as [[brinicle]]s.

Ice stalactites may also form corresponding stalagmites below them and given time may grow together to form an ice column.

===Concrete stalactites===
[[Image:Mostar interchange concrete stalactites.jpg|right|thumb|Concrete stalactites]]
[[File:Calthemite straw stalactites - air movement.jpg|thumb|left|upright|[[Calthemite]] soda straw stalactites under a concrete slab]]
{{Main|Calthemite}}

Stalactites can also form on [[concrete]], and on plumbing where there is a slow leak and where there are [[Water hardness|calcium, magnesium or other ions]] in the water supply, although they form much more rapidly there than in the natural cave environment.  These secondary deposits, such as stalactites, stalagmites, flowstone and others, which are derived from the lime, mortar or other calcareous material in [[concrete]], outside of the "cave" environment, can not be classified as "[[speleothem]]s" due to the definition of the term.<ref name=Hill&Forti1986 /> The term "[[calthemite]]" is used to encompass these secondary deposits which mimic the shapes and forms of speleothems outside the cave environment.<ref name=SmithGK2016>Smith, G K. (2016). "Calcite straw stalactites growing from concrete structures". Cave and Karst Science 43(1), pp4-10.</ref>

The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is due to the presence of [[calcium oxide]] in cement.  Concrete is made from aggregate, sand and cement. When water is added to the mix, the calcium oxide in the cement reacts with water to form [[calcium hydroxide]] (Ca(OH)<sub>2</sub>). The [[chemical formula]] for this is:<ref name="Classroom">{{Citation | last1 =Braund | first1 =Martin  | last2 =Reiss  | first2 =Jonathan | publication-date =2004 | title =Learning Science Outside the Classroom | pages=155–156| publisher =Routledge | isbn =0-415-32116-6 | year =2004
}}</ref>

::{{chem|Ca||O||(s)}} + {{chem|H|2|O||(l)}} → {{chem|Ca||(OH)|2||(aq)}}

Over time, any rainwater that penetrates cracks in set (hard) concrete will carry any free [[calcium hydroxide]] in [[Solution (chemistry)|solution]] to the edge of the concrete. Stalactites can form when the solution emerges on the underside of the concrete structure where it is suspended in the air, for example, on a ceiling or a beam.  When the solution comes into contact with [[air]] on the underside of the concrete structure, another [[chemical reaction]] takes place.  The solution reacts with [[carbon dioxide]] in the air and [[precipitate]]s [[calcium carbonate]].<ref name="Classroom"/>

::{{chem|Ca||(OH)|2||(aq)}} + {{chem|C||O|2||(g)}} → {{chem|Ca||C||O|3||(s)}} + {{chem|H|2|O||(l)}}

When this solution drops down it leaves behind particles of calcium carbonate and over time these form into a stalactite.  They are normally a few centimeters long and with a [[diameter]] of approximately {{convert|4 to 5|mm|abbr=in}}.<ref name="Classroom"/>  The growth rate of stalactites is significantly influenced by supply continuity of {{chem|Ca||2+}} saturated solution and the drip rate. A straw shaped stalactite which has formed under a concrete structure can grow as much as 2&nbsp;mm per day in length, when the drip rate is approximately 11 minutes between drops.<ref name=SmithGK2016 /> Changes in [[leachate]] solution [[pH]] can facilitate additional chemical reactions, which may also influence [[calthemite]] stalactite growth rates.<ref name=SmithGK2016 />