Stalactite

Definitions. In addition to showing varying degrees of chemical purity, limestone assumes a number of widely divergent physical forms, including marble, travertine, chalk, calcareous mad, coral, shell, ooHtes, stalagmites, and stalactites. AH these materials are essentially carbonate rocks of the same approximate chemical composition as conventional limestone (2—4). 

Stalactites and stalagmites are conical, icicle-like shapes of pure CaCO that form on roofs and floors, respectively, of caverns. These are precipitated from cold groundwater that drips from limestone crevices. 

FIGURE 10.2 Stalactites hang from the roof of a cave and stalagmites grow from the floor. Both are made of insoluble calcium carbonate formed from the soluble hydrogen carbonate ions in groundwater. 

The story does not end with cavern formation. Many limestone caves, such as Carlsbad Caverns, contain spectacuiar formations that inciude staiagmites, stalactites, and limestone columns. We need to examine the equiiibria more cioseiy to understand how these structures form. 

The amount of calcium carbonate precipitating from any particular drop is imperceptibly small. Nevertheless, over the years, these deposits grow into translucent hollow tubes of CaC03 called soda straws (see photo inset). Soda straws lengthen as water drops fall from their tips. These delicate structures can reach lengths of several feet. In time, water flowing over the outside of the tube adds width to the growing formation, and the soda straw matures into the familiar stalactite. 

Over the eons, the flow and evaporation of water inside a cavern creates a stunning array of rock sculptures. Stalagmites grow upward from the floor, sometimes joining stalactites to form massive columns. Limestone dams create beautiful pools of water. Limestone draperies fall like curtains from water flowing around overhanging rock. Delicate mineral flowers sprout from the walls. All these features result from the aqueous solubility equilibrium of calcium carbonate. 

Zero( ) age deposits and waters. Perhaps the most sensitive test of initial conditions for speleothems and analogous carbonate deposits is to undertake U-series measurements of the most recent calcite in actively growing samples such as straw stalactites and the waters that feed them. Surprisingly, few such measurements have been published. 

Table 1. U, Th data for active straw stalactite and drip water from Conch Bar Cave, Middle Caicos...

The most susceptible material for post-depositional loss or addition of radionuclides is the outer layer of samples that have been exposed to moisture for a long duration. Stratigraphic consistency between ages of the outermost material and that deposited prior to this provides valuable constraints on the technique. Four ages were derived for a band of clear, white calcite deposited on a stalactite from 53.6 m below sea level in a blue hole of Andros, Bahamas (Richards et al. 1994). Isotopic data for the outermost surface, which had been exposed to sea water for at least 8 ka was indistinguishable from the internal material (Fig. 8). 

Figure 8. TIMS ages of latest phase of growth of GB-89-23-2 (Richards et al. 1994). Outermost sample (ABTl n = 4) indistinguishable from ages of older material up to 10 mm inside outer surface of this stalactite. GB-89-23-2 was collected from a depth of 53.6 m below present sea level in Stargate, Andros, Bahamas, and was submerged since inundation by rising sea levels during the last deglaciation.

Figure 12. Uranium-lead ( U/ Pb- ° Pb/ °" Pb) isochron plot for the WHCl stalactite from Peak District, England (Richards et al. 1998).

Figure 13. Tera-Wasserbnrg style three-dimensional concoidia plot for the WHCl stalactite (Richards et al. 1988). Shown is the projection onto the U/ ° Pb- ° Pb/ ° Pb plane (see Fig. 10 caption). The intercept of the three-dimensional fit with this plane is shown by the shaded error ellipse. A) Three-dimensional regression, disequilibrium concordia (5 " U(0) = 320 dashed line], and plane intercept. B) Detail of intercept with 2 U/ ° Pb- ° Pb/ Pb plane, disequilibrium concordia (2o error), regression line, and the secular equilibrium concordia.

A priori considerations. An extensive review of the form and mineralogy of the secondary cave deposits is provided by Hill and Forti (1997), but also see Ford and Williams (1989). By far the most useful speleothems for geological purposes are the most common forms stalactites, stalagmites and flowstones composed of calcite, and occasionally aragonite. Other deposits that have been used to derive useful information include travertines, gypsum crusts, calcite veins and scalenohedral calcite (dog-tooth spar). 

Figure 7.1 Cave formations are caused when rainwater and carbon dioxide mix and form a weak carbonic acid, which then dissolves the calcium carbonate of limestone beneath the earth, allowing for cave formation. The photo shows stalactites and stalagmites and other formations at Luray Caverns in Virginia.

Stalagmites are cave formations that start on the floor and grow upward. These formations are caused when water that contains dissolved limestone drips from the ceiling of the cave and lands on the same spot. Sometimes, water dripping off of a stalactite will actually form a stalagmite directly below it. Eventually, a column forms when the stalactite and stalagmite meet. This feature can take thousands or even millions of years to form. 

Calcium carbonate is the primary component of seashells, antacids, marble and limestone (e.g. stalactites and stalagmites in caves), blackboard chalk, scale in water pipes, and calcium supplements for people and animals. It is also used to capture S02 gas in fossil fuel burning boilers, thereby helping to prevent acid rain, and as a soil additive to provide pH adjustment and calcium to farmers soil. 

The example of chicken breath was inspired by the article, From chicken breath to the killer lakes of Cameroon uniting seven interesting phenomena with a single chemical underpinning by Ron DeLorenzo, Journal of Chemical Education, 2001, 78(2), 191. The article also discusses boiler scale, the way that carbon dioxide partitions between hzzy drink and the supernatant gases (see p. 165), and stalactites and stalagmites. 

Stalactites, 15 29 Stalagmites, 15 29 Stalk fibers, 21 18 Stamp-pad inks, 14 328 Standard cell potential, 15 750 Standard deviation, 13 257, 20 699 Standard electrode potential selected ions, 7 799t... 

Carbonic acid (HjCO ) is produced by dissolving carbon dioxide in water. When formed under pressure, it is the gas used in carbonated drinks. In nature, it dissolves the limestone in caves, resulting in the formation of stalactites and stalagmites. It is corrosive as are other acids, although it is considered a rather weak acid. 

In addition to limestone, calcium is also found in other rocks, coral, shells, eggshells, bones, teeth, and stalactites and stalagmites. 

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