What is the chemistry of the formation of stalactites?

Question

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Answer 1

The Formation of Stalactites and Stalagmites

Alan Band/Fox Photos/Getty Images

Educational signs in the Luray Caverns in Virginia promote public understanding of the

difference between stalactites and stalagmites.

We can trace the words stalactite and stalagmite back to the Greek word "stalassein,"

which means "to drip." This is fitting because it describes how both are formed in nature.

Although they look lifelike and a little creepy, stalactites and stalagmites grow simply

because of water running over and through inorganic material.

Limestone caves, where most stalactites and stalagmites are found, are mainly composed of

calcite, a common mineral found in sedimentary rocks. Calcite molecules are made of

calcium and carbonate ions, and are referred to as CaCO3, or calcium carbonate. When

rainwater falls over a cave and trickles through rocks, it picks up carbon dioxide and

minerals from limestone. If we add water, carbon dioxide and calcium carbonate together,

we get this equation:

H20 + CO2 + CaCO3 = Ca (HCO3)2

Ca (HCO3)2 is known as calcium bicarbonate, and the water carries the substance, basically

dissolved calcite, through the cracks of the roof of a cave. Once water comes into contact

with the air inside the cave, however, some of the calcium bicarbonate is transformed back

into calcium carbonate, and calcite starts to form around the crack. As water continues to

drip, the length and thickness of the calcite grows, and eventually a stalactite forms on

the ceiling. It can take a very long time for most stalactites to form -- they usually

grow anywhere between a quarter-inch and an inch every century.

Dorling Kindersley/Getty Images

A cross-section of a cave, complete with stalactites

and stalagmites.

It doesn't end there -- we haven't talked about stalagmites yet. Of course, stalagmites

don't just emerge from the ground. The water dripping from the end of a stalactite falls

to the floor of a cave and deposits more calcite into a mound. Soon enough, a stalagmite

will form in a conelike shape. This is why you usually find stalactites and stalagmites in

pairs, and sometimes they'll even grow together to form one big column. There are many

limestone caves around the world famous for their displays of dripstone, including

Carlsbad Caverns in New Mexico, Timpanogos Caves in Utah, Mammoth Cave in Kentucky and

Jenolan Caves and Buchan Caves in Australia.

And if you ever forget which one's which, there are several classic memory tricks to get

stalactites and stalagmites straight. Here are a few:

Stalactites have to hold on "tight" to the top of the cave

Stalactites hang from the ceiling like ladies' tights

Stalactite has a "t" in it, as in "t" for "top"

Stalactite has a "c" in it, as in "c" for "ceiling," and stalagmite has a "g" in it, as in

"g" for "ground"

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Answer 2

I am not an expert, but here is what i think I know... Primarily stalactites are made of limestone (calcium carbonate) it is soluble in water and forms as the dripping water leaves a coating that evaporates over time. Stalactites could also include any other water soluble minerals that are present in the water and left behind through this process.

Then again I could be completely wrong about this.