Formation of Stalactites and Stalacmites

Stalactites are formed by the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions.

Limestone is calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution.

The chemical formula for this reaction is:

CaCO3(s) + H2O(l) + CO2(aq) → 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:

Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)

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.

Concrete stalactites

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

The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide in concrete. This calcium oxide reacts with any rainwater that penetrates the concrete and forms a solution of calcium hydroxide.

The chemical formula for this is:

CaO(s) + H2O(l) → Ca(OH)2(aq)

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 air and another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate.

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(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 half a centimeter.