At this point, we’ve learned about man-made byproducts and nature-made byproducts. But how about one that’s a little of both? I’m talking about calthemites, which are secondary deposits that form in those man-made caves such as parking garages, mines, and tunnels.

Calthemites grow both on and under these structures in forms that mimic natural cave speleothems like stalactites, stalagmites, flowstone, and so on. They are often the result of an hyperalkalinic solution of pH 9-14 seeping through a concrete structure to the point of coming into contact with the air on the underside. Here, carbon dioxide in the air facilitates the necessary reactions to secondarily deposit calcium carbonate.

These calcium carbonate deposits are usually white, but can be colored red, orange, or yellow thanks to iron oxide. If copper pipes are around, copper oxide can cause calthemites to be blue or green. As pretty as all that sounds, I didn’t find any evidence of these parking garage growths having been turned into jewelry. So there’s your million-dollar idea.

Calthemite Chemistry

The calthemite class also includes secondary deposits in man-made caves and tunnels where there is no concrete lining. Instead, the deposit is derived from limestone or dolomite or some other calcareous rock from which the thing was hollowed out.

Concrete stalactites and such are formed so due to their chemistry, which differs from those formed in limestone caves. They come as a result of calcium oxide in cement. If you’ll recall, concrete is a mixture of sand, aggregate, and cement. When water is added, calcium oxide in the cement reacts and forms calcium hydroxide. Under certain conditions, this can separate out into calcium and hydroxide ions.

The calcium hydroxide readily reacts with carbon dioxide to form calcium carbonate. This happens as soon as the concrete starts to set — the calcium carbonate takes over the mixture, using up all the carbon dioxide within. Atmospheric carbon dioxide continues to react just outside the surface. But it can’t penetrate very far, and so some free calcium hydroxide remains within the concrete.

Any external water source that’s able to seep into the micro cracks and air voids in the set concrete will carry that carbon hydroxide to the underside of the structure. When it hits the atmosphere, carbon dioxide will diffuse into the solution. Over time, the reaction deposits straw-shaped stalactites.

Of course, not all calthemites are flowstone or stalactites. Some times the flow rate is so fast that the water drips to the ground, where stalagmites can form. Of course, depending on the location, they can be trod upon and driven over and thus continually ground down into nothing noticeable.

A Growth Mindset

The growth rates of the various types of calthemites highly depends on the drip rate and supply of calthemite solution to the calcium carbonate deposition site. Growth also depends on the available carbon dioxide, as it dictates how much calcium carbonate can be created, and how quickly. Conversely, evaporation and ambient temperature appear to have little influence on growth rate.

Calthemite stalactites, on average, grow much faster than natural cave speleothems — up to nearly 200 times as fast. There’s a calthemite straw out there that has been recorded over several consecutive days as growing 2 mm per day thanks to a drip rate of one every 11 minutes.

If the drip rate exceeds one drop per minute, a straw cannot form. Instead, the solution falls to the ground and the calcium carbonate is deposited as a stalagmite. On the other end, a drip rate greater than ~25-30 minutes, there’s a chance that the tip of the straw will calcify and become blocked. New calthemite straws can often grow next to dormant ones when the solution finds a path of lesser resistance.

Although both are composed of calcium carbonate, calthemite straw walls are quite a bit thinner than those of their natural cave speleological brethren. This is because the chemistry differs in creating the straws. On average, calthemite straws are 40% the mass per unit length of cave straws with equivalent diameter.

Beautiful Bones

This type of accidental semi-man-made ite has a kind of beauty I didn’t expect to find when I started writing this article. I’ll definitely be looking out for these formations the next time I’m in a parking garage, and I might even be tempted to break one off.

Unfortunately, not every byproduct can be beautiful. Some are just terrifying. Stay tuned!