The History and Chemistry of Rock–not the Rolling Stones, the Dolomites

The Brentan Dolomites, the only dolomitic group west of the Adige River, photographed by the author at Madonna di Campiglio

If you take a sample of rock from one of the main peaks of Northern Italy’s Dolomite Mountains and add hydrochloric acid, the effervescence will be quite weak. The streaming of carbon dioxide bubbles will be far more vigorous if you add the same acid to rocks from most Alpine peaks. Both samples contain carbonate, the source of CO₂ ,  but the rates differ because the Dolomites consist of mostly CaMg(CO₃)₂ *, a mineral known as dolomite, whereas the Alps are mostly limestone, which contains calcite mineral CaCO₃.

How does dolomite form?

Dinosaur tracks, south of Trento in the Dolomites. They date back to 200 million years ago, around the time that the dolomite material formed. Source: https://www.visittrentino.it/en/dinosaur-tracks_md_2373

First, there has to be some calcium carbonate in the environment. In warm areas, especially in reservoirs of magnesium rich- water in shallow lagoons, magnesium ions will penetrate the calcite and get incorporated into the crystal to yield dolomite:

Mg²⁺_(aq) + 2 CaCO_{3 (s)} –> CaMg(CO₃)_{2 (s)} + 2 Ca²⁺_(aq)

The material of the Dolomite peaks  was made in the Upper Triassic Period, about 200 million years ago when Pangea was splitting up and 70% of species were becoming extinct due to massive volcanism and ensuing global warming. It helped dinosaurs extend a long-lasting advantage over mammals.

How does calcium carbonate form?

Artificial micelles embedded in calcite simulate a similar process in nature. From Nature.

It’s well known that algae, bacteria and especially molluscs are capable of precipitating calcium carbonate. But not to be to reductionist, it should be pointed out that the approximately 5% of a shell that does not consist of CaCO3 makes the shell much stronger. The minor component consists of a protein micelle embedded within the calcium carbonate crystal. Furthermore, the crystal growth itself depends on the protein matrix binding to Ca²⁺.

But where does the carbonate ion come from?

Calcium carbonate is not very water-soluble, so carbonate cannot be obtained directly from limestone in the ocean. Molluscs and bacteria rely on urea hydrolysis. With the help of an enzyme, urea reacts with water to produce both carbamic acid and the alkaline molecule ammonia. Carbamic acid, in turn , can be hydrolyzed to produce carbonic acid (H2CO₃) and more ammonia. Since carbonic acid is in equilibrium with hydrogen carbonate and H+ ions, the hydroxide from the aqueous ammonia-equilibrium can then make carbonate according to the following:

H⁺_(aq) + HCO₃ – _(aq)  +2 OH^–_(aq) –>  CO₃ ^2-_(aq) + 2 H ₂O

What are other sources of hydrogen carbonate?

Also found in baking soda and in every living cell, hydrogen carbonate ion abounds in the sea. And most of it does not come from the hydrolysis of urea. That biochemical cycle serves mostly to provide the ammonia which raises the pH in order to precipitate carbonate. So where does most of the hydrogen carbonate come from? When carbon dioxide dissolves in rainwater, it forms carbonic acid which weathers a variety of rocks, ranging from feldspar to mica.

 

Scanning electron micrograph of a coccosphere of Emiliania huxleyi, composed of plate-shaped calcium carbonate coccoliths. From http://aem.asm.org/content/71/5.cover-expansion

Feldspars are characterized by Si₃O₈^4-, while micas contain Si₃O₁₀^8- . When they react with carbonic acid, they liberate positive ions, transform the silicate polyatomic to a different silicate in the form of clay and also release quartz and hydrogen carbonate ion. Rivers then deliver the ions to the sea where some serve as a reactant for calcium carbonate production in shell material and coccoliths, which originated at the time that the Dolomites’ material was formed.

How did dolomite from the sea become mountains that now reach the clouds?

The Earth’s most common intrinsic igneous rock is granite, which contains quartz along with two forms of feldspar and mica. Granite, has large crystals, suggesting that they cooled slowly underground. The solidified rock only became exposed by uplifting through plate tectonics. The same mechanism is also what lifted the dolomite material out of the sea as the African plate collided with the European one, a process that’s been ongoing for 40 million years and will continue to do so, perhaps closing off the Mediterranean.  The collision not only created the Dolomites but the Alps and the Pyrenees.

Other Sources

Physical Geology, fourth edition, 1988. Plummer and McGeary.

Britannica Macropedia. Minerals. Last Printed Edition, 2010

http://onlinelibrary.wiley.com/doi/10.1111/sed.12001/abstract;jsessionid=EF3999A595CC9E41472559F36FA1CC15.f02t04

https://en.wikipedia.org/wiki/Brenta_group

• * postscript:

 

*In Lake Huron’s Bruce peninsula there are a combination of minerals present. The more erosion-resistant dolomite is at times underneath layers of other sedimentary rock. The less resistant layers have been carved out of the cliffs, leading to the attractive formation known as the Grotto.

 

Again, Guess What’s Being Described

We’re not looking for the name of the clouds in the picture. If we were, we might mistake them for cirrus clouds, whose quantity is probably affected by our mystery.

Noctilucent clouds above Estonia. From Wikipedia

In fact, the bluish clouds are noctilucent clouds composed of ice particles that crystallise around meteoric dust. In the late 1800s, the astronomer Otto Jesse examined simultaneous pictures that were taken 35 km apart, and by comparing the clouds to the position of a star, he obtained a parallax angle. Using a tangent ratio, he calculated that they were 82 000 metres above us, at about 13 times the altitude of cirrus. For a while it was believed that an increasing number of  noctilucent clouds had been forming, serving as a canary for a problem caused by our mystery. But with more data it turns out that the frequency of the formation of this cloud-type oscillates.

We are looking for something that combines with water to convert mica and feldspar into salt, sand and clay. An 18th century Scottish doctor described it as a diamond dissolved in vital air. Both its physical and chemical properties make it essential to life. Until the middle of the 20th century, it was also believed to be the source of that vital air, but that belief turned out to be mistaken.

It is part of a great cycle. Rain, oceans and the most common protein on earth remove it from the air. Volcanoes, mitochondria, and certain human activities return it to the atmosphere. Our mystery absorbs part of the electromagnetic spectrum by stretching asymmetrically and bending in two different ways, so if found in excess, it disturbs the planet far more profoundly and objectively than the way the rock band INXS perturbs my peace of mind.

I was offended by a Fraser Institute brochure that was once once distributed to Canadian schools. It claimed that because the percentage of the mystery is so small relative to the rest of its constituents, it could not possibly be harmful—an odd argument to make, given, for example, the lethal does of botulism toxin. Others such as Bjorn Lomborg do not dispute that its increasing quantity is significant but pretend that the consequences are exaggerated. I wish he knew what he was talking about. Scholar Howard Friel’s  The Lomborg Deception systematically tears apart Lomborg’s thesis; it had gained credibility only because most readers had not dug into the book’s sources. Others feel so threatened by the social and political solutions being proposed to reduce the amount of the mystery that they fund all sorts of organizations to misrepresent the truth. Here is a list of those receiving such funds:

From desmogblog.com

The German word for our mystery has 17 letters, and it is derived from their word for coal, kohlen.  When burnt, coal produces more of the answer to our puzzle than any other fossil fuel.

Greenhouse gases based on Co2equivalents. The impact from fossil fuels is even greater than the pie chart suggests because almost 1/3 of the human-produced methane is released from extracting fossil fuels. The latter also contribute some of the N2O. Compared to CO2, certain fluorine-containing gases are, on average, about 10 000 times more efficient at absorbing infrared. Luckily, those gases along with methane and nitrous oxide are not as abundant as CO2. Source: IPCC(2014) but based on 2010 global emissions.

 

 

Other sources:

Cirrus Clouds and Climate Change http://www.nature.com/nclimate/journal/v1/n1/full/nclimate1068.html

Lectures on the Elements of Chemistry: Delivered in the University of Edingburg …, Volume 1
By Joseph Black

Noctilucent clouds—not a canary for climate change http://onlinelibrary.wiley.com/doi/10.1029/2003EO280001/epdf