The Science of Sustainability

Smelly Rocks: Researchers Reveal The Source of "Stinkspar" Stench

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antozonite

I call fluorine the Tyrannosaurus rex of the elements because it reacts with everything. That's why chemists thought the element was only found in nature as ions bound in minerals like fluorite (CaF2). Now researchers in Germany have found bubbles of diatomic fluorine (F2) trapped in a special type of fluorite called antozonite.

In 1816, scientists noticed antozonite releases a powerful stench when crushed, earning it the nicknames “fetid fluorite” and “stinkspar.” Seventy years later, Henri Moissan studied a sample of antozonite and concluded that the rock contained elemental fluorine, chemist Florian Kraus told Nature News. And if anyone should know what fluorine smells like, Moissan would. He won the 1906 Nobel Prize in chemistry in part for synthesizing fluorine gas.

But mineralogists weren’t convinced by Moissan’s source of the stink, proposing instead that the rock held fluoride-, selenium- or phosphorous-containing compounds. Solving this almost-200-year old mystery took a mix of old-fashioned chemical analysis and modern instruments.

I marvel at how the first chemists first learned about the properties and reactivity of each element. Their laboratory equipment seems crude by today’s standards, yet their logic was quite clever. Chemists knew that the chemical formula for hydrofluoric acid was HF before Moissan prepared fluorine gas!

Here’s how they figured that out. French chemist Antoine Lavoisier puzzled over the composition of acids around 1776, when the element oxygen was a new element on the chemical scene. Lavoisier figured out that oxygen was crucial to combustion (that’s a story for another day), so he thought the element must be in acids too. But others weren’t so sure.

In the early 1800s, English chemist Humphry Davy noticed that when he mixed ammonia (a base) and sulfuric acid (H2SO4), a salt formed. He heated that salt [(NH4)2SO4] until it gave off water vapor and nitrogen gas.

Davy then mixed ammonia and hydrofluoric acid to get another salt. But no water evaporated when he heated that powder. So he concluded that hydrofluoric acid did not contain oxygen like sulfuric acid.

Davy continued working with HF, electrifying it with a large battery to test the reactivity of fluorine. But it was Moissan who finally synthesized fluorine gas in 1886 by electrifying a solution of hydrofluoric acid and a potassium fluoride salt. Pale yellow fluorine gas bubbled up from one of the electrodes submerged in the solution.

Without the instruments of today, chemists analyzed the products based on their observations and knowledge of other reactions. Many of these scientists may not have been able to predict what would happen as they split matter into its chemical elements. Would a reaction catch fire? Explode? Spit out gas that burned their lungs?

These scientists certainly had a dangerous task when they started working with fluorine. It probably didn’t take them long to figure out that hydrofluoric acid etches glass. But they may not have known – as we do now – that spilling that acid on their body would steal calcium from their bones while it burned their skin. In a 1897 lecture to the board of regents of the Smithsonian Institution, Moissan says several of his colleagues became ill after breathing HF vapors.

Fast forward to today. Kraus, a professor at Technical University of Munich in Germany, learned about the mysterious odor of stinkspar while preparing for a lecture and questioned the mineralogists’ theories. “A fluorine chemist knows at once how fluorine smells,” he told Nature News. Kraus thought modern chemical analysis could finally settle the debate.

He collected some stinkspar from a mine in Wölsendorf, about 100 miles from Munich, and crushed some of it in his lab. Kraus compared the odor from the rock to that of pure F2 and other fluorine-containing gases – “at very low concentrations and only very briefly,” reports Chemistry World.

Then he asked Jörn Schmedt auf der Günne, a colleague at Munich University, to test a chunk of the rock using a technique called nuclear magnetic resonance spectroscopy. This allowed the researchers to identify particular elements in the undamaged rock. The analysis revealed a sharp signal for F2.

The scientists think radioactive elements in the rock may split CaF2 into calcium clusters and F2. “It is not surprising that chemists doubted the existence of elemental fluorine in fetid fluorite,” they said in a statement. “The fact that elemental fluorine and calcium, which would normally react with each other at once, are found here side by side is indeed hard to believe.”

Careful experimentation solves chemical mysteries, basic or complex. Now that scientists peer into materials and atoms with powerful instruments, they depend less on their senses to identify elements than the first chemists did. Though it’ll always be hard to forget a stinky smell.

Reference: Angewandte Chemie International Edition, 2012. DOI: 10.1002/anie.201203515

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Melissae Fellet

About the Author ()

Melissae Fellet is a freelance science writer obsessed with electrons, atoms and molecules. Writing about chemistry, physics and technology, she hopes to reveal how the invisible building blocks of matter influence things like plastics, perfumed shampoos and the speedy computer chips we use everyday. She holds a BS in biochemistry and microbiology from the University of Florida and a PhD in chemistry from Washington University in St. Louis. She spends sunny days at her home in Santa Cruz either watching otters in the bay or tromping around the redwood forests.