In its element: Fluorine

Fluorine, naturally

 

Fluorine-19 NMR spectroscopy has been used to reveal how despite its renowned reactivity, elemental fluorine can exist in nature.

Fluorine prefers company, it is inherently difficult to prevent it from reacting with other substances. Glass cannot resist it and bricks burn when exposed to fluorine. This is perhaps why the notion that it might occur freely in nature has been an almost moot chemical point. Natural fluorine is usually found as fluorite minerals. Fluorine must be handled carefully but it does has several industrial applications including corrosion prevention and fuel tank diffusion barriers. It is also used to make sulfur hexafluoride for high-voltage switch insulators.

However, there is an odd form of calcium fluoride: antozonite, also known by the curious name of "fetid fluorite", and stinkspat in German. Ever since this material was first discovered two centuries ago scientists have debated the origins of its foul odour. Crush a chunk of antozonite and it produces an intense smell the nature of which has not until now been clearly identified.

Among the chemists who have attempted to explain fetid fluorite have been Friedrich Wöhler (1800-1882) and Justus von Liebig (1803-1873). Indeed, Wöhler proposed that F₂ was responsible for the smell in a letter to Liebig in 1861. Other chemists proposed many the other compounds but no definitive identification was made during the subsequent decades, not through olfactory tests, chemical analyses and mass spectrometric studies. Arsenic, chlorine, iodine, ozone, selenium, and sulfur, phosphorus compounds, hypochlorous acid and hydrofluorocarbons, and of course diatomic fluorine itself, have all been held responsible in their time for the pungent odour.

Direct evidence

Now, researchers at the Technische Universitaet Muenchen (TUM) and the Ludwig-Maximilians-University Munich (LMU) have pinpointed the source of antozonite's fetid air and it turns out to be produced by elemental fluorine trapped within inclusions in the mineral crystal. Writing in the international edition of the journal Angewandte Chemie, the team explains how they picked up the fluorine scent using NMR. They provide the first direct evidence that the mineral has inclusions containing diatomic fluorine and that the gas is not formed by the action of crushing that usually produces the smell.

Florian Kraus, head of the Fluorine Chemistry Work Group at the Department of Chemistry of TUM, and Joern Schmedt auf der Guenne, head of the Emmy-Noether Work Group for Solid State NMR at the Department of Chemistry of the LMU Munich and their colleagues used fluorine-19 NMR spectroscopy to prove beyond any reasonable doubt that fluorine is present in the inclusions and that the mineral need not be destroyed to show that the element is present in situ. This evidence should settle the debate surrounding "stinking fluorspar".

Just right

"It is not surprising that chemists doubted the existence of elemental fluorine in fetid fluorite," the researchers explain. "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." It seems that within antozonite, the conditions are just right for elemental fluorine to exist. The actual source of the gas, however, is even more intriguing as it is apparently generated by minute uranium impurities which constantly ionise the fluorite and split it into calcium and elemental fluorine. The fluorine remains in minute inclusions, separated from the calcium by the essentially inert fluorite. The calcium forms clusters that give antozonite its dark colour.