Because of its extreme properties, until now chemists were convinced that fluorine cannot occur in nature in its elemental form, but only as a fluoride ion, for instance in minerals such as fluorite (CaF2), also known as fluorspar. A certain variety of it, the so-called fetid fluorite or antozonite from the Maria mine in Woelsendorf in the Upper Palatinate, Germany, has been an object of contention in science for some 200 years. When crushed, it emits an unpleasant, pungent smell.
A number of eminent chemists, among them Friedrich Woehler (1800-1882) and Justus von Liebig (1803-1873), proposed various substances to explain the odor. Over the years, scientists resorted to olfactory tests, chemical analyses and complex mass spectrometer studies – coming to very different conclusions.
Next to elemental fluorine, substances like iodine, ozone, phosphorus compounds, arsenic, sulphur, selenium, chlorine, hypochlorous acid and hydrofluorocarbons were made responsible for the smell. Direct evidence that this fluorspar has inclusions of fluorine and that the gas does not form during crushing was lacking hitherto.
Now, finally, a team led by Dr Florian Kraus of the Technische Universitaet Muenchen and Dr Joern Schmedt auf der Guenne of the Ludwig-Maximilians-University has succeeded in directly proving the presence of fluorine in antozonite beyond any doubt. Their results appear in a paper in the journal Angewandte Chemie.
Using NMR spectroscopy, the scientists were able to identify the fluorine in-situ and thereby put an end to the long discussions about the cause for the odor of stinking fluorspar.
“It is not surprising that chemists doubted the existence of elemental fluorine in fetid fluorite,” the researchers said. “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.”
However, in the case of antozonite there are very special conditions: the elemental fluorine is generated through minute uranium inclusions in the mineral, which constantly emit ionizing radiation and thus split the fluorite into calcium and elemental fluorine.
The fluorine remains in minute inclusions, separated from the calcium by the non-reactive fluorite and thus retains its elemental form. The ionizing radiation also leads to the formation of calcium clusters, which give antozonite its dark color.
Bibliographic information: Jörn Schmedt auf der Günne, Martin Mangstl, Florian Kraus.. Occurrence of Difluorine F2 in Nature—In Situ Proof and Quantification by NMR Spectroscopy. Angewandte Chemie International Edition. Article first published online: ; doi: 10.1002/anie.201203515