Riddle of the stinking stones solved
Fluorine is the most reactive of the elements, and is so corrosive that even bricks cannot withstand fluorine gas. 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). A certain variety of it, the so-called “fetid fluorite” from the “Maria” mine in Wölsendorf in the Upper Palatinate, has been an object of contention in scientific circles for some 200 years. When crushed, it emits an unpleasant, pungent smell.
Many distinguished chemists, including Friedrich Wöhler (1800-1882) and Justus von Liebig (1803-1873), have suggested various substances as the basis for the smell. In addition to elemental fluorine, various substances like chlorine and diverse fluorocarbons have been proposed as sources. In collaboration with colleagues from the Technical University in Munich (TUM), LMU researchers have now detected elemental fluorine directly in fluorspar, and refuted the idea that it is generated only when the mineral is crushed.
Radiation is the culprit
The work was carried out by a team led by chemist Jörn Schmedt auf der Günne, Head of the Emmy Noether Junior Research Group for Solid-State NMR at LMU, and by Florian Kraus, who directs the Research Group on Fluorochemistry at the TUM. Using a technique known as 19F nuclear magnetic resonance spectroscopy , they were able to identify the fluorine “in-situ”, i.e. non-destructively in its natural environment, and thereby put an end to the long discussions about the cause for the odor of “fetid fluorite”.
“It‘s no wonder that chemists have for so long resisted the notion that elemental fluorine might be present in fetid fluorite,” explain the researchers. “The idea that elemental fluorine and calcium, which would be expected to react with each other instantly, could coexist in the rock fabric is scarcely credible.” However, in fluorspar, another factor comes into play. The elemental fluorine is generated by tiny uranium-containing inclusions in the mineral. These inclusions continuously emit ionizing radiation, which cleaves the fluorite into its constituents, calcium and fluorine. The fluorine can survive if it is trapped in minuscule inclusions. (Angewandte Chemie, 4. July 2012) göd