It's pretty much inconceivable that fluorine (F2), perhaps the most reactive element known, should occur uncombined. But that's what a team of German scientists reported in Angewandte Chemie this week after analyzing the black fluorite of Wölsendorf, a mineral that had puzzled mineralogists for two centuries because of its peculiar smell when crushed.
The black Wölsendorf variety of fluorite (CaF2) was given the name "antozonite" in 1859 by Franz Zippe, who took the odor for ozone. Researchers had guessed in the 1850s that fluorine gas was responsible instead, and that became the working hypothesis for a long, long time. But proof was difficult because fluorine is so reactive that detecting it, in the few seconds it exists after crushing, had to rely on the chemist's nose. In a nice bit of nuclear magnetic resonance spectroscopy, the German team proved the presence of fluorine inside unbroken fluorite specimens.
Apparently the presence of abundant uranium is the key. Its natural radiation, which turns fluorite purple by splitting calcium from fluorine, leads to tiny islands of pure Ca and F2 that are separated by fluorite and remain unable to reach each other.