New data on the structure of norbergite: location of hydrogen by X-ray diffraction

Two crystals of OH-rich norbergite from the Alpujarride Complex Betic Cordilleras) have been analyzed, and their structure fined by single-crystal X-ray diffraction analysis. The new data confirm the earlier refinement on a composition near the F end-member, and add new insight to the crystal chemistry of this humite-group mineral. In particular, hydrogen atoms were located on the difference-Fourier map. Their position and the consequent structural constraints to the (OH)F-1 substitution suggest that OH-rich norbergite requires very high pressure to crystallize, thus explaining the normal occurrence of the fluorine-dominant compositions.