Na-rich dravite in the marbles from Friesach, Carinthia, Austria: Chemistry and crystal structure
Pertlik, Franz; Ertl, Andreas; Kö rner, Wilfried; Brandstä tter, Franz; Schuster, Ralf
published: Jun 13, 2003
Chemical analyses and a crystal structure determination of light green tourmaline crystals from a marble near Friesach, Carinthia, show that this tourmaline belongs to a near end-member dravite with the highest Mg-content at the Y site of all previously investigated dravites. The composition was determined using electron microprobe data and atomic absorption spectroscopy. Fluorine was determined by F-sensitive electrodes. In combination with the structure refinement, including crystal chemical considerations on the average bond-lengths, these data yield the formula (Na0.66Ca0.18□0.16) (Mg2.46Al0.54Fe2+0.01 Ti0.01) (Al5.64Mg0.36) (BO3)3 [Si6O18] [(OH)3.51F0.12O0.37]. This dravite crystallizes in space group R3m (No. 160) with a = 15.9459(3), c = 7.1723(3)Å. The structure refinement yielded an R value of 0.023 for 898 single crystal X-ray data.