Na-rich dravite in the marbles from Friesach, Carinthia, Austria: Chemistry and crystal structure

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-contentat 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 (Na 0 . 6 6 Ca 0 . 1 8 □ 0 . 1 6 ) (Mg 2 . 4 6 Al 0 . 5 4 Fe 2 + 0 . 0 1 Ti 0 . 0 1 ) (Al 5 . 6 4 Mg 0 . 3 6 ) (BO 3 ) 3 [Si 6 O 1 8 ] [(OH) 3 . 5 1 F 0 . 1 2 O 0 . 3 7 ]. This dravite crystallizes in space group R3m (No. 160) with a = 15.9459(3), c = 7.1723(3) A. The structure refinement yielded an R value of 0.023 for 898 single crystal X-ray data.