Rossmanite, [](LiAl₂)Al₆(Si₆O₁₈)(BO₃)₃(OH)₄, a new alkali-deficient tourmaline; description and crystal structure

Abstract

Rossmanite is a new tourmaline species from near Roz na , western Moravia, Czech Republic. It forms pale pink columnar crystals about 25 mm long and 5 mm thick, elongated along c with striations parallel to c on the prism faces. It is brittle, H 7, D meas 3.00 g/cm 3 , D calc 3.06 g/cm 3 . In plane-polarized light, it is colorless. Rossmanite is uniaxial negative, 1.645(1), 1.624(1), trigonal, space group R3m, in the hexagonal setting a 15.770(2), c 7.085(1) A , V 1525.8(4) A 3, Z 3. The strongest six Xray diffraction lines in the powder pattern are at d 3.950 A with I 100% for (hkl) (220); 2.552 A , 93%, (051); 1.898 A , 72%, (342); 4.181 A , 58%, (211); 2.924 A , 56%, (122); and 3.434 A , 53%, (012). Analysis by a combination of electron microprobe, SIMS, H-line extraction, and crystal-structure refinement gave SiO 2 38.10 wt%, Al 2 O 3 44.60, Na 2 O 1.43, Li 2 O 1.13, B 2 O 3 10.88, H 2 O 3.70, F 0.20, O F 0.08, sum 99.96 wt%, Fe, Mg, Ca, Mn, Ti, F, K not detected. The formula unit (31 anions) is X ( 0.57 Na 0.43 ) Y (Li 0.71 Al 2.17 ) Z Al 6 (Si 5.92 O 18 ) (B 2.92 O 9 )(OH) 3.83 F 0.10 O 0.07 , with the ideal end-member formula (LiAl 2 )Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 4 ; thus rossmanite can be derived from elbaite [Na(Al 1.5 Li 1.5 )(Si 6 O 18 )(BO 3 ) 3 (OH) 4 ] by the substitution X 2 Y Al X Na 2 Y Li, where vacancy. The crystal structure of rossmanite was refined to an R index of 1.7% using 1094 observed (5) reflections collected with MoK X-radiation from a single crystal. The structure refinement confirmed the low occupancy of the X site and the presence of Li at the Y site. There is considerable positional disorder at the O1 and O2 sites induced by the local variations in bond-valence distribution associated with -Na disorder at X and Li-Al disorder at Y.