ROSSMANITE, SQUARE(LIAL2)AL-6(SI6O18)(BO3)(3)(OH)(4), A NEW ALKALI-DEFICIENT TOURMALINE - DESCRIPTION AND CRYSTAL-STRUCTURE

Rossmanite is a new tourmaline species from near Rozna, western Moravi a, Czech Republic. It forms pale pink columnar crystals about 25 mm lo ng and 5 mm thick, elongated along c with striations parallel to c on the prism faces. It is brittle, H = 7, D-mean = 3.00 g/cm(3), D-calc = 3.06 g/cm(3). In plane-polarized light, it is colorless. Rossmanite i s uniaxial negative, omega = 1.645(1), epsilon = 1.624(1), trigonal, s pace group R3m, in the hexagonal setting a = 15.770(2), c = 7.085(1) A ngstrom, V = 1525.8(4) Angstrom(3), Z = 3. The strongest six X-ray dif fraction lines in the powder pattern are at d = 3.950 Angstrom with I = 100% for (hkl) = (220); 2.552 Angstrom, 93%, (051); 1.898 Angstrom, 72%, (342); 4.181 Angstrom, 58%, (211); 2.924 Angstrom, 56%, (122); an d 3.434 Angstrom, 53%, (012). Analysis by a combination of electron mi croprobe, SIMS, H-line extraction, and crystal-structure refinement ga ve SiO2 38.10 wt%, Al2O3 44.60, NaO 1.43, Li2O = 1.13, B2O3 = 10.88, H 2O = 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 7)Al-Z(6)(Si5.92O18 )(B2.92O9)(OH)(3.83)F0.10O0.07, with the ideal end-member formula squa re(LiAl2)Al-6(Si6O18)(BO3)(3)(OH)(4); thus rossmanite can be derived f rom elbaite [Na(Al1.5Li1.5)(Si6O18)(BO3)(3)(OH)(4)] by the substitutio n (X)square(2), + Al-Y --> Na-X(2) + Li-Y, where square = vacancy. The crystal structure of rossmanite was refined to an R index of 1.7% usi ng 1094 observed (5 sigma) reflections collected with MoK alpha X-radi ation from a single crystal. The structure refinement confirmed the lo w occupancy of the X site and the presence of Li at the Y site. There is considerable positional disorder at the 01 and 02 sites induced by the local variations in bond-valence distribution associated with squa re-Na disorder at X and Li-Al disorder at Y.