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University of Manitoba, Department of Geological Sciences, Winnipeg, MB, Canada
Rossmanite is a new tourmaline species from near Rozna, 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, omega = 1.645(1), epsilon = 1.624(1), trigonal, space group R3m, in the hexagonal setting a = 15.770(2), c = 7.085(1) Aa, V = 1525.8(4) Aa 3 , Z = 3. The strongest six X-ray diffraction lines in the powder pattern are at d = 3.950 Aa with I = 100% for (hkl) = (220); 2.552 Aa, 93%, (051); 1.898 Aa, 72% (342); 4.181 Aa, 58%, (211); 2.924 Aa, 56%, (122); and 3.434 Aa, 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 identical with F 0.08, sum = 99.96 wt%, Fe, Mg, Ca, Mn, Ti, F, K not detected. The formula unit (31 anions) is x ([] (sub 0.57) Na (sub 0.43) ) Y (Li (sub 0.71) Al (sub 2.17) ) Z Al 6 (Si (sub 5.92) O 18 ) (B (sub 2.92) O 9 ) (OH) (sub 3.83) F (sub 0.10) O (sub 0.07) , with the ideal end-member formula [](LiAl 2 )Al 6 (Si 6 O 18 ) (BO 3 ) (OH) 4 ; thus rossmanite can be derived from elbaite [Na(Al (sub 1.5) Li (sub 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 (5sigma ) reflections collected with MoKalpha 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.
This record provided courtesy of AGI/GeoRef.
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