Chemical composition and crystal structure refinement of schorl from the Lundazi pegmatite field, Zambia


Authors: Ondruš P, Vrána S, Mašláň M

Published in: Bulletin of Geosciences, volume 77, issue 1; pages: 23 - 28; Received 22 October 2001; Accepted in revised form 11 February 2002;

Keywords: pegmatite, tourmaline, schorl, chemical composition, crystal structure, refinement, cell parameters, pleochroism, Mossbauer spectroscopy, XRD powder diffraction,

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Present work characterizes tourmaline from one of the major pegmatite fields in Zambia - the Lundazi pegmatite belt.
Microprobe analysis gave (wt%): SiO2 34.65, TiO2 0.85, Al2O3 29.80, Fe2O3 4.94, FeO 9.47, MnO 0.29, MgO 4.06, Li2O < 0.01, CaO 0.59, Na2O 2.38, K2O 0.08, F 0.32, H2O+ 3.24, B2O3 10.32, O=F -0.14, totalling 100.45. The empirical formula, calculated on the basis of 31 (O, OH, F) per formula unit gives: (Na0.78Ca0.11K0.020.09)Σ1.00 (Fe2+1.33Fe3+0.63Mg0.61Ti0.11Mn0.040.12)Σ3.00 (Al5.59Mg2+0.41)Σ6.00 [(Si5.84Al0.16)Σ6.00O18.00](BO3)3.00(OH)3.00[(OH)0.64O019F0.17]Σ1.00 resulting in simplified formula: Na(Fe2+, Fe3+, Mg)3(Al,Mg)6(Si6O18)(BO3)3.00(OH)3.00 (OH, O, F). Analysis of Mossbauer spectra confirms both Fe2+ and Fe3+ in the Y site; Fe3+/Fe2+ is 0.47. The water was determinate using Karl Fischer titration, B2O3 was calculated to stoichiometry. Refined crystal structure data (atomic coordinates, inter-atomic distances and angles) are in agreement with data for schorl. Mean polyhedron distances are (Å): X-O 2.646, Y-O 2.078, Z-O 1.939, Si-O 1.644, B1-O 1.382. The refined unit-cell parameters are: a = 15.9856 (2), c = 7.1892 (1) Å, V = 1591.00(4) Å3, space group is R3 m, Z=3. Dm = 3.18(1) g/cm3, Dx = 3.195(2) g/cm3. Streak is light grey, lustre is glassy. Pleochroism in powder mounts: ε = light brownish red, ω = dark bluish grey (thin fragments), ε = strong brownish red, ω = opaque (thick fragments). The studied sample belongs to hydroxy-tourmaline subgroup with 19 atomic % of oxy subgroup (Fe3+/Fe2+ = 0.47 in the Y site and ratio O/(OH-) = 0.30 in the W site).