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Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany

^* e-mail: Bernd.Marler{at}ruhr-uni-bochum.de

Two excess-boron olenite samples which had been synthesized from a reaction mixture of 0.625 Na₂O · 4.5 Al₂O₃ · 6.0 SiO₂ · 3.0 B₂O₃ + excess H₂O at 600°C / 25 kbar (sample 1) and 650°C / 20 kbar (sample 2) were structurally analyzed by a Rietveld refinement. The investigated tourmalines possess space group symmetry R3m with lattice parameters of a = 15.5996(8) Å, c = 7.0224(6) Å for sample 1 and a = 15.6329(8) Å, c = 7.0365(6) Å for sample 2. ₂₉Si MAS NMR spectroscopy showed no octahedral Si, so that the octahedral Y and Z sites are exclusively occupied by Al₃₊ ions. The average T-O distances are 1.573 Å and 1.590 Å for samples 1 and 2, respectively, indicating that B₃₊ ions (B-O = 1.470 Å) partly replace Si₄₊ ions (Si-O = 1.620 Å) at the T-position. The refined occupancy factors give a chemical composition of Na_0.8Al_2.9Al₆[Si_3.8B_2.2O₁₈](B₃O₉)(OH,O)₄ for sample 1 and Na_0.7Al_2.9Al₆[Si_4.2B_1.8O₁₈](B₃O₉)(OH,O)₄ for sample 2, approximately confirming an earlier chemical analysis of sample 1. The difference in charge which is generated by the partial replacement of Si⁴⁺ ions by B³⁺ ions is compensated by protons leading to OH-contents near 4. It is interesting to note that sample 1 synthesized at a higher pressure and lower temperature contains a larger amount of tetrahedral boron.

Key-words: olenite, tourmaline, structure, Rietveld refinement, excess boron, tetrahedrally coordinated boron.

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