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 Na0.8Al2.9Al6[Si3.8B2.2O18](B3O9)(OH,O)4 for sample 1 and Na0.7Al2.9Al6[Si4.2B1.8O18](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.

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