Two excess-boron olenite samples which had been synthesized from a reaction mixture of 0.625 Na 2 O · 4.5 Al 2 O 3 · 6.0 SiO 2 · 3.0 B 2 O 3 + excess H 2 O 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) A, c = 7.0224(6) A for sample 1 and a = 15.6329(8) A, c = 7.0365(6) A for sample 2. 29 Si MAS NMR spectroscopy showed no octahedral Si, so that the octahedral Y and Z sites are exclusively occupied by Al 3+ ions. The average T-O distances are 1.573 A and 1.590 A for samples 1 and 2, respectively, indicating that B 3+ ions (B-O = 1.470 A) partly replace Si 4+ ions (Si-O = 1.620 A) at the T-position. The refined occupancy factors give a chemical composition of Na 0.8 Al 2.9 Al 6 [Si 3.8 B 2.2 O 18 ](B 3 O 9 )(OH,O) 4 for sample 1 and Na 0.7 Al 2.9 Al 6 [Si 4.2 B 1.8 O 18 ](B 3 O 9 )(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 Si 4+ ions by B 3+ 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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Marler et al. (2002) studied this question.