Dumortierite from the Gföhl unit, Lower Austria : chemistry, structure, and infra-red spectroscopy

For dumortierite from Gföhl, Lower Austria, crystal structure refinement in combination with chemical analyses gives the optimized formula (Al_0.78□_0.12Mg_0.09Ti_0.01) (Al_5.70□_0.20Ti_0.06Fe_0.04) (Si_2.85Al_0.15) B O₁₆ [O_1.17(OH)_0.81F_0.02], with a = 4.6900(3), b = 11.7875(6), c = 20.1823(11) Å. For dumortierite from Weißenkirchen in der Wachau, Lower Austria, crystal structure refinement in combination with the chemical analyses gives the optimized formula (Al_0.74□_0.13Mg_0.12Ti_0.01) (Al_5.70□_0.20Ti_0.08Fe_0.02) (Si_2.83Al_0.17) B O₁₆ [O_1.14(OH)_0.85F_0.01], with a = 4.6948(2), b = 11.8037(5), c = 20.2106(8) Å. Complete chemical characterizations, including light elements (B, H, Li, Be), were combined for the first time with high-quality structure refinements with R-values in the range 0.018–0.019, allowing assignment of site-occupancies with confidence. The optimizations demonstrate that all Mg occupies the M1 sites. Mg substitution at M1 plays a prevailing role in the high number of absorption bands. Contrary to M2, M3 and M4, the M1 site has substantial vacancies. Whereas the average M-O distances of the M2 and M3 sites are similar in each sample, the M-O distances of the M4 sites in both samples are distinctly lower than M2 and M3. This is a clear indication that, unlike the M2 and M3 sites, the M4 sites contain almost no vacancies and no significant substitutions of Fe or Ti. Significant amounts of tetrahedrally-coordinated A1 were only found at the T1 sites (0.15 apfu) but not at the T2 sites. The MgO content, ∼0.7–0.8 wt%, is relatively high for “normal” dumortierite, but similar to dumortierites from the Saxonian Erzgebirge, Germany. Both dumortierite samples from the Gföhl unit contain relatively high OH contents (0.81–0.85 pfu), and measureable amounts of both F and BeO, with 0.05–0.06 wt% and 0.01–0.02 wt%, respectively. The compositions of these dumortierite crystals from different localities are very similar and may therefore reflect similar PT conditions and fluids during crystallization in these pegmatites of the same geological unit (Gföhl unit). FTIR spectroscopy reveals spectra in the 4000–3000 cm⁻¹ domain that are markedly different from those of dumortierite samples of different origin. The lack of a band at 3675 cm⁻¹ in the sample from Gföhl and the importance of the 3696 cm⁻¹ band in both investigated dumortierite samples suggest a possible ordering of Mg and □ at the M1 site, in accord with the conclusions from the structure study.