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

Autor(en)

Andreas Ertl, Yves Fuchs, John M. Hughes, Ralf Schuster, Franz Brandstätter, Stefan Prowatke

Abstrakt

For dumortierite from Gfoehl (Lower Austria), crystal structure refinement in combination with chem. analyses gives the optimized formula (Al0.78.box.0.12Mg0.09Ti0.01)(Al5.70.box.0.20Ti0.06Fe0.04)(Si2.85Al0.15)BO16[O1.17(OH)0.81F0.02], with a = 4.6900(3), b = 11.7875(6), c = 20.1823(11) .ANG.. For dumortierite from Weissenkirchen in der Wachau, Lower Austria, crystal structure refinement in combination with the chem. analyses gives the optimized formula (Al0.74.box.0.13Mg0.12Ti0.01)(Al5.70.box.0.20Ti0.08Fe0.02)(Si2.83Al0.17)BO16[O1.14(OH)0.85F0.01], with a = 4.6948(2), b = 11.8037(5), c = 20.2106(8) .ANG.. Complete chem. characterizations, including light elements (B, H, Li, Be), were combined for the 1st 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 most of the absorption bands. Contrary to M2, M3, and M4, the M1 site has substantial vacancies. Whereas the av. 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 amts. of tetrahedrally-coordinated Al1 were only found at the T1 sites (0.15 apfu) but not at the T2 sites. The MgO content, .apprx.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 Gfoehl unit contain relatively high OH contents (0.81-0.85 pfu), and measureable amts. of both F and BeO, with 0.05-0.06 wt% and 0.01-0.02 wt%, resp. The compns. of these dumortierite crystals from different localities are very similar and may therefore reflect similar PT conditions and fluids during crystn in these pegmatites of the same geol. unit (Gfoehl 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 Gfoehl and the importance of the 3696 cm-1 band in both investigated dumortierite samples suggest a possible ordering of Mg and .box. at the M1 site, in accord with the conclusions from the structure study.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Université Paris-Est-Marne-la-Vallée, Miami University, Geologische Bundesanstalt, Naturhistorisches Museum Wien (NHM), Universität Heidelberg

Journal

European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences

Band

17

Seiten

173-183

Anzahl der Seiten

11

ISSN

0935-1221

Publikationsdatum

2005

Peer-reviewed

Ja

ÖFOS 2012

105116 Mineralogie

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/4dfcd8e0-a955-4c85-a84c-e69f2448b179