Mg-bearing (U)HP tourmaline with a symmetry deviation from the trigonal crystal system

Autor(en)

Andreas Ertl, Peter Bačík

Abstrakt

Preliminary single-crystal structure refinements on (ultra?) high-pressure tourmalines associated with talc and dolomite from Piedmont, Italy, show that, with increasing Mg-Al disorder between the Y and Z sites, the amount of [4]B increases, while the <T-O> distance decreases. Simultaneously a symmetry deviation from the trigonal crystal system (possibly to the monoclinic crystal system) increases. This is the first time that cation disorder could be related to [4]B in Mg-rich tourmaline.

Tourmalines, which were formed under UHP conditions, have an excellent positive correlation between total [6]Al and the determined temperature of tourmaline formation from the different localities (Ertl et al. 2010). Additionally, there is a negative correlation between F content and the temperature of UHP tourmaline formation. However, no significant amounts of [4]B were found in these investigated tourmaline samples. Pronounced correlations between PT conditions and the [4]B content were described: Towards lower temperatures higher [4]B contents are found in tourmaline (Ertl et al. 2008). This is consistent with previous investigations on the B coordination in melts and that above a pressure of ca.
1000–1500 MPa the highest observed [4]B content does not change significantly at a given temperature.

Tourmaline samples from Piedmont are the first high-pressure tourmalines, which show a symmetry deviation from the trigonal crystal system. Further investigations are necessary to learn, if such a deviation can be explained by the Mg-Al disorder and/or by the presence of B at the tetrahedral site. It is also not clear, if such a disorder is dependent on the temperature or on other factors.

References:
Ertl A, Tillmanns E, Ntaflos T, Francis C, Giester G, Körner W, Hughes JM, Lengauer C, Prem M (2008): Tetrahedrally coordinated boron in Al-rich tourmaline and its relationship to the pressure–temperature conditions of formation. Eur. J. Mineral., 20, 881-888.
Ertl A, Marschall HR, Giester G, Henry DJ, Schertl H-P, Ntaflos T, Luvizotto GL, Nasdala L, Tillmanns E (2010): Metamorphic ultra high-pressure tourmalines: Structure, chemistry, and correlations to PT conditions. Am. Mineral., 95, 1- 10.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Naturhistorisches Museum Wien (NHM), Univerzita Komenského v Bratislave

Publikationsdatum

07-2022

Peer-reviewed

Ja

ÖFOS 2012

105113 Kristallographie, 105116 Mineralogie

ASJC Scopus Sachgebiete

Earth and Planetary Sciences (miscellaneous)

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/b1b3e167-df83-4844-93b6-3e440ba67f13