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Tetrahedrally coordinated boron in Al-rich tourmaline and its relationship to the pressure–temperature conditions of formation

¹ Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstrasse 14, 1090 Wien, Austria
² Department für Lithosphärenforschung, Universität Wien, Geozentrum, Althanstrasse 14, 1090 Wien, Austria
³ Harvard Mineralogical Museum, 24 Oxford Street, Cambridge, MA 02138, USA
⁴ Department für Umweltgeowissenschaften, Universität Wien, Geozentrum, Althanstrasse 14, 1090 Wien, Austria
⁵ Office of the Provost, The University of Vermont, Burlington, VT 05405, USA

^* Corresponding author, e-mail: andreas.ertl{at}a1.net

An Al-rich tourmaline from the Sahatany Pegmatite Field at Manjaka, Sahatany Valley, Madagascar, was structurally and chemically characterized. The combination of chemical and structural data yields an optimized formula of ^X(Na_0.53Ca_0.090.38)^Y(Al_2.00Li_0.90Mn²⁺_0.09Fe²⁺ _0.01)^Z Al₆ (BO₃)₃^T[Si_5.61B_0.39]O₁₈^V(OH)₃^W[(OH)_0.6O_0.4], with a = 15.777(1), c = 7.086(1) Å (R₁ = 0.017 for 3241 reflections). The T–O distance of ~ 1.611 Å is one of the smallest distances observed in natural tourmalines. The very short Y–O distance of ~ 1.976 Å reflects the relatively high amount of Al at the Y site. Together with other natural and synthetic Al-rich tourmalines, a very good inverse correlation (r² = 0.996) between ^[4]B and the unit-cell volume was found. ^[4]B increases with the Al content at the Y site approximately as a power function with a linear term up until ^[4]B Si 3 apfu and ^YAl 3 apfu, respectively, in natural and synthetic Al-rich tourmalines. Short-range order considerations would not allow for ^[4]B in solid solution between schorl and elbaite, but would in solid solutions between schorl, "oxy-schorl", elbaite, liddicoatite, or rossmanite and hypothetical ^[4]B-rich tourmaline end-members with only Al³⁺ at the Y site. By plotting the ^[4]B content of synthetic and natural Al-rich tourmalines, which crystallized at elevated PT conditions, it is obvious that there are pronounced correlations between PT conditions and the ^[4]B content. Towards lower temperatures higher ^[4]B contents are found in tourmaline, which is consistent with previous investigations on the coordination of B in melts. Above a pressure of ~ 1000–1500 MPa (depending on the temperature) the highest observed ^[4]B content does not change significantly at a given temperature. The PT conditions of the formation of ^[4]B-rich olenite from Koralpe, Eastern Alps, Austria, can be estimated as 500–700 MPa/630 °C.

Key-words: Al-rich tourmaline, Manjaka, tetrahedrally coordinated boron, crystal structure, chemical analyses, PT conditions.