Original paper

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

Ertl, Andreas; Tillmanns, Ekkehart; Ntaflos, Theodoros; Francis, Carl; Giester, Gerald; Körner, Wilfried; Hughes, John M.; Lengauer, Christian; Prem, Markus

Abstract

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(Na0.53Ca0.09□0.38) Y(Al2.00Li0.90Mn2+0.09Fe2+0.01) ZAl6 (BO3)3 T[Si5.61B0.39]O18 V(OH)3 W[(OH)0.6O0.4], 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.

Keywords

al-rich tourmalinemanjakatetrahedrally coordinated boroncrystal structurechemical analysespt conditions