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

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) Z Al6 (BO3)3 T [Si5.61B0.39]O18 V (OH)3 W [(OH)0.6O0.4], with a = 15.777(1), c = 7.086(1) A ( R 1 = 0.017 for 3241 reflections). The 〈 T –O〉 distance of ~ 1.611 A is one of the smallest distances observed in natural tourmalines. The very short 〈 Y –O〉 distance of ~ 1.976 A 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 ( r 2 = 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 Y Al ≈ 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.