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

An Al-rich tourmaline from the Sahatany Pegmatite Field at Manjaka, Sahatany Valley, Madagascar, was structurally and chem. characterized. The combination of chem. and structural data yields an optimized formula of X(Na0.53Ca0.09.box.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) .ANG. (R1 = 0.017 for 3241 reflections). The T-O distance of .apprx. 1.611 .ANG. is one of the smallest distances obsd. in natural tourmalines. The very short Y-O distance of .apprx. 1.976 .ANG. reflects the relatively high amt. 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 vol. was found. [4]B increases with the Al content at the Y site approx. as a power function with a linear term up until [4]B Si 3 apfu and YAl 3 apfu, resp., in natural and synthetic Al-rich tourmalines. Short-range order considerations would not allow for [4]B in solid soln. between schorl and elbaite, but would in solid solns. 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 crystd. at elevated PT conditions, it is obvious that there are pronounced correlations between PT conditions and the [4]B content. Towards lower temps. 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 .apprx. 1000-1500 MPa (depending on the temp.) the highest obsd. [4]B content does not change significantly at a given temp. The PT conditions of the formation of [4]B-rich olenite from Koralpe, Eastern Alps, Austria, can be estd. as 500-700 MPa/630 C.