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 chemically characterized. The combination of chemical and structural data yields an optimized formula of (X)(Na(0.53)Ca(0.09)square(0.38)) (Y)(Al(2.00)Li(0.90)Mn(2+) (0.09)Fe(0.01)(2+)) (Z)Al(6) (BO(3))(3) (T)[Si(5.61)B(0.39)]O(18) (V)(OH)(3) (W)[(OH)(0.6)O(0.4)], with a = 15.777(1), c = 7.086(1) angstrom (R(1) = 0.017 for 3241 reflections). The < T-O > distance of similar to 1.611 angstrom is one of the smallest distances observed in natural tourmalines. The very short < Y-O > distance of similar to 1.976 angstrom 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 At content at the Y site approximately as a power function with a linear term up until ([4])B approximate to Si approximate to 3 apfu and (Y)Al approximate to 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 Al(3+) 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 similar to 1000-1500 MPa (depending on the temperature) the highest observed 4 113 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 degrees C.