The [9]-coordinated X site in the crystal structure of tourmaline-group minerals

The [9]-coordinated X site in tourmaline is usually occupied by Na, Ca, K, or is vacant. In contrast to earlier statements in the literature our recent evaluation of 81 tourmaline samples with Al on the Z site has clearly shown that the < X-O > distance, as could have been expected, is positively correlated to the average effective ionic radius of the X-site occupants (r = 0.98 for 81 tourmaline samples, with Al-6 at the Z site and (OH)(3) at the V site). Olenite and oxy-rossmanite samples, in which the V site is not completely occupied by OH, show a significant deviation to this correlation. X-site vacancies (up to similar to 0.7 apfu), as well as a significant variation of < T-O > and < Y-O > distances, do not seem to have a significant effect on the < X-O > distance. Tourmalines of the elbaite-olenite-rossmanite series (with Al-6 at the Z site) show a positive correlation between the < X-O > and the < Z-O > distance (r = 0.80; 40 samples) due to inductive effects in the structure.