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

摘要

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 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 ~0.7 apfu), as well as a significant variation of and distances, do not seem to have a significant effect on the distance. Tourmalines of the elbaiteolenite- rossmanite series (with Al _6 at the Z site) show a positive correlation between the and the distance (r = 0.80; 40 samples) due to inductive effects in the structure.