INFLUENCE OF THE Z' CATION SITE ON THE B-O2 AND B-O8 BOND-LENGTHS IN TOURMALINE GROUP MINERALS

Tourmaline group minerals are commonly assigned chemical formulae and identified based on normalization of chemical analyses. Difficulty in measuring the lighter elements as well as the valence state of iron can lead to uncertainty when differentiating tourmaline species. For this reason, there has been a significant amount of work investigating methods to identify tourmaline species aside from normalization assumptions. Most research to date has focused on variations and site-ordering at the X-, Y- and Z-sites. Of all the sites in the tourmaline crystal structure, however, the only site that seems to remain chemically static is the B-site, as it is only known to host boron. Clark et al. (in press) found that B-O2 and B-O8 bond-lengths in the BO₃ triangle are inversely related. Further, variation in the bond-lengths is directly related to the chemistry at the other cation sites. Statistical analysis by forward stepwise linear regression showed the cation site which showed the best correlation was the Z'-site, which can be occupied by a variety of cations. The reasons for this correlation are still unclear. We are investigating why the chemical variations of the Z'-site have such a strong influence on the geometry of the BO₃ triangle and how the weaker octahedral bonds can influence the stereochemistry of the B-site in the first place. Preliminary results suggest it may be related to the valence of the Z cation. Additionally, as Z and Z' are the same site, we are examining how the difference in positioning in the crystal lattice can result in such a pronounced variation on the influence each bond has on the BO₃ triangle.