THE TILT OF THE BORON TRIANGLE IN TOURMALINE

Past tourmaline research has revealed numerous mysteries regarding this enigmatic mineral group. Research has shown, for example, that the stereochemistry of the boron triangle is dependent upon the chemistry in its cation sites. This is particularly true of the Z and Z’ sites, which are bonded to the same oxygen atom. Although the Z and Z’ sites have the same element in each site, the bond lengths and angles to the shared oxygen are different. Furthermore, even though they are bonded to the same oxygen, prior research has shown that the Z’ site has a greater effect on the overall stereochemistry of the boron triangle than the Z site does. Our current hypothesis for this odd behavior by the Z and Z’ sites is that it is caused by a tilt in the boron triangle relative to the c-axis, potentially caused by differences in the chemistry of the different sites. We are examining a database of 99 tourmaline samples consisting of 14 different species, looking at the bond lengths and angles between the different cation sites relative to the c-axis. Specifically, we are focusing on sites that are situated around the boron site and share oxygens with boron. The structural data is then coupled with the chemistry of each sample. We are using this data to investigate what could be causing the tilt in the boron triangle, the difference in bond length of the Z and Z’ sites, and why the Z’ site has a greater effect on the (BO₃) triangle than the Z site. This research has greater implications regarding how the crystal structure of a mineral can affect its chemistry and vice versa. Understanding the cause of the tilt of the boron triangle relative to the c-axis may allow us to obtain a better insight into how the different sites in a mineral bond, and why they bond in the ways that they do.