EFFECTS OF THE PETROCHEMICAL ENVIRONMENT ON THE X-SITE IN TOURMALINE

The tourmaline group comprises trigonal borosilicates of the cyclosilicate superclass with an end-member formula of XY₃Z₆ (T₆O₁₈) (BO₃)₃ V₃W. Crystallographically, there is only one location large cations can fit, the 9-coordinated X-site which is limited to 1.0 apfu. However, multiple researchers have reported greater than 1.0 apfu of such cations in certain members of the tourmaline group. This anomaly was examined in povondraite (NaFe³⁺₃(Fe³⁺₄Mg₂) Si₆O₁₈ (BO₃)₃ (OH)₃O) by studying the crystal structure for abnormalities in bond lengths. With this information, we explored the possibility of large cations populating another site beyond X. There was evidence which allowed this possibility, but more data were needed. Over-occupation of the X-site is not limited to povondraite, leading us to also investigate other tourmaline species. Using data acquired by EMPA and S-XRD along with published data, we are investigating the limits of the tourmaline structure in terms of the phenomenon of site-expansion and accommodation and how it may be affected by the environment in which each species forms. We are exploring the variation in tourmaline chemistry in combination with data about the petrologic environment to shed light on anomalous site occupation in tourmaline.