POVONDRAITE CRYSTAL CHEMISTRY

The tourmaline super-group has a general chemical formula of XY₃Z₆ (T₆O₁₈) (BO₃)₃ V₃W, and povondraite is a species of tourmaline with a formula of NaFe³⁺₃(Fe³⁺₄Mg₂) Si₆O₁₈ (BO₃)₃ (OH)₃O. Through standard normalization procedures, multiple researchers have reported quantities of greater than 1 apfu in the X-site in povondraite. Several different explanations for this anomaly have been explored, including incorrect Fe²⁺/Fe³⁺ ratios, ^[4]B, and OH+F sums. These could all change the normalized values, however they fail to explain the variance seen in povondraite, as any normalization scheme which would lower the X-site cation value also lowers all other cations, resulting in apparent vacancies in other cation sites, which has never been reported for tourmaline. We have collected S-XRD and TEM data on samples of povondraite and EMPA data from several different labs in order to try to isolate the cause of the reported anomaly.