X-SITE OVER-OCCUPANCY IN POVONDRAITE

The tourmaline super-group has a general chemical formula of XY₃Z₆(T₆O₁₈) (BO₃)₃ V₃W. The only crystallographic site in its lattice that is big enough for larger cations to occupy is the X-site; this site is 9-coordinated and allows for 1 apfu. Povondraite is a species of tourmaline with a formula of NaFe³⁺₃(Fe³⁺₄Mg₂) Si₆O₁₈ (BO₃)₃ (OH)₃O. Through standard normalization, multiple researchers have reported quantities of greater than 1 apfu in the X-site in povondraite. Since searches for missing constituents have returned negative, there are a limited number of explanations for this anomaly. Some possibilities for this over-occupancy include 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. While the most likely cause for this anomaly is analytical error, because it has been reported by multiple researchers it deserves further investigation. Other possibilities for this anomaly include actual vacancy in other cation sites or occupancy of larger cations in sites other than X. Either of these could cause the observed anomaly. We are collecting S-XRD and EMPA data on several povondraite crystals, allowing a linkage between crystallography and chemistry. In order to lower the chances of analytical error, we will be collecting EMPA data at a multiple labs. This should allow ample information to determine the cause of this reported anomaly.