ANOMALOUS X-SITE OCCUPANCY IN POVONDRAITE

Povondraite is a species of tourmaline with an ideal formula of NaFe³⁺₃(Fe³⁺₄Mg₂)Si₆O₁₈(BO₃)₃(OH)₃O. Crystallographically, there is only one location large cations can fit, the 9-coordinated X-site which is limited to 1 apfu. However, multiple researchers have reported greater than 1.0 apfu in the X-site based on standard normalization procedures. Possible explanations for these anomalous X-site cation sums are limited, including Fe²⁺/Fe³⁺ ratios, ^[4]B, and OH+F sums; all of which could result in a change in normalized values. However, these possibilities fail to explain the discrepancy seen in povondraite’s X-site; a search for missing constituents also was negative. Two remaining explanations are possible to account for this anomaly. If a vacancy exists in another site, standard normalization procedures would overcompensate by increasing the calculated amounts of all the cations. Alternatively, this value may represent an accurate amount of Na+Ca+K, with these cations populating another site beyond X. Using published data, we are investigating these possibilities by examining the crystal structure of povondraites for abnormalities in bond lengths.