Twelve Al–V–Cr oxy-tourmalines from the Sludyanka crystalline complex (Russia) were structurally and chemically characterized by single-crystal X-ray diffraction and electron microprobe analysis. Three samples were also optically characterized by optical absorption spectroscopy and refractive index measurements. The data obtained and those of fourteen earlier studied oxy-tourmalines from the same locality show a complete substitution series due to the replacement of Al ↔ V3+ ↔ Cr3+. The cation site distribution results show that Mg has a strong preference for the Z site, whereas the trivalent cations have a preference for Y and Z controlled by their size: YV3+ > YCr>> YAl and ZAl>> ZCr > ZV3+.
In accordance with the bond-valence theory, the observed increase of F content with increasing Cr content may be interpreted by the occurrence of the local arrangements O1(F)–Y(CrCrCr) and Z[Cr(Cr,Mg)]–O3(O2−)–Y(Cr), while the incorporation of V3+ into the oxy-tourmaline structure is controlled by the local arrangement O1(O2−)–Y(VVV).
The unit-cell parameters (a and c) are strictly correlated to each other and can be expressed by a quadratic relation dependent on the mean-bond distances 〈Z–O〉 and 〈X–O〉. A strong positive correlation between (V+Cr) content and mean refractive index is proposed as a reliable guide to derive the composition of Al–V–Cr oxy-tourmalines from refractive-index measurements.