Crystal chemistry of Al-V-Cr oxy-tourmalines from Sludyanka complex, Lake Baikal, Russia

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 oxytourmalines from the same locality show a complete substitution series due to the replacement of Al ↔ V3þ ↔ Cr3þ. 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: YV3þ > YCr >> YAl and ZAl >> ZCr > ZV3þ. 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 V3þ 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–Oi and 〈X–Oi. A strong positive correlation between (V þ 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.