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

Abstract

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 ↔ V 3+ ↔ Cr 3+ . 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: Y V 3+ > Y Cr >> Y Al and Z Al >> Z Cr > Z V 3+ . 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 (O 2− )– Y (Cr), while the incorporation of V 3+ into the oxy-tourmaline structure is controlled by the local arrangement O1 (O 2− )– 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.