Crystal chemistry of Al-V-Cr oxy-tourmalines from Sludyanka Complex, Lake Baikal, Russia
Crystal chemistry of Al-V-Cr oxy-tourmalines from Sludyanka Complex, Lake Baikal, Russia
European Journal of Mineralogy (December 2016) 29 (3): 457-472
- absorption
- Asia
- cations
- cell dimensions
- chromium
- Commonwealth of Independent States
- crystal chemistry
- crystal structure
- electron probe data
- fluorine
- halogens
- Lake Baikal
- metals
- optical properties
- optical spectra
- refinement
- refractive index
- ring silicates
- Russian Federation
- silicates
- spectra
- substitution
- tourmaline group
- valency
- vanadium
- X-ray diffraction data
- Sludyanka Complex
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 (super 3+) <--> Cr (super 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: (super Y) V (super 3+) > (super Y) Cr >> (super Y) Al and (super Z) Al >> (super Z) Cr > (super Z) V (super 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 (super O1) (F)- (super Y) (CrCrCr) and (super Z) [Cr(Cr,Mg)]- (super O3) (O (super 2-) )- (super Y) (Cr), while the incorporation of V (super 3+) into the oxy-tourmaline structure is controlled by the local arrangement (super O1) (O (super 2-) )- (super 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.