The influence of cationic partitioning among crystallographic sites based on bond-valence constraints and the genetic environment geochemistry on the composition of tourmaline-supergroup minerals

Theoretical bond-length calculation from ideal bond valences for each ion and coordination can predict ion site preference in the structure. It revealed that the B-site occupancy is strictly limited to B³⁺, T site can freely accommodate Si⁴⁺ and Be²+, B³⁺ and Al³⁺ substitution requires compression or expansion of TO₄ tetrahedron. Proper bond lengths for octahedral sites were calculated for Al³⁺ (Z-site preference), Ti⁴⁺, Mn³⁺, Ga³⁺, V³⁺, Fe³+ (mixed preference), Mg²⁺, Fe²⁺, Li⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cu²⁺, Sc³⁺ and Zr⁴⁺ (Y-site preference). Another group of cations including U⁴+, Th⁴⁺, Y³⁺, lanthanoids from Tb³⁺ to Lu³⁺ and Ce⁴⁺ have significantly longer bonds than typical Y-O but form too short bonds for the X site. Therefore, they probably prefer octahedron. The usual empirical bond length for the X site is met with Na⁺, Ca²⁺, Sr²⁺, Pb²⁺ and lanthanoids from La³⁺ to Gd³⁺, while K⁺, Rb⁺ and Cs⁺ are too big. However, the tourmaline composition results from interaction of structure with the genetic environment. The proportion of REE, U⁴⁺ and Th⁴⁺ in tourmaline is structurally limited, while e.g. Zr⁴⁺, Sc³⁺, Sr²⁺, Pb²⁺ have only geochemical limits with no obvious structural constraints. Moreover, environmental properties such as pressure or specific local structural arrangements can overcome structural constraints. (authors)