STEREOCHEMICAL CONSTRAINTS IN TOURMALINE: FROM A SHORT-RANGE TO A LONG-RANGE STRUCTURE

Extension of the local bond-valence approach from Mg-Al to Fe(2+)-Fe(3+) short-range arrangements is explored in the structure tourmaline. Stable local arrangements involving trivalent (R(3+)) cations (Al, Fe(3+)) and divalent (R(2+)) cations (Mg, Fe(2+)) around the W and V anion sites in Li-free tourmaline result from short-range bond-valence requirements. The coupling of these stable local arrangements determines the formation of larger clusters of octahedra of general form [WY(3)VZ(2)], which can have either ordered or disordered distributions of R3+ and R2+ cations. These clusters are related through four different expressions: 1) 2 (Y)R(2+) + (Z)R(3+) + (W)(OH)(1-) reversible arrow 2 (Y)R(3+) + (Z)R(2+) + (W)O(2-), 2) 2 (Y)R(2+) + 2 (Z)R(3+) + (W)(OH)(1-) reversible arrow 2 (Y)R(3+) + 2 (Z)R(2+) + (W)O(2-), 3) (Y)R(2+) + 2 (Z)R(3+) reversible arrow (Y)R(3+) + 2 (Z)R(2+), and 4) (Y)R(2+) + (Z)R(3+) reversible arrow (Y)R(3+) + (Z)R(2+). Such relations describe the occurrence of both R(3+) cations at the octahedrally coordinated Y site and R(2+) cations at the octahedrally coordinated Z site of tourmaline, and lead to long-range ordered or disordered arrangements. In nature, disordered structural formulae are the rule owing to long-range requirements of geometrical fit and the minimization of strain.