Stereochemical constraints in tourmaline: from a short-range to a long-range structure

Abstract

Extension of the local bond-valence approach from Mg–Al to Fe2+–Fe3+ short-range arrangements is explored in the structure of tourmaline. Stable local arrangements involving trivalent ( R 3+) cations (Al, Fe3+) and divalent ( R 2+) cations (Mg, Fe2+) 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 R 3+ and R 2+ cations. These clusters are related through four different expressions: 1) 2 Y R 2+ + Z R 3+ + W (OH)1− ⇌ 2 Y R 3+ + Z R 2+ + W O2−, 2) 2 Y R 2+ + 2 Z R 3+ + W (OH)1− ⇌ 2 Y R 3+ + 2 Z R 2+ + W O2−, 3) Y R 2+ + 2 Z R 3+ ⇌ Y R 3+ + 2 Z R 2+, and 4) Y R 2+ + Z R 3+ ⇌ 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.