Structural mechanisms for light-element variations in tourmaline

It is now well known that Si, B and OH+F are variable components of tourmaline, and yet the stereochemical details of their variation in the tourmaline structure are still not well characterized or understood. Application of the valence-sum rule of bond-valence theory to questions of short-range atomic arrangements shows that there are considerable stereochemical constraints associated with the variation of Si, B and OH+F in the tourmaline structure. The occurrence of a trivalent cation (Al, B) at the T site must be locally associated with the occurrence of trivalent cations (Al, Fe (super 3+) ) at the neighboring Y and Z sites, and possibly with Ca at the neighboring X site. In Li-free tourmaline, the occurrence of O (super 2-) at O(1) (i.e., OH+F<4 apfu) must be locally associated with 3Al or 2Al+Mg (or the Fe (super 2+) -Fe (super 3+) analogues) at the adjacent 3Y sites in order for the valence-sum rule to be satisfied on a local scale. In Li-bearing Mg-free tourmaline, O ₂ - at O(1) must be locally associated with 3Al at the adjacent 3Y sites. These requirements provide stringent constraints on the possible substitution schemes whereby additional O (super 2-) (i.e, a deficiency in OH+F) is incorporated into tourmaline.