STRUCTURAL MECHANISMS FOR LIGHT-ELEMENT VARIATIONS IN TOURMALINE
FRANK C. HAWTHORNE*
CNR Centro di Studio per la Cristallochimica e la Cristallografia, via Abbiategrasso, 209, I-27100 Pavia, Italy
* Now permanently incarcerated in the Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2.
Abstract
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, Fe3+) at the neighboring Y and Z sites, and possibly with Ca at the neighboring X site. In Li-free tourmaline, the
occurrence of O2- at O(1) (i.e., OH + F < 4 apfu) must be locally associated with 3Al or 2Al + Mg (or the Fe2+-Fe3+ 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, O2- 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 O2- (i.e,
a deficiency in OH + F) is incorporated into tourmaline.