THE INCORPORATION OF FLUORINE IN TOURMALINE: INTERNAL CRYSTALLOGRAPHIC CONTROLS OR EXTERNAL ENVIRONMENTAL INFLUENCES?

The manner in which F is incorporated into the tourmaline structure depends on internal influences such as crystallographic constraints and on external influences such as temperature, pressure, local mineral assemblage and fluid composition. Tourmaline has a general formula XY(3)Z(6)(T(6)O(18))(BO(3))(3)V(3)W, with the most common site-occupancies being: X = Ca(2+), Na(1+), K(1+), (X)square (vacancy); Y = Li(1+), Mg(2+), Fe(2+), Mn(2+), Al(3+), Cr(3+), Fe(3+); Z = Al(3+), Mg(2+), Fe(3+), Cr(3+); T = Si(4+), Al(3+); B = B(3+); V [O(3)] = OH(1-), O(2-); W [O(1)] = OH(1-), F(1-), O(2-). Of particular importance for understanding F(1)-incorporation in tourmaline is that F(1)-occurs solely at the O(1) site. Substitution of F1-at this site is influenced by the occupancy (total charge) of the X and the Y sites. The X site is generally occupied by cations of variable charge (+1 or +2) or is vacant (zero charge). There are three Y-site cations, which can have charges of +1, +2, +3 or +4. However, the charge of the local bond-valence arrangements of the Y-site cations are most commonly between +6 (e.g., Mg(3)) and +7 (e.g., Mg(2)Al). Because of local bonding of the O(1)-site anion to three neighboring Y-site cations and an X-site cation, the charges at the X and Y sites affect the F1-occupancy at the W site. Disorder of Mg and Al at the Y and Z sites influences the local charge, and consequently, the F content. The accumulated data on tourmaline verify the general crystallographic influences. A summary of > 8800 tourmaline compositions from different lithologic environments illustrates that for tourmaline with an average X-site charge exceeding +0.9, there is a range from 0 to a maximum of 1.0 apfu F. As the X-site charge decreases, the maximal amount of F decreases, with the maximal amount of F being less than 0.2 apfu for those tourmalines with an X-site charge of less than +0.5. Petrological factors superimpose local environmental influences on F concentrations. The presence of minerals with a fixed high F content, such as fluorite, local assemblages of minerals and the degree of fractionation in a magma or fluid control the amount of F that is available to tourmaline within the crystallographic constraints imposed by local X- and Y-site charge.