The incorporation of fluorine in tourmaline: Internal crystallographic controls or external environmental influences?

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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 XY3Z6(T 6O18)(BO3)3V3W, with the most common site-occupancies being: X = Ca2+, Na1+, K 1+, X□ (vacancy); Y= Li1+, Mg2+, Fe2+, Mn2+, Al3+, Cr3+, Fe 3+; Z = Al3+, Mg2+, Fe3+, Cr 3+; T= Si4+, Al3+; B = B3+; V= [0(3)] = OH1-, O2-; W= [O(I)] = OH1-, F 1-, O2-. Of particular importance for understanding F 1- incorporation in tourmaline is that F1- 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 K-site cations, which can have charges of +1, +2, +3 or +4. However, the charge of the local bond-valence arrangements of the K-site cations are most commonly between +6 (e.g., Mg3) and +7 (e.g., Mg 2Al). Because of local bonding of the O(1)-site anion to three neighboring K-site cations and an X-site cation, the charges at the X and K sites affect the F1- occupancy at the W site. Disorder of Mg and Al at the K 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 O 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 K-site charge.

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Canadian Mineralogist

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