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

Abstract

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 □ (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 F 1− 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 F 1− 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.