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

• DOI: 10.3749/canmin.49.1.41
• Publication Date: 2011-03-11T01:03:55Z
• Authors: D. J. Henry; B. L. Dutrow
• Keywords: Tourmaline; Fluorine

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