• journal_title：The Canadian Mineralogist
• Contributor：Darrell J. Henry ; Barbara L. Dutrow
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.1.41
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：1
• firstpage：41
• section：Articles

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(T₆O₁₈)(BO₃)₃V₃W, with the most common site-occupancies being: X = Ca²⁺, Na¹⁺, K¹⁺, ^X□ (vacancy); Y = Li¹⁺, Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Cr³⁺, Fe³⁺; Z = Al³⁺, Mg²⁺, Fe³⁺, Cr³⁺; T = Si⁴⁺, Al³⁺; B = B³⁺; V ≡ [O(3)] = OH¹⁻, O²⁻; W ≡ [O(1)] = OH¹⁻, F¹⁻, O²⁻. Of particular importance for understanding F¹⁻ 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 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., Mg₃) and +7 (e.g., Mg₂Al). 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.