Abstract

The structure of an alkali cation-deficient, vanadium tourmaline has been refined (R = 0.041 for 2727 intensity data) in order to evaluate the effects of tourmaline composition on structural distortion. V-touiinaline, (Na0.44Ca0.36Mg0.18K0.02) (Mg1.87V3+0.76Cr3+0.19Fe2+0.18) (Al5.56Ѵ3+0.38Ті4+0.06)(Si5.63Al0.37) (B2.980.02)O27(O1.10OH2.58F0.32), is rhombohedral with a = 15.967(2) and с = 7.191(1)A; space group R3m. Despite compositional differences the structure is very similar to those of other members of the tourmaline group, most notably a recently refined aluminous dravite. Analysis of tourmaline-group structural data reveals (1) a negative correlation between tetrahedral bond angle variance and mean “alkali”(3a)-oxygen bond length; (2) negative correlations between both 9b and 18c octahedral angle variances and mean 9b-0 bond length; (3) a negative correlation between ditrigonality and 9b octahedral angle variance; (4) a positive correlation between weighted mean octahedral bond length and cell volume; and (5) a positive correlation between Na occupancy in the 3a site and mean За-О bond length. These observations demonstrate a systematic flexibility of the structure in response to diverse cation substitution.

The lack of a distinct coupling between the sizes of the 9b and 18c octahedra in refined tourmaline structures and the extensive and possibly complete substitution of Al3+ in the 9b site in tourmalines suggest that the presence of cations that can vary in size (e.g., Fe2+,3+) in order to create a compatible edge may not be a prerequisite for dravite-elbaite solid solution. In view of the structural flexibility of tourmaline and the ease of proton exchange to maintain charge balance, the apparent immiscibility of dravite and elbaite is thought to reflect extreme fractionation of Mg and Li during petrogenesis and by the tourmaline structure, due to the large difference in the field strengths of these cations. Other major features of tourmaline substitutional chemistry are also rationalized on the basis of cation field strength.

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