Substitution mechanism in tourmalines of the 'fluor-elbaite'-rossmanite series from Wolkenburg, Saxony, Germany

摘要

Single-crystal structure and chemical (EMPA, SIMS, TGA) data were obtained from three pale coloured to colourless Al- and Li-rich tourmaline crystals from a pegmatite from Wolkenburg near Limbach-Oberfrohna, Saxony, Germany. Tourmaline samples of the "fluor-elbaite"-rossmanite solid solution series were chemically (including the light elements) and structurally characterized for the first time. The crystal chemical formulae of these three investigated samples range from a "fluor-elbaite" dominant composition with ~X(Na_(0.65)square_(0.29)Ca_(0.06))~Y(Al_(1.64)Li_(1.12)Mn_(0.10)~(2+)Fe_(0.02)~(2+)square_(0.12)~ZAl_6(BO_3O_3(Si_(5.90)B_(0.10))O_(18)(OH)_3[F_(0.72)(OH)_(0.18)O_(0.10)], over ~X(Na_(0.54)square_(0.45)Ca_(0.01))~Y(Al_(1.90)Li_(0.97)Mn_(0.03)~(2+)Fe~(2+)square_(0.09))~ZAl_6 (BO_3)_3~T(Si_(5.82)B_(0.18))O_(18)~V(OH)_3~W[(OH)_(0.51)F_(.37)O_(0.12)] to Na-rich rossmanite with the composition ~X(square_(0.51)Na_(0.48)Ca(0.01))~Y(Al_(2.02)Li_(0.71)Mn_(0.03)~(2+)square_(0.24))~Z(Al_6(BO_3)_3~T(Si_(5.69)Al_(0.17)B_(0.14))O_(18)~V(OH)_3~W[(OH)_(0.70)F_(0.26)O_(0.04]When the rossmanite component increases, while the "fluor-elbaite" component decreases, the olenite component also increases. In a pegmatitic system where essentially no Fe, Mn, Ti and Mg are available, the Li content is an important factor, which seems to control the amount of Si in tourmaline. Once the Li~(1+) content is lower, Al~(3+) cations (with a higher oxidation /tate) must occupy the Y site. For a charge-balanced formula, other cation sites must therefore have lower bulk charges. This can be achieved by increasing vacancies at the X site and increasing amounts of trivalent cations at the T site. In the Wolkenburg tourmalines of the "fluor-elbaite"-rossmanite series both substitutions are observed. For our investigated tourmalines we can propose a modified (coupled) Tschermaks' substitution with ~Xsquare + ~YAl~(3+) + ~T(A1, B)~(3+)<->~XNa~+ + ~YLi~+ + ~TSi~(4+). The F content is dependent on the charge of the X-site occupants. There is also a good negative correlation between the charge of the X-site occupants and the X-02 distance, which can be used to draw some conclusions about the X-site occupancy when only single-crystal structure data are available.