Elbaite-liddicoatite from Black Rapids glacier, Alaska

摘要

Liddicoatite, ideally Ca(AlLi_2)Al_6(Si0_6)(B0_3)_3(OH)_3F, is an extremely rare species of tourmaline, found in very few localities worldwide. A large (~ 2 cm in cross section), euhedral sample of tourmaline retrieved from atop the Black Rapids glacier, Alaska, is shown to vary from a light pink elbaite in the core region, average composition (Na_(0.4)Ca_0.3)□_(0.3))(Al_(1.75)Li_(1.25)) Al_6(BO_3)_3(Si_6O_(l8))F_(0 4)(OH)_(3.6), to a pale green liddicoatite at the edge of the crystal, (Na_(0.3)Ca_(0.6) □_(0.1))(Al_(1.0)Li_(1.6)Fe_(0.2)Mn_(0.2))Al_6(BO_3)_3 (Si_6O_(18))F_(1.0)(OH)_(3.0)? Detailed electron-microprobe analysis and ~(11)B and ~(27)A1 Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy show that several substitutions were active during growth, with ~□ + ~yAl —~xCa + ~YLi (liddicoatite-rossmanite solid-solution) and 2~yAl+~x□ —2~YM~* + ~XCa accounting for most of the compositional variation. Throughout the tourmaline, there are instances of reversals in the trends of all major constituents, although few compositional gaps are observed. Most notably, a sharp decline in Ca content from ~0.35 to ~0.05 apfu (atoms per formula unit) with increasing distance from the core at ~2 mm from the crystal edge is followed by a sharp rise in Ca content (to 0.55 apfu), along with (Fe + Mn) content (from 0.01 to 0.35 apfu). In the core region, the origin of the Ca in the tourmaline is not clear; the correlation of Ca and F is consistent with both (1) a melt in which Ca was held as complexes with F, or (2) earlier contamination of the melt by a (Ca, F)-rich fluid. Close to the rim, a dramatic increase in Ca, F, Mn and Fe is probably due to late-stage contamination by fluids that have removed these components from adjacent wallrocks.