Elbaite-liddicoatite from Black Rapids glacier, Alaska

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 57-73 - DOI: 10.2451/2011PM0005 Special Issue in memory of Sergio Lucchesi Elbaite-liddicoatite from Black Rapids glacier, Alaska Aaron J. Lussier 1 , Frank C. Hawthorne 1,* , Vladimir K. Michaelis 2 , Pedro M. Aguiar 2 and Scott Kroeker 2 1 Department of Geological Sciences, University of Manitoba, Winnipeg, Canada 2 Department of Chemistry, University of Manitoba, Winnipeg, Canada *Corresponding author: frank_hawthorne@umanitoba.ca Abstract Liddicoatite, ideally Ca(AlLi 2 )Al 6 (SiO 6 )(BO 3 ) 3 (OH) 3 F, 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 6 O 18 )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 6 O 18 )F 1.0 (OH) 3.0 . Detailed electron-microprobe analysis and 11 B and 27 Al Magic-Angle-Spinning Nuclear Magnetic Resonance spectroscopy show that several substitutions were active during growth, with X □ + Y Al → X Ca + Y Li (liddicoatite-rossmanite solid-solution) and 2 Y Al + X □ → 2 Y M* + X Ca 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. Key words: liddicoatite; elbaite; tourmaline; late-stage Ca enrichment; pegmatite; zoning; electron-microprobe analysis; Black Rapids glacier, Alaska; 11 B MAS NMR; 27 Al MAS NMR.

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