ZONING IN A SINGLE CRYSTAL OF LIDDICOATITE (TOURMALINE) FROM MADAGASCAR: CRYSTAL-STRUCTURE REFINEMENT, INFRARED SPECTROSCOPY, AND 11B, 27AL, 29SI MAGIC-ANGLE-SPINNING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Liddicoatite [ideally, Ca (Li₂Al) Al₆ (Si₆O₁₈) (BO₃)₃ (OH)₃ F] is a rare species of calcium tourmaline found predominantly in rare-element, elbaite-subtype pegmatites where uncommon geologic processes have resulted in the concentration of sufficient amounts of Ca and Li (which typically show contrasting geochemical behaviour) in the later stages of pegmatite crystallization. This species commonly shows very conspicuous and complex patterns of color zoning. In the large (20 cm diameter) Madagascar crystal investigated here, over 50 zones are observed; they are trigonal-pyramidal in form and display a diffuse, oscillatory character with an overall gradation in color from purple to light green to dark green.

Variations in crystal structure and chemical composition have been characterized from core to rim. The structures of 24 crystals, a ≈ 15.9, c ≈ 7.1 Å, V ≈ 1560 Å3, R3m (Z = 3), have been refined to R indices of < 2.0%. The data for all samples indicate strong positional disorder of the O(1) and O(2) anions, the result of local positional disorder induced by cations of different size and charge at the Y-site. Calculation of bond-valence sums using bond-length values indicated by the observed positional disorder results in superior agreement with the valence-sum rule for the O(1)–Y₃ arrangement. Fourier-Transform infrared spectroscopy was used to assess short-range order of Li, Mn²⁺ and Al at the X- and Y-sites; these data suggest clusters of composition, LiMnMn, LiAlAl and LiLiAl. Also, MAS NMR data indicates variable amounts of tetrahedrally coordinated B throughout the crystal.

Electron-microprobe analysis shows that the chemical composition varies according to Ca + Li ↔ Na + Mn,²⁺, regardless of oscillatory zoning patterns; this results in a restricted variation in Al content throughout the sample, contrary to the elbaite - liddicoatite solid-solution series. EMP data also suggest the simultaneous operation of at least two zoning mechanisms during the crystallization process, resulting in independent compositional trends for Mn²⁺, Fe²⁺, and Ti⁴⁺. Further, significant concentrations of Pb (0.10-0.55 wt.% PbO) are also observed.