Tourmaline from the elbaite-schorl series from the Himalaya Mine, Mesa Grande, California, U.S.A.: A detailed investigation

Abstract

            Chemical, structural, infrared, optical and Mössbauer spectroscopic data were obtained on tourmalines from gem pockets in the Himalaya mine, San Diego County, California, U.S.A., including a strongly color zoned crystal. Ca and Li abundances increase from core to rim whereas Mn²⁺ and F others increase, peak, and decrease. When lepidolite begins crystallizing, the F content in tourmaline decreases. The black core of this crystal is a Mn-bearing “oxy-schorl”. A yellowish green, intermediate Mn-rich “fluor-elbaite” zone contains a relatively high Mn content with ~6 wt% MnO. The nearly colorless “fluor-elbaite” rim has the highest Li content of all zones. There is an inverse correlation between the lattice parameter a (for values ³ 15.84 Å) and the Li content (r² = 0.96). Mössbauer studies from the different zones within this crystal show that the relative fraction of Fe³⁺ increases continuously from the Fe-rich core to the Fe-poor near-rim zone, reflecting the increasing fugacity of oxygen in the pegmatite pocket. There is a high positive correlation between the lattice parameter a (for values ³ 15.84 Å) and the (Fe²⁺ + Mn²⁺) content in tourmalines from the elbaite-schorl series (r² = 0.99). Lower values than 15.84 Å for a can be suggested to result from an increasing ^[4]B content in samples that usually have a (Fe²⁺ + Mn²⁺) content of < 0.1 apfu.

            Positive correlations between Al at the Y site and ^[4]B (r² = 0.93), and between (Mn²⁺ + Fe²⁺) and ^[4]Al (r² = 0.99) were found  in tourmalines from the Himalaya Mine. These correlations indicate that, in the short-range order configurations, ^YAl is coupled with ^[4]B, whereas Mn²⁺ and Fe²⁺ are coupled with ^[4]Al.

            To obtain the most accurate OH data for the investigated tourmaline samples the OH determinations in this study were undertaken by different methods (SIMS, hydrogen manometry, continuous-flow mass spectrometry). Our analyses indicate that some elbaites contain a mixed occupation of F, OH and O at the W site. We conclude that the approximate assumption OH = 4 - F is only valid for elbaitic tourmalines with FeO + MnO < 8 wt%.

            Finally, the conditions of formation for tourmaline-bearing cavities in the Himalaya dike are discussed. Whether gel or glass, the transition from low- to high-viscosity of the pocket-forming medium occurs before the primary crystallization within the pockets has ceased. At the pocket stage in the Himalaya dike, the Li content of residual hydrosilicate melt was evidently high enough to promote a continuous transition from schorl-foitite of the marginal units into elbaite-rossmanite-liddicoatite solid solutions in the final stages of consolidation of the pegmatite.

Zoned Tourmaline from the Himalaya Mine, near Mesa Grande, California