A548 Goldschmidt Conference Abstracts 2007

Yellow Mn-rich elbaite from the Bixbyite in bixbyite – Precipitates as Canary mining area, Zambia indication for fast diffusion pathways 1 2 BRENDAN M. LAURS , WILLIAM B. SIMMONS , STEFAN LAUTERBACH AND HANS-JOACHIM KLEEBE 3 2 BJÖRN ANCKAR AND ALEXANDER U. FALSTER Technical University of Darmstadt, Institute for Applied 1Gemological Institute of America (GIA), 5345 Armada Geosciences, Geomaterial Science, Schnittspahnstr. 9. Drive, Carlsbad, CA, 92008, USA ([email protected]) D-64287 Darmstadt, Germany 2University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA, 70148, USA ([email protected]) Large Bixbyite crystals (Mn,Fe)2O3, space group Ia3;¯, 3GeoloGem Consulting, Driveslättvägen 102, Alingsås 44191, occur only at one location in the Thomas Range (Utah), where Sweden ([email protected]) crystals up to 4 cm have been discovered. In contrast to the commonly small isomorphic crystals, The only commercial source of yellow gem tourmaline is most of the larger Bixbyite crystals show distinct re-entrant the Canary mining area, located 32 km west-southwest of facets at halfway of every edge of the cube, linked by a band Lundazi in eastern Zambia. Since 1983, tourmaline has been of parallel linear features. The structure of these linear features mined in colors typically ranging from yellow-green to yellow was studied using high-resolution transmission electron to orange and brown; much of the orange-to-brown material is microscopy (HRTEM). HRTEM revealed atomically sharp heated to attain a “golden” or “canary” yellow color. interfaces between lamellae running along {100} of the Significant production has come from eluvial and alluvial Bixbyite structure and the matrix. deposits, but the largest find (~3 tonnes) occurred at a nearby Regular formed precipitates aligned parallel to these granitic pegmatite, in a large cavity that also contained lamellae show the same chemical composition by EDS as the abundant quartz crystals and “cleavelandite” feldspar. The host crystal and electron diffraction indicates that the pegmatite is lens-shaped and discordantly intrudes biotite precipitates have the same crystal structure as the host crystal. schist; it strikes east-west (~100°) and dips moderately south. We present a model explaining the formation of small By the time of the authors’ visit in 2004, most of the ‘child crystals’ with identical structure and composition in the pegmatite had been removed via a large open pit that was host Bixbyite matrix, which makes use of the assumption that partially filled with water. Visible portions of the footwall the observed lamellae act as fast diffusion highways. showed a medium-grained border zone (~15 cm thick) and in places a discontinuous aplitic zone that abruptly transitioned into a coarse-grained intermediate zone consisting mainly of K-feldspar and subordinate sodic plagioclase. Quartz was leached from the rock, resulting in vugs containing black tourmaline and local albite (An0-1.5, as clear overgrowths on K-feldspar). Conspicuous veins (up to 10 cm thick) of black tourmaline ± K-feldspar + albite cross-cut the pegmatite. Electron-microprobe analysis of 23 samples of yellow-to- brown and yellow-green tourmaline from the associated secondary deposits showed they were elbaite with 1.14-7.59 wt.% MnO and 0.04-0.54 wt.% TiO2; iron was typically absent but exceptionally ranged up to only 0.21 wt.% FeO. Previous studies have recorded up to 9.18 wt.% MnO in this elbaite (e.g., J. E. Shigley et al., Am. Min., 71, 1986, pp. 1214- 1216), approaching the tsilaisite end member. Gem tourmaline typically consists of pink, green, or blue elbaite that is mined from the lepidolite-bearing inner zones of complex LCT pegmatites. In contrast, Mn-rich (and Ti- bearing) yellow-to-brown elbaite from the Canary mining area appears to have formed in a relatively simple pegmatite that was enriched in B and H2O, but not Li. Early crystallization of abundant schorl consumed the available Fe, inhibiting subsequent crystallization of garnet and biotite; this allowed Mn to be conserved until the late-stage formation of gem tourmaline. Following pegmatite crystallization, the influx of an unusual B-, Fe-, K-and Na-rich fluid (apparently from an external source) resulted in local tourmaline ± K-feldspar + albite veining and quartz dissolution; the same event also brecciated some of the yellow elbaite with black tourmaline ± K-feldspar + albite assemblages.