UNUSUAL (Na,Mg)-ENRICHED BERYL AND ITS BREAKDOWN PRODUCTS (BERYL II, BAZZITE, BAVENITE) FROM EUXENITE-TYPE NYF PEGMATITE RELATED TO THE OROGENIC ULTRAPOTASSIC TREBIC PLUTON, CZECH REPUBLIC

摘要

An intragranitic, euxenite-type NYF pegmatite, Koichovice II, derived from the ultrapotassic orogenic Tebí syenogranite pluton, contains pale green prismatic crystals of primary beryl I, up to 3 cm long, arranged in radial aggregates. They are typically enclosed in massive quartz at the contact with albite or K-feldspar. Beryl I is locally altered to the secondary mineral assemblage beryl II + bavenite + bazzite + smectite. Electron-microprobe and LA–ICP–MS data yielded: slight indications of heterogeneity in beryl I, with high Na (0.20–0.32 apfu), Mg (0.24–0.38 apfu), moderate Fe_tot (0.06–0.14 apfu), Sc (0.06 apfu, 0.05–0.68 wt.% Sc₂O₃), Cs (0.01–0.15 apfu, 0.23–3.65 wt.% Cs₂O), and very low Li ( 80 ppm); secondary beryl II with high Na (0.24–0.34 apfu), low Fe_tot (0.03–0.05 apfu), Sc (0.02 apfu, 0.22 wt.% Sc₂O₃), and Cs (0.005 apfu; 0.14 wt.% Cs₂O); bazzite with high Na (0.38–0.48 apfu), Mg (0.41–0.64 apfu), Fe_tot (0.09–0.33 apfu), Sc (1.13–1.22 apfu, 12.79–13.80 wt.% Sc₂O₃), and moderate Ca (0.10–0.16 apfu, 0.91–1.53 wt.% CaO). Low-temperature measurements (25 K) of 57Fe Mössbauer spectra of beryl I yielded: O,CHFe2+ ~80%, TFe2+ ~10% and OFe3+ ~10%. The substitutions: (1) CH OR3+ = CHR+ OR2+ where R+ = Na, Cs, K, Rb; R2+ = Mg, Fe2+, Mn, Ca; R3+ = Al, Sc, Fe3+, and (2) CH 2OR3+ = CHCa 2OR2+ [or modified CH 2OR3+ = CH(Ca,Fe2+) 2OR2+] were revealed. A sum of Na + K + Rb + Cs > 0.5 apfu found in bazzite from Koichovice, Kazakhstan and Tørdal, Telemark, Norway, suggests the existence of new phases related to bazzite via the substitution (1) with the general formula R+Be₃R3+R2+Si₆O₁₈ and idealized formulas Na+Be₃ScMg2+Si₆O₁₈ (Koichovice) and Na+Be₃ScFe2+Si₆O₁₈ (Kazakhstan, Norway). Low Fe/(Fe + Mg) values, in the range 0.14–0.29 in beryl I, are similar to those in the other Fe,Mg minerals (biotite, tourmaline, amphibole) from the pegmatites of the Tebí pluton. The (Na,Mg,Fe)-enriched crystals of beryl examined are similar to various, but mostly non-pegmatitic environments rather than to NYF pegmatites, and support a rather unique bulk-composition and compositional evolution of minerals from granitic pegmatites of the Tebí pluton. The assemblage of secondary minerals after beryl I, including common bavenite and elevated Ca in bazzite, suggest a high activity of Ca in the late fluids, which is perhaps enhanced by very low contents of P and F in pegmatites of the Tebí pluton. We suggest a temperature slightly below 250–350°C and neutral to slightly alkaline conditions for the formation of the secondary assemblage beryl II + bavenite + bazzite + smectite.