Chemical composition and species attribution of tourmalines from a rare-metal pegmatite vein with scapolite, Sangilen Upland, Tuva |
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Authors: | L G Kuznetsova A A Zolotarev O V Frank-Kamenetskaya I V Rozhdestvenskaya Yu M Bronzova J Spratte and A Ertl |
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Abstract: | A detailed study of the chemical composition and substitutions in calcium tourmalines from a scapolite-bearing rare-metal
pegmatite vein from the Sol’bel’der River basin has shown that their species attribution is determined by occupancy of octahedral
site Y. The composition of the yellow tourmaline most abundant in the central part of the pegmatite bodyis rather constant
and characterized by the ideal formula Ca(Mg2Li)Al6(Si6O18)(BO3)3(OH)3F. Variations in the chemical composition of zonal tourmaline crystals from the contact part of the pegmatite are controlled
by abrupt change in the chemical medium during their formation. The yellow cores of these crystals are close in composition
to tourmaline from the central part of the pegmatite vein. The Mg content abruptly decreases toward the crystal margin: Mg2+ → Fe2+, 2Mg2+ → Li+ + Al3+, and Mg2+ + OH → Al3+ + O2−. The composition of dark green marginal zones in tourmaline is characterized by the ideal formula Ca(Al1.5Li1.5)Al6(Si6O18)(BO3)3 (OH2O)(F). The results indicate specific formation conditions of pegmatite. The crystallochemical formulas of the studied tourmalines
allow us to regard them as new mineral species in the tourmaline group. |
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