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Milarite-group minerals from the NYF pegmatite Velká skála, Písek district, Czech Republic: sole carriers of Be from the magmatic to hydrothermal stage

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Year of publication 2017
Type Article in Periodical
Magazine / Source European Journal of Mineralogy
MU Faculty or unit

Faculty of Science

Keywords milarite; agakhanovite-(Y); EPMA; LA–ICP-MS; CL; compositional trends; beryllium; NYF pegmatite
Description Four distinct paragenetic, morphological and compositional types of milarite-group minerals were distinguished in the NYF pegmatite Velka skala, Pisek district, Czech Republic (Moldanubian Zone). The simply zoned (Kfs, Plg, Ab, Qz> Bt> Ap> Tur) pegmatite dike, similar to 60 cm thick, is enclosed in durbachite (K, Mg-rich melasyenite). The milarite-group minerals include: yellowish, subhedral, prismatic grains of homogeneous milarite I (milarite s. s.); prismatic subparallel aggregates of white to grayish milarite II (zoned milarite s. s. to Sc-rich milarite, up to similar to 20 mol% of the oftedalite component) from small pockets; colorless hexagonal prismatic crystals of milarite III (milarite s. s.) lining open fractures and microscopic fillings in recrystallized K-feldspar locally associated with REE-bearing minerals (allanite, aeschynite, titanite); very rare milarite IV (milarite s. s. to agakhanovite) as prismatic crystals and thin overgrowths with complex oscillatory zoning, <= 200mm in size, on crystals of milarite II and milarite III. Their mineral assemblages, textures and compositions suggest primary (magmatic) for milarite I and hydrothermal origin for milarite II, III, and IV. The simplified compositional evolution in these milarites is: Al+Ca -> Be+Sc -> Be+Y, HREE -> Be+LREE. Milaritegroup minerals are the sole carriers of Be in the pegmatite, no relics or pseudomorphs after other Be-rich minerals were found; thus milarite is now established as an additional primary Be-rich mineral in granitic pegmatites. Mineral assemblages, chemical composition of minerals and close relation to the host durbachite suggest an NYF affinity of the Velka skala pegmatite as well as a close relation to euxenite-type pegmatites of the Trebic Pluton. Milarite-group minerals from Velka Skala exhibit very similar compositional evolution to those found in the Heftetjern pegmatite, Tordal, Norway.
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