Milarite-group minerals from the NYF pegmatite Velká skála, Písek district, Czech Republic: sole carriers of Be from the magmatic to hydrothermal stage
Novák, Milan; Cícha, Jaroslav; Čopjaková, Renata; Škoda, Radek; Vašinová Galiová, Michaela
European Journal of Mineralogy Volume 29 Number 4 (2017), p. 755 - 766
published: Sep 1, 2017
ArtNo. ESP147052904025, Price: 29.00 €
Four distinct paragenetic, morphological and compositional types of milarite-group minerals were distinguished in the NYF pegmatite Velká skála, Písek district, Czech Republic (Moldanubian Zone). The simply zoned (Kfs, Plg, Ab, Qz > Bt > Ap > Tur) pegmatite dike, ~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 ~20 mol% of the oftedalite component) from small pockets; colorless hexagonal prismatic crystals of milarite III (milarite s. s.) lining open fractures and microscopicfillings 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, 200μm 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 Velká skála pegmatite as well as a close relation to euxenite-type pegmatites of the Třebíč Pluton. Milarite-group minerals from Velká Skála exhibit very similar compositional evolution to those found in the Heftetjern pegmatite, Tørdal, Norway.