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

Abstract

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, ∼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 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, ≤200 D:\Palash\HWNLM\07_06m 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 . Milarite-group 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 a NYF affinity of the Velka skala pegmatite as well as a close relation to euxenite-type pegmatites of the Třebic Pluton. Milarite-group minerals from Velka Skala exhibit very similar compositional evolution to those found in the Heftetjern pegmatite, Tordal, Norway.