Primary (magmatic) and hydrothermal milarite-group minerals from Velká skála pegmatite, Písek pegmatite district, Czech Republic

English Abstract:

Milarite group includes framework silicates with the general formula A₂ B₂ C D T2₃ T1₁₂ O₃₀ (H₂O) where A = Ca, Na, Y, REE, Sc; B = □, Na, K, Ca; C = K; D = □; T2 = Be, Li, B, Al; T1 = Si. Several distinct textural and compositional types of milarites were found out in the Velká skála pegmatite. Simply zoned, subvertical pegmatite dike, up to ~1 m thick, is enclosed in quartz melasyenite and its mineral assemblage is: Pl+Kfs+Qz+Ab˃Bt (Fe~Mg) + accessory titanite, pyrite, fluorapatite, allanite-(Ce), zircon and cheralite. Homogeneous milarite I, close to the ideal formula Ca₂K(Be₂Al)Si₁₂O₃₀ (H₂O), forms pale yellowish, subhedral, elongated grains, up to 2.5 cm long, enclosed in coarse-grained Ab+Qz unit and locally in part developed into small pockets. Prismatic subparallel aggregates of white to grayish milarite II, up to 1 cm long, typically occur in small pockets lined with Ab+Qz+Kfs+REE-minerals; inner part of individual fibers are enriched in Sc (≤0.17 apfu). Very rare milarite III forms short prismatic crystals, up to 0.2 mm long, and overgrowths, up to 20 μm thick, on crystals of milarite II. It is highly heterogeneous with complex oscillatory zoning reflecting variable Y (≤0.51 apfu), and REE (≤0.45 apfu), rarely Ce >Y, and it evolves from milarite to agakhanovite-(Y) and "agakhanovite”-(Ce). The general compositional trend in milarites is characterized by simplified evolution Al+Ca → Be+Sc → Be+Y,REE. The Velká skála locality is the only granitic pegmatite containing several distinct types of milarite from a magmatic stage with milarite I, which substitutes beryl or gadolinite as a Be-dominant primary mineral in the pegmatite, to hydrothermal stage with milarite II and milarite III. The mineral assemblages of the Velká skála pegmatite imply alkaline conditions with low activity of B, P and F typical for most F-poor NYF pegmatites such as some pegmatites enclosed in melasyenites of the Třebíč pluton, Czech Republic.