Mineral assemblages and compositional variations in bavenite–bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic

Abstract

Mineral assemblages and chemical composition of bavenite-bohseite were examined at 15 granitic pegmatites from the Bohemian Massif, Czech Republic. Three types distinct in their relation to primary Be-precursors, mineral assemblages, morphology, and origin were recognised. (i) Primary hydrothermal bavenite-bohseite crystallised in miarolitic pockets from residual pegmatite fluids. Secondary bavenite-bohseite occurs in two distinct types: (ii) a proximal type spatially restricted to pseudomorphs after a primary Be-mineral (beryl > phenakite, helvine-danalite), and (iii) a distal type on brittle fractures and fissures of host pegmatite. Their mineral assemblages are highly variable: (i) axinite-(Mn), smectite, calcite, pyrite, (ii) bertrandite, milarite, secondary beryl, bazzite, K-feldspar, muscovite-illite, scolecite, gismondine-(Ca), analcime, chlorite, and (iii) muscovite, albite, quartz, epidote, pumpellyite-(Mg), pumpellyite-(Fe 3+ ), titanite, chlorite. The electron probe micro-analyses showed in addition to major constituents (Si, Ca, Al) also minor concentrations (in apfu) of Na (≤0.24), Fe (≤0.10), Mn (≤0.10), and F (≤0.36). The (i) hydrothermal miarolitic bavenite-bohseite is mostly Al-rich (2.00-0.67 apfu) relative to (ii) proximal bavenite-bohseite and bohseite after beryl, phenakite and helvine-danalite (1.56-0.46, 0.70-0.05, 1.02-0.35 apfu, respectively) and

Chrysoberyl and associated beryllium minerals resulting from metamorphic overprinting of the Maršíkov–Schinderhübel III pegmatite, Czech Republic

“…Experimental studies indicate a lower stability of beryl at ~300–400°C, whereas euclase is stable at ~300–450°C, and bertrandite below 250°C at a pressure up to 300 MPa (Franz and Morteani 1981, 2002; Barton 1986; Barton and Young, 2002; Grew, 2002, and reference therein). However, a recent study of secondary Be minerals and their mineral assemblages from granitic pegmatites (Novák et al , 2023) suggests the stability of bertrandite up to T ≈ 300°C. Consequently, this late retrograde metamorphic event occurred at T ≈ 500 to 200°C and P ≤ 250 MPa.…”

“…The most fractionated pegmatites show an affinity to the LCT (lithium-caesium-tantalum) family and beryl-columbite subtype of the rare-element class according to the classification of Černý and Ercit (2005), as were documented by their mineral assemblages (e.g. Černý et al, 1992Novák 1988Novák , 2005Novák et al, 2003Novák et al, , 2023Chládek and Zimák, 2016;Chládek et al, 2020Chládek et al, , 2021Dolníček et al, 2020aDolníček et al, , 2020b.…”