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Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic
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Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic
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Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic
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Journal Article
Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic
2023
Overview
The paragenesis and composition of bavenite-bohseite were investigated in fifteen 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: (1) primary hydrothermal bavenite-bohseite crystallised in miarolitic pockets from residual pegmatite fluids; and secondary bavenite-bohseite in two distinct types: (2) a proximal type restricted spatially to pseudomorphs after a primary Be mineral (beryl > phenakite, helvine-danalite); and (3) a distal type on brittle fractures and fissures of host pegmatite. The mineral assemblages are highly variable: (1) axinite-(Mn), smectite, calcite and pyrite; (2) bertrandite, milarite, secondary beryl, bazzite, K-feldspar, muscovite-illite, scolecite, gismondine-Ca, analcime, chlorite; and (3) muscovite, albite, quartz, epidote, pumpellyite-(Mg), pumpellyite-(Fe3+), titanite and chlorite. Electron microprobe analyses showed, in addition to major constituents (Si, Ca and Al), minor concentrations (in apfu) of Na (≤0.24), Fe (≤0.10), Mn (≤0.10) and F (≤0.36). The type 1 hydrothermal miarolitic bavenite-bohseite is mostly Al-rich (2.00-0.67 apfu) relative to type 2 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 type 3 distal bavenite-bohseite typically after beryl (1.63-0.09 apfu). Raman spectroscopy revealed that the distance between the OH- vibrational modes decreases with increasing bohseite component. The Al content of secondary type 2 proximal bavenite-bohseite is controlled by the composition of the Be precursor whereas type 3 distal bavenite-bohseite with beryl as the Be precursor is more variable and the composition is governed mainly by the composition of fluids. Calcium, a crucial component for bavenite-bohseite origins, was derived from residual pegmatite fluids (Vlastějovice, Vepice IV or Trebíc Plutons) or external sources (e.g. Drahonín IV, Věžná I or Marsíkov). Primary type 1 hydrothermal bavenite-bohseite from miarolitic pockets might have crystallised at T ≈ 300-400°C and P ≈ 200 MPa, whereas the secondary type 2 and 3 bavenite-bohseite formed at T ≈ 300-100°C and P ≈ 200-20 MPa.
