Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif

[Marco E. Ciriotti]

Prosssima pubblicazione.

Referenza:
▪ Novák, M., Dolníček, Z., Zachař, A., Gadas, P., Nepejchal, M., Sobek, K., Škoda, R., Vrtiška, L. (2023): Mineral assemblages and compositional variations in bavenite-bohseite from granitic pegmatites of the Bohemian Massif, Czech Republic. Mineralogical Magazine, 87, (in press).

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-(Fe3+), 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 (iii) distal bavenite-bohseite typically after beryl (1.63–0.09 apfu). Raman spectroscopy revealed that the distance between the OHvibrational modes decreases with increasing bohseite component. The Al content of the secondary (ii) proximal bavenite-bohseite is controlled by the composition of its Be-precursor whereas (iii) distal bavenite-bohseite with beryl as the Be-precursor is more variable and its composition is governed mainly by the composition of fluids. Calcium, a crucial element for the bavenite-bohseite origin, was sourced from residual pegmatite fluids (Vlastějovice, Vepice IV, Třebíč Pluton) or from external sources (e.g., Drahonín IV, Věžná I, Maršíkov). Primary (i) hydrothermal bavenite-bohseite from miarolitic pocket may have crystallised at T ~300–400 ºC and P ~200 MPa whereas the secondary (ii) proximal and (iii) distal bavenite-bohseite approximately at T ~300–100 °C and P ~200–20 MPa.

NdR: molto, molto interessante... Pieno di informazioni.