Beryl a fenakit z granitových pegmatitov tunela Sitina v Bratislave
Granitic pegmatite vein (ca. 15 cm thick) containing Be-minerals occurs in granite from the Sitina tunnel in Bratislava, SW Slovakia. Dark grey quartz, white to dark grey microcline, milky-white albite (partially variety cleavelandite) and muscovite are main minerals of the pegmatite. Accessory minerals include garnet (almandine), chlorite and mica (biotite) and Be-bearing minerals: beryl and phenakite. Beryl shows increased concentrations of Mg (up to 0.35 apfu), Fe (up to 0.14 apfu) and Na (up to 0.48 apfu). Average crystallochemical formula of beryl is following (n=12): Na0.46Be3.00 Al1.52Mg0.32Fe0.12Si6.04O18. The Fe,Mg-rich composition and high c/a lattice ratio (0.9916) indicate a presence of octahedral beryl type. Phenakite is a part of younger generation of quartz crossing the beryl crystal.