Cu-rich members of the beudantite-segnitite series from the krupka ore district, the krušné hory mountains, Czech Republic

Abstract

Copper-rich members of the beudantite-segnitite series (belonging to the alunite supergroup) were found at the Krupka deposit, Krušné hory Mountains, Czech Republic. They form yellow-green irregular to botryoidal aggregates up to 5 mm in size. Well-formed trigonal crystals up to 15 m in length are rare. Chemical analyses revealed elevated Cu contents up to 0.90 apfu. Comparably high Cu contents were known until now only in the plumbojarosite-beaverite series. The Cu2+ ion enters the B3+ position in the structure of the alunite supergroup minerals via the heterovalent substitution Fe3+Cu2+-1→ (AsO4)3-(SO4)2--1. The unit-cell parameters (space group R-3m) a = 7.3265(7), c = 17.097(2) Å, V = 794.8(1) Å3 were determined for compositionally relatively homogeneous beudantite (0.35-0.60 apfu Cu) with the following average empirical formula: Pb1.00(Fe2.46Cu0.42Al0.13 Zn0.01)3.02 [(SO4)0.89(AsO3OH)0.72 (AsO4)0.34(PO4)0.05] 2.00 [(OH)6.19F0.04]6.23. Interpretation of thermogravimetric and infrared vibrational data is also presented. The Cu-rich members of the beudantite-segnitite series are accompanied by mimetite, scorodite, pharmacosiderite, cesàrolite and carminite. These minerals are characterized by refined unit-cell parameters and by quantitative chemical analyses. The assemblage of supergene minerals corresponds to the following crystallization sequence: mimetite → beudantite-segnitite → pharmacosiderite, scorodite, cesàrolite, and carminite.