New crystal-chemical data on zincoberaunite from Krásno near Horní Slavkov (Czech Republic)

Abstract

A study of zincoberaunite from Krásno near Horní Slavkov (Czech Republic) provided new chemical and structural data of this rare member of the beraunite group. The studied material is monoclinic, space group C2/c, with a = 20.3440(19) Å, b = 5.1507(3) Å, c = 19.1361(15) Å, β = 93.568(8)°, V = 2001.3(3) Å3, Z = 4. Based on refined site occupancies and bond-valence considerations, the structural formula is (Zn0.81Al0.19)(OH)2(Fe0.61Al0.39)(OH)2(H2O)2(Fe1.52Al0.48)(H2O)2(Fe1.72Al0.28) (OH)(PO4)3.83(AsO4)0.17(H2O)2. Electron-microprobe analyses support the obtained results. However, keeping the same cation occupancy at the M2–M4 sites, the ratio of Al3+ to Me2+ at the M1 site requires the presence of divalent cations as follows: (Zn0.57Fe0.24Al0.19)Σ1.00(Fe3.85Al1.15)Σ5.00[(PO4)3.89(AsO4)0.10(SiO4)0.01]Σ4.00[(OH)4.59F0.24 O0.17]Σ5.00(OH2)4.00·2H2O. Individual prismatic zincoberaunite crystals exhibit a chemical zonation manifested by increasing Fe and decreasing Zn and Al contents from cores to rims. The mineral composition is close to the Zn–Al-rich members of the beraunite group known from the same locality, but in this case, dominant occupancy of Zn at the M1 site was confirmed. With its increased aluminium content, zincoberaunite from Krásno differs significantly from the holotype specimen described from Hagendorf South pegmatite in Germany. The most prominent Raman bands are in good agreement with data published for related members of the beraunite group. Structure refinement (R = 3.56 % for 1906 observed unique reflections) revealed three different types of OH or H2O, which play distinct role in structure bonding.