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

[摘要] A study of zincoberaunite from Krásno near Horní Slavkov (Czech Republic) provided new chemical and structural dataof 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 samecation occupancy at the M2–M4 sites, the ratio of Al3+ to Me2+ at the M1 site requires the presence of divalent cationsas 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 decreasingZn and Al contents from cores to rims. The mineral composition is close to the Zn–Al-rich members of the beraunitegroup known from the same locality, but in this case, dominant occupancy of Zn at the M1 site was confirmed. With itsincreased aluminium content, zincoberaunite from Krásno differs significantly from the holotype specimen described fromHagendorf South pegmatite in Germany. The most prominent Raman bands are in good agreement with data publishedfor 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.

[学科分类] 自动化工程

[关键词] zincoberaunite;phosphate;crystal structure;Raman spectra;Krásno near Horní Slavkov;Czech Republic