Chemical composition and Raman spectroscopy of cornubite and its relation to cornwallite in Miedzianka, the Sudety Mts., Poland
Janeczek, Janusz; Ciesielczuk, Justyna; Dulski, Mateusz; Krzykawski, Tomasz
published: Aug 1, 2016
ArtNo. ESP154019303002, Price: 29.00 €
Cornubite, Cu5 (AsO4)2 (OH)4, occurs in association with chrysocolla, cornwallite-pseudomalachite and philipsburgitekipushite series, and malachite in weathered amphibolites in dumps around a historic mining town of Miedzianka (former Kup- ferberg) in the Sudety Mountains, SW Poland. The averaged chemical composition of cornubite determined by EPMA is expressed by the formula: (Cu5.07 Zn0.04)5.11 (As0.945 P0.01 V0.005 Si0.025 O4 )2 (OH)4. Unit cell dimensions are: a = 6.155(1) Å, b = 6.253(1) Å, c = 6.806(1) Å, α = 93.14°, β = 111.49°, γ = 107.08°, V = 229.08 Å 3. The strongest Raman band at 825 cm –1 is resolved into four over - lapping components centered at 830, 825, 823, and 792 cm–1. In the region of OH stretching vibrations there is a single Raman band at 3337 cm –1 resolved into four overlapping bands centered upon 3527, 3337, 3323 and 3104 cm –1. The calculated dOH... O distances of ca. 2.94 Å for band at 3527 cm–1 and of ca. 2.76 Å for band at 3337 cm–1 suggest relatively weak H-bonds in cornubite. The polarized Raman spectrum shows strong dependence on the orientation of AsO4 tetrahedra in cornubite. The strongest change in the polarizability is observed along the y-axis of the cornubite crystal. Different Cu-sites with different dihedral angles may affect the geometry of the adjacent AsO4 tetrahedra. Stability relationship between cornubite and cornwallite (dimorphs of Cu5 (AsO4)2 (OH)4) in Miedzianka is explained in terms of (PO4)aq 3–activity. In the presence of PO4 3–;ions in aqueous solution, cornwallite is stable due to its ability to form solid solution with pseudomalachite.