Tschörtnerite, a copper-bearing zeolite from the Bellberg volcano, Eifel, Germany

Abstract

The new mineral tschortnerite, ideally Ca₄(K,Ca,Sr,Ba)₃Cu₃(0H)₈[Si₁₂Al₁₂0_4S]·ϰH₂0, ϰ > 20, occurs as well-formed cubes up to a maximum size of 0.15 mm in a Ca-rich xenolith at the Bellberg volcano near Mayen, Eifel, Germany. The light blue, transparent crystals are optically isotropic, n = 1.504(2). Microprobe analysis (in weight percent) gave CaO 13.10, CuO 9.64, SrO 4.49, BaO 1.93, K₂O 1.37, Fe₂O₃ 0.30, Al₂O₃ 25.21, SiO₂ 30.25, H₂O (calc, by difference) 13.71. The empirical formula based on 48 O atoms within the tetrahedral net is Ca_5.60Sr_1.04K_0.70Ba_0.30Cu_2.90Fe_0.09Al_11.85Si_12.06O₄₈(OH)_8.44·14.01H₂O. Tschörtnerite is cubic, space group Fm3̅m [a = 31.62(1) Å, V = 31614 ų, Z = 16]. The density is -D_meas = 2.1 g/cm³, D_calc = 2.10 g/cm³. Single-crystal X-ray investigations showed that tschortnerite is a zeolite; the structure contains interconnection of double six-rings, double eight-rings, sodalite cages, truncated cubo-octahedra, and previously unknown 96-membered cages (tschörtnerite cage). A new structural unit is the [Cu₁₂(OH)₂JCa₈O₂₄(H₂O)₈ cluster centered within the truncated cubo-octahedron. The cluster is formed by a rhomb- dodecahedron-like arrangement of comer connected CuO₄ squares, the eight CaO₇ poly- hedra are branched. The sodalite cage houses Ca₄(OH)₄O₁₂ clusters of edge-sharing CaO₆ octahedra. Half-occupied (K,Ca,Sr,Ba) positions were located in the basal and top face of the double eight-rings, i.e., the border to the tschortnerite cage. Within the large tschort­nerite cage only H₂O molecules were localized.