Schlegelite, Bi₇O₄(MoO₄)₂(AsO₄)₃, a new mineral from Schneeberg, Saxony, Germany

Author： Krause Werner Bernhardt Heinz-Jürgen Effenberger Herta

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0935-1221

Source： European Journal of Mineralogy, Vol.18, Iss.6, 2006-11, pp. : 803-811

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Abstract

Schlegelite is a new bismuth molybdate arsenate from the dumps of the Pucher Richtschacht, near Schneeberg, Saxony, Germany. Associated minerals are quartz, petitjeanite, pucherite, bismuth and sillenite. Schlegelite forms dense spherical aggregates that are composed of small lath-like crystals up to 0.3 mm in length. Crystallographic forms are {010} (dominant), {001} and {100}; the elongation is parallel to [100]. The colour is yellow, the streak is pale yellow; Vickers hardness is VHN₂₅ = 285 kg/mm², corresponding to a Mohs' hardness of 31/2; good cleavage parallel to {010} and {001}. Schlegelite is biaxial negative, 2V = 40(5)°, n_x = 2.22, n_y = 2.255 (calc.), and n_z = 2.26 (calculated from reflectance data); non-pleochroic; orientation X = a, Y = b, Z = c. Electron-microprobe analyses gave Bi₂O₃ 70.20, PbO 0.48, CaO 0.05, P₂O₅ 0.51, As₂O₅ 15.38, V₂O₅ 0.21, MoO₃ 12.13, total 98.96 wt.-%. The empirical formula based on 24 oxygen atoms is Bi_6.78Pb_0.05Ca_0.02O_3.51(MoO₄)_1.90(PO₄)_0.16(VO₄)_0.05(AsO₄)_3.01, the idealized formula is Bi₇O₄(MoO₄)₂(AsO₄)₃. Schlegelite is orthorhombic, space group Pnca, with a = 5.299(1), b = 16.133(4), c = 23.948(5) Å, V = 2047.2(7) Å 3 , Z = 4, D_calc = 7.23 g/cm³. The crystal structure refinement converged for 2378 reflections and 166 variable parameters at R1 = 0.045. The atomic arrangement is characterized by ribbons of Bi atoms shortly linked by oxo-oxygen atoms parallel to [100]. The ribbons are connected to AsO₄ and MoO₄ tetrahedra forming layers parallel to (001). The chemical composition of schlegelite is remarkable as minerals containing both the anion groups, arsenate and molybdate, are rare; furthermore, Bi arsenates with a second anion group were found only occasionally. The mineral name is for Fritz Schlegel, a dedicated mineral collector and finder of the new species, in recognition of his contributions to the mineral assemblage of the Schneeberg area in Saxony, Germany.