Crystal structure and chemical formula of schmiederite ,Pb2Cu2(OH)4(SeO3)(SeO4), with a comparison to linarite, PbCu(OH)2(SO4)

Abstract

Based on a X-ray structure analysis it was proved that the mineral schmiederite contains both selenite and selenate groups [a = 9.922(3)A,b = 5.712(2)A,c = 9.396(3)A, β = 101.96(3)°, space group P21/m,Z = 2 {Pb2Cu2(OH)4(SeO3)(SeO4)},R w = 0.055 for 1131 reflections up to sin θ/λ = 0.65 A−1]. The crystal structure is closely related to that of linarite [a = 9.701(2) A,b = 5.650(2) A,c= 4.690(2)A, β = 102.65(2)°, space group P21/m,Z = 2 {PbCu(OH)2(SO4)},R w = 0.034 for 1991 reflections up to sin θ/λ = 1.0 A−1]. The Pb atom in linarite and the Pb(1) atom in schmiederite have each three Pb-O bonds < 2.45 A with trigonal pyramidal arranged ligands; the Pb(2) atom in schmiederite has only one such near O atom. The Cu atoms are approximately square planar coordinated by hydroxil groups. In addition two further O atoms complete the coordination figure to a strongly distorted octahedron. All the anion groups have the usual geometry.