Ianbruceite, ideally [Zn_2(OH)(H_2O)(AsO_4)](H_2O)_2, a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure

摘要

Ianbruceite, ideally [Zn_2(OH)(H_2O)(AsO_4)](H_2O)_2, is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as thin platy crystals up to 80 μm long and a few urn thick, which form flattened aggregates up to 0.10 mm across, and ellipsoidal aggregates up to 0.5 mm across. It is associated with coarse white leiteite, dark blue kottigite, minor legrandite and adamite. lanbruceite is sky blue to very pale blue with a white streak and a vitreous lustre; it does not fiuoresce under ultraviolet light. It has perfect cleavage parallel to (100), is flexible, and deforms plastically. The Mohs hardness is 1 and the calculated density is 3.197 gcm~(-3). The refractive indices are α = 1.601, β = 1.660, γ = 1.662, all ±0.002; 2V_(obs) = 18(2)~°, 2V_(calc) = 20~°, and the dispersion is γ < υ, weak. Ianbruceite is monoclinic, space group P2_1/c, a=11 .793(2), b = 9.1138(14), c = 6.8265(10) A, β = 103.859(9)~°, V = 712.3(3) A~3, Z = 4, a:b:c = 1.2940:1:0.7490. The seven strongest lines in the X-ray powder diffraction pattern [d (A),I, (hkl)] are as follows: 11.29, 100, (100); 2.922, 17, (130); 3.143, 15, (202); 3.744, 11, (300); 2.655, 9, (230); 1.598, 8, (152); 2.252, 7, (222). Chemical analysis by electron microprobe gave As_2O_5 36.27, As_2O_3 1.26, Al_2O_3 0.37, ZnO 49.72, MnO 0.32, FeO 0.71, K_2O 0.25, H_2O_(calc) 19.89, sum 108.79 wt.%; the very high oxide sum is due to the fact that the calculated H_2O content is determined from crystal-structure analysis, but H_2O is lost under vacuum in the electron microprobe.The crystal structure of ianbruceite was solved by direct methods and refined to an R_1 index of 8.6%. The As is tetrahedrally coordinated by four O anions with a mean As-O distance of 1.687 A: Zigzag [~([5])Zn~([6])Znψ_7] chains extend in the c direction and are linked in the b direction by sharing corners with (AsO_4) tetrahedra to form slabs with a composition [Zn_2(OH)(H_2O)(AsO_4)]. The space between these slabs is filled with disordered (H_2O) groups and minor lone-pair stereoactive As~(3+). The ideal formula derived from chemical analysis and crystal-structure solution and refinement is [Zn_2(OH)(H_2OXAsO_4)KH_2O)_2. .