Redefinition of lemanskiite: new mineralogical data, crystalstructure and revised formula NaCaCu₅(AsO₄)₄Cl∙3H₂O

ZubkovaN. V., PekovI. V., ChukanovN. V., KasatkinA. V., KsenofontovD. A., YapaskurtV. O., BritvinS. N., PushcharovskyD. Yu.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 6. P. 43-61

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Language: Russian

Abstract

Refinement of the crystal structure of lemanskiite (R = 0.019), studied for the first time, has allowed redefining this mineral: to carry out refinement of its formula; redefine crystal system, space group and parameters of its unit-cell. X-ray powder diffraction pattern of lemanskiite has been correctly identified, the IR spectrum was obtained for its pure sample—free from pollutions by other phases. It is shown that lemanskiite is not a dimorph of lavendulan NaCaCu₅(AsO₄)₄Cl  5H₂O but contains less H₂O in comparison with the latter. Studied sample of lemanskiite has been picked up from the oxidation zone of Perseverancia deposit (Guanaco, Antofagasta, Chile). Its empirical formula is Na_0.98(Ca_0.98Sr_0.03)_∑1.01Cu_5.07As_3.97O_15.97Cl_1.033H₂O. The idealized formula of lemanskiite may be written as NaCaCu₅(AsO₄)₄Cl 3H₂O. The mineral is monoclinic, P2₁/m, a = 9.250(2), b = 10.0058(10), c = 10.0412(17) Å, b = 97.37(3)°, V = 921.7(3) ų, Z = 2. Lemanskiite represents a new structure type in the lavendulan group. Crystal structure of lemanskiite is based on the heteropolyhedral layers built by clusters of four distorted Cu-centered tetragonal pyramids, linked by edges, and eight AsO₄ tetrahedra connected also with the Cu-centered squares not involved in the clusters. Na-centered trigonal prisms and Ca-centered seven-fold polyhedra, connected with heteropolyhedral layers from both sides of each layer, are linked in the interlayer space by shared edges.

Key words: lemanskiite, lavendulan, lavendulan group, arsenate, crystal structure, oxidation zone, Perseverancia mine, Chile.