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Ekatite, (Fe³⁺, Fe²⁺, Zn)₁₂(OH)₆[AsO₃]₆[AsO₃HOSiO₃]₂, a new mineral from Tsumeb, Namibia, and its crystal structure

Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D- 70569 Stuttgart, Germany. – e-mail: paul.keller{at}po.uni-stuttgart.de

Ekatite, ideally (Fe³⁺, Fe²⁺, Zn)₁₂(OH)₆[AsO₃]₆[AsO₃HOSiO₃]₂, is a new mineral from Tsumeb, Namibia. It is associated with chalcocite and occurs as small sprays of fine needles, up to 2 mm long and less than 0.2 mm in diameter. Ekatite is brownish black, translucent with a brown streak and vitreous lustre. It is brittle without cleavage and its Mohs' hardness is 3. The mineral is non-fluorescent, optically uniaxial positive with indices of refraction 1.99 and 2.08, pleochroic with = dark brownish black and = medium brown. Microprobe analysis revealed the following chemical composition (in wt%): (total Fe given as) Fe₂O₃ 50.79, ZnO 3.78, As₂O₃ 42.50, and SiO₂ 2.10. The empirical formula based on 20 cations and the results of structure determination is: (Fe³⁺ _5.95Fe²⁺ _5.14Zn_0.81)_11.90(OH)_6.00(As_1.01O₃)_6.00[(As₃)_1.43(HOSiO₃)_0.61] _2.04. Single crystal X-ray studies give the following parameters: hexagonal, space group P6₃mc; a = 12.773(2), c = 5.051(1) ų, V = 713.7(4) ų with Z = 1; D_calc = 4.061 g/cm³. The strongest reflections of the powder pattern are [d_obs in Å, (I_vis) and (hkl_hex)]: 11.11 (3) (100), 6.37 (5) (110), 3.220 (10) (211, 220), 2.766 (3) (400), 2.420 (7) (401, 410), 1.867 (3) (402), 1.672 (3) (521), and 1.508 (3) (701, 531). The crystal structure was solved by direct methods and refined to R = 0.043 using 980 observed unique reflections. It contains dimers of face-sharing (Fe³⁺ Fe²⁺ Zn)O₄(OH)₂ octahedra, which are linked together by common edges to form infinite double chains. They are connected by corners to form a three-dimensional framework with two types of channels running along the c-axis. The larger is hexagonal and lined by six AsO₃-groups, in such a way that the lone-pairs of As³⁺ point to its centre. Trigonal pyramids of AsO₃ and HOSiO₃tetrahedra, in random distribution, occupy the smaller triangular channel, with either the lone pairs or the OH oriented along the c-axis. Ekatite is structurally related to phosphoellenbergerite, ellenbergerite and holtedahlite, as well as to several transition-metal tellurites, selenites, phosphites and Zn₇(OH)₃(SO₄)(VO₄)₃.

The name is in honour of Dieter Ekat (1935–1996), a Namibian mining engineer.

Key-words: ekatite, new mineral, crystal structure, Fe arsenite/silicate, Tsumeb, Namibia.

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