Ekatite, (Fe (super 3+) ,Fe (super 2+) ,Zn) (sub 12) (OH) (sub 6) [AsO (sub 3) ] (sub 6) [AsO (sub 3) , HOSiO (sub 3) ] (sub 2) , a new mineral from Tsumeb, Namibia, and crystal structure

Ekatite, ideally (Fe (super 3+) ,Fe (super 2+) ,Zn) (sub 12) (OH) (sub 6) [AsO (sub 3) ] (sub 6) [AsO (sub 3) , HOSiO (sub 3) ] (sub 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 approximately 3. The mineral is non-fluorescent, optically uniaxial positive with indices of refraction omega approximately 1.99 and epsilon approximately 2.08, pleochroic with omega = dark brownish black and epsilon = medium brown. Microprobe analysis revealed the following chemical composition (in wt%): (total Fe given as) Fe (sub 2) O (sub 3) 50.79, ZnO 3.78, As (sub 2) O (sub 3) 42.50, and SiO (sub 2) 2.10. The empirical formula based on 20 cations and the results of structure determination is: (Fe (super 3+) (sub 5.95) Fe (super 2+) (sub 5.14) Zn (sub 0.81) ) (sub Sigma 11.90) (OH) (sub 6.00) (As (sub 1.01) O (sub 3) ) (sub 6.00) [AsO (sub 3) ) (sub 1.43) (HOSiO (sub 3) ) (sub 0.61) ] (sub Sigma 2.04) . Single crystal X-ray studies give the following parameters: hexagonal, space group P6 (sub 3) mc; a = 12.773(2), c = 5.051(1) Aa, V = 713.7(4) Aa (super 3) with Z = 1; D (sub calc) = 4.061 g/cm (super 3) . The strongest reflections of the powder pattern are [d (sub obs) in Aa, (I (sub vis) ) and (hkl (sub 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 (Fe (super 3+) , Fe (super 2+) , Zn)O (sub 4) (OH) (sub 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 AsO (sub 3) -groups, in such a way that the lone-pairs of As (super 3+) point to its centre. Trigonal pyramids of AsO (sub 3) and HOSiO (sub 3) tetrahedra, 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 Zn (sub 7) (OH) (sub 3) (SO (sub 4) )(VO (sub 4) ) (sub 3) . The name is in honour of Dieter Ekat (1935-1996), a Namibian mining engineer.