Original paper
Ekatite, (Fe3+,Fe2+,Zn)12(OH)6[AsO3]6[AsO3,HOSiO3]2, a new mineral from Tsumeb, Namibia, and its crystal structure
Keller, Paul
European Journal of Mineralogy Volume 13 Number 4 (2001), p. 769 - 777
published: Jul 18, 2001
Abstract
Ekatite, ideally (Fe3+,Fe2+,Zn)12(OH)6[AsO3]6[AsO3,HOSiO3]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 ω ~1.99 and ~2.08, pleochroic with ω = dark brownish black and ε = 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+5.14Zn0.81)Σ11.90(OH)6.00(As1.01O3)6.00[(AsO )1.43(HOSiO3)0.61]Σ2.04. Single crystal X-ray studies give the following parameters: hexagonal, space group P63mc; a = 12.773(2), c = 5.051(1) Å, V = 713.7(4) Å3 with Z = 1; Dcalc = 4.061 g/cm3. The strongest reflections of the powder pattern are [dobs in Å, (Ivis) and (hklhex)]: 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 HOSiO3 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 Zn7(OH)3(SO4)(VO4)3. The name is in honour of Dieter Ekat (1935-1996), a Namibian mining engineer.
Keywords
ekatite • new mineral • crystal structure • fe arsenite • silicate • tsumeb • namibia