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Iron bearing tetrahedrite and tennantite at 25ºC and 250ºC

Karen Friese 1Andrzej Grzechnik 1Emil Makovicky 2Tonci Balic-Zunic 2Sven Karup-Moller 3

1. Dpto. Física Materia Condensada, Universidad del País Vasco (UPV/EHU), Facultad de Ciencia y Tecnología, Apdo. 644, Bilbao 48080, Spain
2. Department of Geography and Geology, University of Copenhagen, Copenhagen DK-1350, Denmark
3. Institute for Environment and Resources, Danish Technical University, Lyngby DK-2800, Denmark


Phases in the tetrahedrite-tennantite solid solution series with the general composition (Cu,Ag)10(Fe,Zn,Hg,Cu,…)2(Sb,As)4S13 are among the most frequent complex sulfides in ore deposits. The crystal structures of the tetrahedrite-tennantite family can be derived from the  sphalerite structure type, where a part of the tetrahedra is replaced by other coordination polyhedra. Cu(1)S4 tetrahedra form a tetrahedral framework with large cavities which host clusters of Cu(2) with three-fold coordination and lone electron pairs of Sb (As).  These metalloids are positioned as (Sb,As)S3 pyramids in the cavity walls.

The four phases investigated here do not deviate from the ideal stochiometry

M12(Sb,As)4S13,  yet some of the Cu atoms are substituted by Fe. Their compositions are Cu11.4Fe0.6Sb4S13, Cu10.2Fe1.8Sb4S13, Cu11.9Fe0.1As4S13, and Cu10.8Fe1.2As4S13. At low substitution, iron is incorporated as Fe3+. With increasing substitution, Fe3+ is converted into Fe2+, the conversion being complete at a composition of Cu10Fe2(Sb,As)4S13.

Rietveld refinement of x-ray diffraction data from low-Fe and high-Fe samples of both  compounds, obtained at SNBL/ESRF (Grenoble) (λ=0.3748 Å) at 25o and 250oC,  suggests that the coefficient of thermal expansion is highest for samples with low iron substitution. Splitting of triangular Cu(2) positions increases with temperature and the half-occupied Cu(2) positions from different coordination triangles approach each other, down to 2.70-2.75 Å in tennantite at 250oC. There is insignificant residual electron density between the split Cu(2)-sites in tetrahedrite at 25oC and its increase with temperature is moderate. The inter-site density is substantially higher in tennantite and increases considerably with temperature, especially in the low-Fe sample.

tetten_2_1.jpg Fobs and difference-Fourier maps (minima/maxima in eÅ-3)  of  Cu11.9Fe0.1As4S13. Central point x(Cu2),0,0; distances in the three directions 2.5x2.5x2.5 Ǻ. Contour lines 2 (Fobs) and 0.1 (difference-Fourier).


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Related papers

Presentation: Poster at 11th European Powder Diffraction Conference, Poster session, by Karen Friese
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-04-15 11:21
Revised:   2009-06-07 00:48