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The valence of iron in synthetic Fe-bearing tennantite was investigated by the Mössbauer method. Tennantite compositions were weighed out in the reference system Cu 12 As 4 S 13-Cu 14 As 4 S 13-Cu 10 Fe 2 As 4 S 13 at the levels with 0.0, 0.5, 1.0, 1.5 and 1.7 Fe apfu. The tennantite compositions obtained were characterized by electron-microprobe analysis and powder-diffraction data. Compositions with a low-level substitution of iron for copper contain Fe 3+. Evidence of divalent iron appears at the level of 1.0 Fe apfu, and for Cu-rich charges, even at 0.5 Fe apfu. It becomes dominant in Cu-rich tennantite at ~1.0 Fe apfu, whereas in Cupoor tennantite, only at ~1.4 Fe apfu. The balance of the iron is in both cases represented by mixed-valence iron, with the values of isomer-shift and quadrupole splitting intermediate between those for Fe 2+ and Fe 3+. Effects of electron delocalization and net charge-transfer in superexchange interactions on the effective valence of iron were modeled using the relation between isomer shift and valence proposed by Goodenough & Fatseas for sulfides.

Mössbauer experiments on a Li-acmite single crystal section with the crystallographic

Abstract. M6ssbauer measurements on synthetic iron orthosilicate Fe2SiO 4 (fayalite) were carried out in the antiferromagnetic spin state below TN-~65K. The M6ssbauer parameters isomer shift c~, inner magnetic field H(0), angle O between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter r/were determined as a function of temperature, These parameters could be attributed to the two crystallographic sites MI and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments./-/(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T= 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous M6ssbauer refinements published elsewhere.Single crystal M6ssbauer spectroscopy revealed the orientation of the crystallographic c-axis (Pnma) with respect to the axes of the efg. A second parameter set which also holds true for the powder refinement yields the angle between c and H(0) on M1 to be temperature-dependent in quantitative agreement with previous neutron diffraction results and solves an old discrepancy discussed elsewhere. On M2, the angle between c and H(0) is found to be much smaller than 90 ~ in contradiction to symmetry requirements.

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