Crystal structures of iron bearing tetrahedrite and tennantite at 25 and 250°C by means of Rietveld refinement of synchrotron data (original) (raw)
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Physics and Chemistry of Minerals, 2008
Electronic and magnetic properties of tennantite subfamily of tetrahedrite-group minerals have been studied by copper nuclear quadrupole resonance (NQR), nuclear magnetic resonance (NMR) and SQUID magnetometry methods. The temperature dependences of copper NQR frequencies and line-width, nuclear spin-lattice relaxation rate T 1 -1 and nuclear spin-echo decay rate T 2 -1 in tennantite samples in the temperature range 4.2-210 K is evidence of the presence of field fluctuations caused by electronic spins hopping between copper CuS 3 positions via S 2 bridging atom. The analysis of copper NQR data at low temperatures points to the magnetic phase transition near 65 K. The magnetic susceptibility in the range 2-300 K shows a Curie-Weiss behavior, which is mainly determined by Fe 2+ paramagnetic substituting ions.
MOSSBAUER STUDY OF Fe-BEARING SYNTHETIC TENNANTITE
The Canadian Mineralogist, 2003
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.
Physics and Chemistry of Minerals, 1993
L-edge X-ray absorption spectroscopy employing a synchrotron radiation source has been used to study the electronic structure and valency of Cu in the chemically and structurally complex tetrahedrite group of minerals. Mechanical mixtures of Cu2+O and Cu+FeS2 were used to estimate the relative cross sections of Cu 2+ and Cu+; the absorption of Cu 2+ at 931 eV is 25 times greater than that of Cu + at 945 eV. Using this calibration, Cu 2 +/Cu ratios were found to vary from 0.00 to 0.054 in the tetrahedrite samples studied; the highest proportion of Cu 2 + occurs in synthetic tetrahedrites with a composition close to Cu12Sb~S~3. This study reveals the utility of the technique for determining the valence state of copper in complex minerals, allowing the crystal chemistry to be more fully charaeterised.
CRYSTAL CHEMISTRY OF TETRAHEDRITE SOLID-SOLUTION: EPR AND MAGNETIC INVESTIGATIONS
The Canadian Mineralogist, 2002
One hundred and thirty samples from the collection of the Natural History Museum in Florence, labeled as tetrahedrite, have been extensively studied by means of SEM, XRD, EPMA, DTA, EPR and SQUID techniques. Wide-ranging substitutions, confirmed by compositional data, play a fundamental role in the thermal behavior, "stabilizing" natural tetrahedrite with respect to the synthetic equivalent. In order to determine the valence state and site occupancies of Cu and Fe, electron paramagnetic resonance (EPR) and magnetic measurements were performed on selected samples of natural tetrahedrite. EPR measurements were performed down to 130 K, and magnetic susceptibility measurements, in the range 2-300 K. EPR magnetic parameters were determined on the basis of spectral simulations. The behaviour of --1 versus temperature was interpreted by means of Heisenberg's model, thus yielding values for the Curie and Weiss constants. All samples are characterized by the presence of small amounts of Cu 2+ and Fe 2+ ; Fe 3+ was detected only in metal-deficient samples. Both Cu and Fe occupy the tetrahedral site; whereas in some samples the former appears aggregated in dimers, the latter is randomly distributed over the lattice. The results of the study confirm the crystal-chemical formula of the tetrahedrite samples investigated.
The crystal structure of cesbronite, Cu3TeO4(OH)4: a novel sheet tellurate topology
Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials, 2018
The crystal structure of cesbronite has been determined using single-crystal X-ray diffraction and supported by electron-microprobe analysis, powder diffraction and Raman spectroscopy. Cesbronite is orthorhombic, space group Cmcm, with a = 2.93172 (16), b = 11.8414 (6), c = 8.6047 (4) Å and V = 298.72 (3) Å3. The chemical formula of cesbronite has been revised to CuII 3TeVIO4(OH)4 from CuII 5(TeIVO3)2(OH)6·2H2O. This change has been accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, Proposal 17-C. The previously reported oxidation state of tellurium has been shown to be incorrect; the crystal structure, bond valence studies and charge balance clearly show tellurium to be hexavalent. The crystal structure of cesbronite is formed from corrugated sheets of edge-sharing CuO6 and (Cu0.5Te0.5)O6 octahedra. The structure determined here is an average structure that has underlying ordering of Cu and Te at one of the tw...
Physical Review B, 2009
Extended x-ray absorption fine structure has been measured at the K edge of copper in CuScO 2 and CuLaO 2 to investigate the local origin of negative thermal expansion ͑NTE͒ along the c axis. A positive expansion of the Cu-O bond has been measured within the entire temperature interval to be contrasted with the negative expansion of the distance between average atomic positions measured by diffraction; the inadequacy of the riding correction to diffraction data has been evidenced. The weak temperature dependence of the parallel mean-square relative displacement ͑MSRD͒ indicates a rather stiff Cu-O bond, while the large perpendicular MSRD measures an intense relative motion of copper with respect to oxygen atoms perpendicular to the c axis. The comparison between the two compounds enlightens the relevance of the correlation of perpendicular motion to quantitatively explain the apparent NTE of the Cu-O bond.
Zeitschrift Fur Kristallographie, 2005
The crystal structures of 12 argyrodite type copper compounds with the general formula Cu (12Àn) B nþ Q 2À 6Ày X À y (B ¼ P, As, Si, Ge; Q ¼ S, Se; X ¼ Cl, Br, I) were refined. The positions of the copper atoms were refined by using a non-harmonic approach. All polymorphic argyrodites were investigated in their cubic high temperature modification crystallizing in spacegroup F 4 43m (No. 216). A comprehensible way to describe the complex structures was developed based on a topological description of the rigid anion-and B-cation substructure as an arrangement of Frank-Kasper polyhedra. An analysis of the joint probability density function and of the one particle potentials for the copper atoms was performed to get a detailed insight in the copper distribution in these argyrodites. They can be divided into four types based on their different distribution of copper. This classification corresponds to the physical properties of the argyrodites, especially to their ionic conductivities, which show a significant dependence on the composition.
The Canadian Mineralogist, 2006
The crystal structure of Ag-bearing paděraite, ideally Cu 7 (Ag 0.33 Pb 1.33 Bi 11.33 ) ⌺13 S 22 , a 17.585(4), b 3.9386(9), c 28.453(7) Å,  105.41(1)°, V 1899.8(8) Å 3 , space group P2 1 /m and Z = 2, from a skarn deposit at Băiţa Bihor in Romania, has been solved by direct methods and refi ned to an R 1 index of 7.72% for 2429 unique refl ections [F o ≥ 4(F o )] measured with MoK␣ X-radiation on a three-circle diffractometer equipped with a CCD detector. The crystal structure of Ag-free, Cu-enriched paděraite, ideally Cu 7 (Cu 0.33 Pb 1.33 Bi 11.33 ) ⌺13 S 22 , a 17.573(2), b 3.9426(4), c 28.423(3) Å,  105.525(2)°, V 1897.3(6) Å 3 , space group P2 1 /m and Z = 2, from a rare-metal granitic pegmatite at Swartberg, South Africa, has been solved by direct methods and refi ned to an R 1 index of 5.04% for 3200 unique refl ections [F o ≥ 4(F o )]. In the crystal structures, there are 12 Bi sites, one Pb site, seven triangular and fl at-tetrahedral Cu sites and 22 S positions. The crystal structure, in most features corresponding to that described earlier, can be represented as a intergrowth of kupčíkite-like "K slabs" and "Q slabs", which on their own make up a structure related to nordströmite or kobellite. We provide the chemical formulae and unit cells of potential KQ n homologues of paděraite, as well as of regular intergrowths of paděraite and cuprobismutite homologous series.