R. Vochten

The effect of crystallinity and Al substitution on the magnetic structure and morin transition in hematite

Abstract An aluminous hematite with 6.5 at% substitution and obtained by thermal decomposition at 500°C of an aluminous goethite, has been annealed at many different temperatures between 550 and 900°C. The resulting compounds have been characterized by X-ray diffraction and electron microscopy. XPS and zeta-potential measurements indicate an enrichment in Al of the surface of the particles at high sintering temperatures. Mossbauer spectra were recorded as a function of temperature and the results compared with those obtained for as-prepared aluminous hematites as a function of the Al substitution. This shows that the amount of Al leaving the hematite lattice at high temperatures is insignificant. A careful evaluation of the Mossbauer data of thermally treated, substituted and non-substituted hematite has allowed to distinguish the effects of crystallinity from those of the Al substitution. It is further suggested that the two low-temperature phases observed in the Mossbauer spectra, coexist within the particles and that the magnetic exchange interaction is on the basis of the canted spin directions.

Transformation of schoepite into the uranyl oxide hydrates: Becquerelite, billietite and wölsendorfite

SummaryThe transformation of schoepite into becquerelite, billietite and lead diuranate hydrate by direct reaction of Ca2+, Ba2+ and Pb2+ ions on solid schoepite at 60°C is demonstrated. The phases obtained in the experiments were identified by chemical and powder X-ray diffraction analyses.The solubilities of becquerelite and billietite were determined at 25° C as a function of pH. The distribution of the different uranyl hydroxyl complexes is computed and discussed. From the solubilities and uranyl hydroxyl distributions the Ksps of becquerelite and billietite are calculated.ZusammenfassungDie Umwandlung von Schoepit in Becquerelit, Billietit und Bleidiuranat-Hydrat durch die direkte Einwirkung von Ca2+, Ba2+ und Pb2+-Ionen auf festen Schoepit wird bei 60°C demonstriert. Die in den Experimenten erhaltenen Phasen wurden durch chemische Analyse und Röntgen-Pulvermethoden identifiziert.Die Löslichkeiten von Becquerelit und Billietit wurden bei 25° C als Funktion des pH bestimmt. Die Verteilung der verschiedenen Uranylhydroxyl-Komplexe wurde berechnet und wird diskutiert. Aus den Löslichkeiten `und den Uranylhydroxyl-Verteilungen werden die Ksps von Becquerelit und Billietit berechnet.

Temperature dependence of the hyperfine parameters of synthetic P2(1)/c Mg-Fe clinopyroxenes along the MgSiO3-FeSiO3 join

Abstract Transmission 57Fe Mössbauer measurements were acquired in the temperature range 11-745 K from a suite of nine synthetic Ca-free P21/c Mg-Fe clinopyroxenes (cpx) along the MgSiO3-FeSiO3 join. The paramagnetic Mössbauer spectra (MS) consist of one doublet produced by Fe2+ ions at an almost regular octahedral M1 site and a second doublet at a more distorted octahedral M2 site. The temperature dependencies of the Fe2+ center shifts were fit to equations derived from the Debye model for the lattice vibrations, allowing the determination of the characteristic Mössbauer temperatures for the two Fe sites. The temperature variations of the M1 and M2 quadrupole splitting ∆EQ(T) are consistent with the higher distortions of the M2 octahedra. Applied-field MS revealed that the principal component of the electric field gradient, Vzz, is positive, implying a tetragonal compression of both octahedral sites. The crystal-field model was used to analyze ∆EQ(T) and to calculate the energy gaps ∆1 and ∆2 of the first excited electronic states within the 5D orbital term, both at M1 and M2. The various physical quantities derived from the MS are discussed in terms of the Fe/(Fe + Mg) ratio.

Solubility and spectrochemical characteristics of synthetic chernikovite and meta-ankoleite

Abstract Chernikovite and meta-ankoleite were synthesized with a relatively high crystallinity and the compounds were identified by means of chemical analysis and X-ray diffraction. The infrared spectra were recorded and the bands assigned. From the luminescence spectra, the band-gap energy for both compounds was calculated as 2.35 e V, indicating that they must be considered as insulators. The dependence of the solubilities of these compounds on the acidity of the solution was studied, and the dominant ionic species were determined. The pKsp values of chernikovite and meta-ankoleite were found to be 22.73±0.24 and 24.30±0.81 respectively.

Transformation of curite into metaautunite paragenesis and electrokinetic properties

Genesis of metaautinute [Ca(UO2/PO4)2 · 7H2O] starting from curite hints at the existence of an intermediate hydrogen autunite stage [HUO2PO4 · 4H2O]. The substitution of protons in hydrogen autunite by Ca2+ ions is proved by electrokinetic measurements. As a consequence of the similarity between X-ray powder patterns of hydrogen autunite and meta-autunite a glycolation method has been applied in order to distinguish the two species.The cell dimensions have been determined from Guinier X-ray diffraction patterns. Both minerals are tetragonal with a=6.981±0.005 Å and c=8.448±0.005 Å for metaautunite and a=7.084±0.005 Å and c=8.777±0.005 Å for hydrogen autunite.For both minerals, the zeta-potential is mostly negative and is strongly influenced by temperature, pH and concentration of cations in the suspension. The surface conductivity has been calculated from the value of the zetapotential. The electrokinetic properties of metaautunite are very similar to those of metatorbernite.

Formation of meta-torbernite starting from curite: Crystallographic data and electrokinetic properties

A discussion is given concerning the transformation of curite into meta-torbernite in an acid environment. Crystallographic data together with differential thermal analysis show that the end-product obtained by this method is identical with meta-torbernite. Measurements of the zeta-potential on these meta-torbernite specimens result in negative values, which are strongly influenced by pH and NaCl concentrations. The surface conductivity is calculated from the measured zeta-potential.

An57Fe mössbauer effect study of ankerite

A natural sample of ankerite has been characterized by chemical analysis, X-ray diffraction and differential thermal analysis. The composition was found to be (Ca1.11Mg0.50Fe0.33Mn0.09) [Co3]1.99.57Fe Mössbauer effect measurements were performed at temperatures between 4.2 and 400 K. At low temperatures (T < 25 K) relaxation effects are clearly dominant. The temperature dependence of the center shift is remarkably well reproduced by a model based on the Debije approximation of the lattice vibrations. In contrast, the temperature dependence of the quadrupole splitting cannot be described by any reasonable crystal field model. It is argued that an orbit-lattice coupling might explain the observed quadrupole splittings. A spectrum recorded in an applied field of 6 T reveals a positive electric field gradient from which an orbital doublet ground state is concluded. Highly anisotropic field reductions are derived but cannot be quantitatively explained due to a lack of knowledge concerning the magnetic structure of ankerite. The line widths decrease significantly with increasing temperature which is only partly due to the decreasing Mössbauer fraction.

Synthesis, solubility, electrokinetic properties and refined crystallographic data of sabugalite

Sabugalite has been synthesized directly from pure chemicals. From chemical, differential thermal and thermogravimetric analyses, its formula is calculated as HA1(UO2/PO4)2·16H2O. The natural relationship between hydrogen autunite, autunite and sabugalite was investigated by means of ion exchange experiments, and its infrared spectrum, electrokinetic properties and solubility studied. An increase in solubility results in a more positive zeta-potential. The cell dimensions have been determined from Guinier-Hägg diffraction data. Synthetic sabugalite crystallizes in the monoclinic system with space group C2/m and cell parameters: a=19.426 Å; b=9.843 Å; c=9.850 Å; α=γ=90°; β=96.161°; V=1,872.54 Å3 and Z=2.

TEMPERATURE-DEPENDENCE OF THE MOSSBAUER PARAMETERS OF THE FE-NI PHASES IN THE SANTA CATHARINA METEORITE.

The temperature variation in the range 8–760K of the hyperfine parameters of the Fe−Ni phases in the Santa Catharina meteorite has been determined. It is suggested that the disordered 50–50 Fe−Ni phase actually consists of two distinct fractions, i.e. a completely disordered phase and one with intermediate long-range ordering parameter. The single-line subspectrum of the 28%-Ni phase was found to display magnetic ordering below approximately 25K.

External surface adsorption of uranyl–hydroxo complexes on zeolite particles in relation to the double-layer potential

The natural zeolites used in this study and identified by X-ray diffraction were scolecite, chabazite, heulandite and stilbite. Thermal analysis (TG and DSC) and water adsorption isotherms were also used as characterization techniques. The zeta potential was measured by a microelectrophoretic technique on zeolite suspensions containing dilute uranyl solutions.The external adsorption of the uranyl species started at pH 4 and the highest values were reached between pH 6 and 7, when the adsorption could be ascribed to single (UO2)3(OH)+5 species. The zeta potential of zeolite particles in uranyl-free suspensions became more negative with increasing pH, explained by hydroxylation of the zeolite surface, resulting in a negative electrical double layer. For heulandite and stilbite a similar zeta potential vs. pH curve was obtained in the presence of uranyl species. The curves obtained in uranyl-containing solutions for scolecite and chabazite, which adsorb the greatest amount of uranyl species, showed a spectacular increase of the zeta potential to positive values between pH 4 and 5 but at higher pH the zeta-potential decreased to negative values. This was probably due to complicated electrostatic interactions between the external surface of the zeolite and the uranyl species in solution.