G. Lamarche

Mineralogy of a natural As-rich hydrous ferric oxide coprecipitate formed by mixing of hydrothermal fluid and seawater: Implications regarding surface complexation and color banding in ferrihydrite deposits

Abstract We characterized the most As-rich natural hydrous ferric oxide (HFO) material ever reported using powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), X-ray fluorescence spectroscopy (XRF), light element analysis using gas chromatography (GC), visible-infrared (vis-IR) diffuse reflectivity, 57Fe Mössbauer spectroscopy, and superconducting quantum interference device (SQUID) magnetometry. We find that the natural As-HFO material is very similar to synthetic coprecipitated As-HFO materials, but is significantly different from all known natural and synthetic As-free HFO materials and ferrihydrite samples. The pXRD patterns show systematic differences with patterns for 2-line ferrihydrite, that are interpreted as evidence for significant populations of oxygen-coordinated Fe-As pairs. Observations by TEM, combined with energy dispersive spectroscopy (EDS) microanalysis, show agglomerations of nanophase primary particles and no evidence for other Fe- or As-bearing phases. Mössbauer spectroscopy shows octahedrally coordinated Fe3+, with a large fraction (~20%) of the octahedral Fe environments that are significantly distorted by the presence of As, compared to the Fe local environments in As-free ferrihydrite and HFO samples. The loss on ignition (LOI) is quantitatively consistent with OH + H2O, measured by GC, which, in turn, is consistent with ~1 nm diameter primary particles having all their surface cations (Fe3+, As5+, Si4+, C4+) coordinated on the free surface side by OH- and OH2. The banding into adjacent yellowish and reddish layers that occurs in the As-HFO deposits was studied by performing mineralogical analyses of the separated adjacent layers of two couplets of yellowish and reddish material. The yellowish samples were found not to contain secondary crystalline phases (as did the reddish samples, in small amounts) and to be relatively As-rich, C- and Si-poor. The observed anticorrelations between As and Si and between As and inorganic C suggest that natural HFOs, which usually contain significant molar amounts of Si, may not be as efficient at surface complexing As (and P) as their Si and C-free synthetic counterparts, unless formed by co-precipitation with the As (or P). The yellowish and reddish layers were also clearly resolved by both Mössbauer spectroscopy and magnetometry. Complexation of arsenate onto the HFO core was found to significantly increase the average quadrupole splitting (QS) obtained from Mössbauer spectroscopy by an amount that could not be explained by other chemical differences and that is consistent with an ~1 nm diameter particle size and somewhat smaller HFO core. The Munsell hue YR index (5-10 YR) was found to be strongly correlated to the average QS, thereby establishing that the color differences, corresponding to the measured shifts of the main visible band edge, are due to the local distortions in the [6]Fe3+ environments that are induced by As complexation, via their influence on the relevant ligand field transitions. SQUID magnetometry allows the following observations. (1) The superparamagnetic to superferromagnetic transitions occur at 25 K and lower in As-HFO, compared to 55 K in synthetic 2-line ferrihydrite, suggesting a smaller magnetic primary particle (or core) size for As-HFO and inter-particle magnetic interaction reduction by surface complexed As, Si, and C. (2) The ratio of supermoment magnitude to magnetic particle size (m2/n, where m is the net number of Fe3+ atomic moments per supermoment and n is the number of Fe3+ cations per particle or HFO core) decreases with increasing As content in the sequence synthetic-HFO > reddish-As-HFO > yellowish-As-HFO. (3) The magnetic susceptibility magnitudes for As-HFO and synthetic 2-line ferrihydrite differ by a factor of 10 and suggest different supermoment formation mechanisms (m2/n < 1 vs. m2/n > 1, respectively) related to differences in intra-particle cationic and anionic disorder and magnetic particle size.

X-ray diffraction analysis of dense PPO membranes

The results obtained from X-ray diffraction of dense poly (2,6-dimethyl-1,4-phenylene oxide) membranes, prepared from different casting solvents, show that the physical properties of solvents have a significant effect on the conformation of the membrane. The diffraction intensities obtained by X-ray analysis are correlated with the selectivity (CO2/CH4) and the permeability of CO2 already reported for these membranes [1]. In view of the results of this study, it seems that when crystallization is low, the state of the amorphous polymer may have an equally important role in the permeability of membranes for gas separation. It was observed that the membranes prepared by trichloro-ethylene and chloroform solvents have similar X-ray diffraction patterns and are of homogeneous morphology. The peak widths suggest small crystallites on the average, thus evenly dispersed in the membrane.

Magnetism of synthetic and natural annite mica: ground state and nature of excitations in an exchange-wise two-dimensional easy-plane ferromagnet with disorder

Abstract We have studied the magnetism of annite mica (the Fe-end-member layer silicate having ideal structural formula: {K + }[Fe 3 2+ ] 3+ Si 3 4+ 〉 O 10 2- (OH - ) 2 ) using SQUID magnetometry, Mossbauer spectroscopy, and neutron d iffraction on well characterized samples of both a synthetic powder and a natural single crystal. In disagreement with previous predictions, we find T c > 10 K and as large as T c = 58 K in the true end member and a metamagnetic ground state spin structure (predominantly ferromagnetic layers with in-plane moments, stacked antiferromagnetically along the c ∗ -axis) that is stable despite significant amounts of Fe 3+ , diamagnetic octahedral ions, and Mn 2+ in the natural sample. We propose that the antiferromagnetic stacking is part of the intrinsic zero-field magnetic domain structure and that it is entirely due to dipole-dipole forces. The most striking features are that a paramagnetic Fe 2+ temperature dependent fraction persists far below T c and that the Fe 3+ moments abruptly disorder significantly below T c , with the tetrahedral Fe 3+ going first as temperature is increased. Plausible causes for the latter intrinsic features are discussed.

Low temperature magnetic behaviour of CuFeS2 from neutron diffraction data

Abstract Measurements of neutron diffraction spectra were made at temperatures 4.2, 25, 45, 65, 85, 150 and 300 K on powdered polycrystalline synthetic CuFeS2. Standard Rietveld profile analysis using GSAS showed that at all temperatures, a good fit to the data could be obtained with both the chemical and magnetic space groups taken as I 4 2d, and an antiferromagnetic configuration of the Fe spins, as reported previously by Donnay et al. However, graphs of lattice parameters a and c against temperature showed a distinct change in slope close to 50 K, indicating the presence of a transition. Graphs of integrated intensity of diffraction lines against temperature showed corresponding discontinuities in the case of magnetic and mixed nuclear-magnetic lines but not for nuclear lines, indicating that the transition was magnetic. Detailed calculations of predicted magnetic intensities showed that the intensity variations could be explained by Cu spins, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic form at lower temperatures. The analysis gave a value of ∼ 0.05 μB for the magnetic moment of the Cu ions.

Temperature effects on the optical energy gap values of CdxZnyMnzTe alloys

Wavelength modulated reflectance measurements were made on a wide range of CdxZnyMnzTe alloys to determine the variation of the energy gap E0 as a function of temperature in the range 10–300 K. It was shown that in the vicinity of the spin‐glass transition temperature Tg, there is a magnetic contribution ΔE which increases E0, and ΔE was determined as the difference between the experimental values and a simple Manoogian–Leclerc extrapolation from higher temperatures. These values of ΔE were fitted to the relation (dt/dt)(ΔE)=−Pt−μ where t=‖T−Tg‖/Tg and good agreement found with the theoretical predictions of Alexander et al. for the effect on the band gap of a critical point such as Tg. As predicted, different behavior was observed close to Tg and at temperatures outside the critical region, μ having a value close to zero in the first range and close to 0.5 in the second. The values of P were very similar for both ranges, extrapolating to zero at the nearest‐neighbor percolation limit of z=0.18 and showin...

Magnetic properties of CdxZnyMnzTe alloys

Low‐field measurements of magnetic susceptibility χ have been made as a function of temperature in the range 4–150 K on polycrystalline samples of the alloy system CdxZnyMnzTe (x+y+z=1). Values of Tg, the spin‐glass transition temperature, have been obtained from the cusp in the χ vs T curves and values of the Curie‐Weiss θ obtained by extrapolation of the 1/χ vs T line at the higher temperatures. When the various values of Tg vs composition are fitted to a straight line they all extrapolate to a percolation limit of 0.18±0.01. However, it is shown that the plot proposed by Escorne et al. of ln Tg vs az−1/3, where a is the lattice parameter, also gives a straight line variation within the limits of experimental error. This indicates that for z≤0.2, the variation of Tg with z must deviate from linearity. The extrapolated value of 0.18±0.01 is attributed to nearest neighbors only and for z≤0.2 more distant neighbors become relatively more important. For both the Tg vs z and ln Tg vs az−1/3, a change in slop...

Study of the structure of asymmetric cellulose acetate membranes for reverse osmosis using electron spin resonance (ESR) method

Electron spin resonance (ESR) technique was used to study the structure of the skin layer of asymmetric cellulose acetate membranes for reverse osmosis. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy-free radical) was used as a spin probe that was brought into the membranes either by reverse osmosis experiments with feed solutions involving TEMPO, or by blending TEMPO into casting solutions. It was found that the mobility of TEMPO in the asymmetric membrane shrunk at 90°C was the same as TEMPO in a dense homogeneous membrane prepared from the same casting solution as that used for the preparation of the asymmetric membranes. On the other hand, TEMPO was more mobile in the asymmetric membranes when they were shrunk at lower temperatures and less mobile in a dense homogeneous membrane when the latter is prepared from a casting solution without pore former (magnesium perchlorate and water). Reverse osmosis experiments were also performed using feed aqueous solutions of sodium chloride and/or TEMPO.

Effects of crystallographic ordering on the magnetic behaviour of (AgIn)1-zMn2zTe2 and (CuIn)1-zMn2zTe2 alloys

Abstract Measurements of magnetic susceptibility in the temperature range 4.2–300 K were made on polycrystalline samples of the (AgIn) 1 - z Mn 2 z Te 2 and (CuIn) 1 - z Mn 2 z Te 2 alloys, and the data used to give values of spin-glass transition te mperature T g and Curie-Weiss paramagnetic temperature θ. For any sample for which the X-ray powder photograph indicated an apparently single phase condition, either zinc-blende or chalcopyrite, the susceptibility data could show up to three separate T g values. These different magnetic conditions are attributed to crystallographic ordering of the Mn ions on the chalcopyrite and zinc-blende lattices, the three observed T g values corresponding to disordered zinc-blende, ordered zinc-blende and ordered chalcopyrite. The value of θ obtained from the 1/χ vs. T plot is shown to be a weighted mean of the separate values of θ for the phases present. The relative sizes of the T g peaks and the values of θ for any given sample gives an indication of the amount of each phase present. These amounts were varied by using different methods of heat treatment and it was shown that the magnetic behaviour was consistent with the T ( z ) phase diagram for the two alloy systems.

Simple top‐loading cryostat insert for a SQUID magnetometer

A simple arrangement to introduce samples into a low‐temperature cryostat from above and to regulate their temperature is described. This approach has been developed in designing a low‐field SQUID magnetometer to operate from 4.2 K to room temperature. Some details on the assembly of the system and the procedures used in carrying out the measurements are given, together with an illustrative example of the performance of the magnetometer.

Crystal structures of I2 · Mn · IV · VI4 compounds

Polycrystalline samples of 18 quaternary compounds of the form I{sub 2} {center dot} Mn {center dot} IV {center dot} VI{sub 4} with I = Cu,Ag, IV = Si,Ge,Sn, and VI = S,Se,Te were made from the elements using the melt and anneal technique. By using various annealing temperatures, it was shown that while the compounds involving I = Cu, IV = Si and Ge, and VI = S and Se showed only one structure which was stable down to room temperature, for most of the other compounds, two different structures could be found, depending upon the annealing temperatures used. Also for all of the Te compounds, annealing or slow cooling below 400C produced a two- or multiphase condition. However, single-phase conditions produced by rapid quenching from higher temperatures were quasistable at room temperature. Lattice parameter values were determined for all of the single-phase forms and values were calculated for the effective lattice parameter a{sub e} (=(V/N){sup 1/3}). It was found that with a{sub e} plotted against the molecular weight W, five straight lines were obtained, each characteristic of a particular structure and magnetic behavior.