P. F. Schofield

Synthesis and Rietveld crystal structures refinement of mackinawaite, tetragonal FeS

Abstract Mackinawite, tetragonal FeS, has been synthesised by reacting iron with Na2S solutions. A Rietveld structure refinement of X-ray powder diffraction data, recorded using X-rays monochromated from synchrotron radiation with a wavelength of 0.6023 A, has been performed. The structure has been refined in the tetragonal space group, P4/nmm, and has the following cell parameters: a = 3.6735(4), c = 5.0328(7) Å, V = 67.914(24) A3. Our refinement shows that the FeS4 tetrahedron in mackinawite is almost perfectly regular, with a much smaller distortion than has been previously reported. An improved X-ray diffraction data set is provided.

Transformation of mackinawite to greigite; an in situ X-ray powder diffraction and transmission electron microscope study

Abstract Synthetic mackinawite (tetragonal FeS) has been found to transform rapidly to greigite (Fe3S4) above ∼373 K during heating experiments, as observed by in situ X-ray diffraction. Using monochromatic synchrotron radiation (λ = 0.60233 Å), we measured the unit-cell parameters of both synthetic mackinawite between 293 and 453 K and of greigite formed from this mackinawite between 293 and 593 K. The coefficients of thermal expansion for mackinawite are α1 = α2 = (1.36 ± 6 0.11) × 10-5, α3 = (2.98 ± 0.12) × 10-5, and αvol = (5.67 ± 0.19) × 10-5 between 293 and 453 K. The coefficients of thermal expansion for greigite are α1 = α2 = α3 = (1.63 ± 0.15) × 10-5, and αvol = (4.86 ± 0.25) x 10-5 between 293 and 593 K. On further heating in situ, we observed the reaction greigite → pyrrhotite + magnetite. Partial transformation of mackinawite to greigite was also observed using transmission electron microscopy (TEM) following in situ heating. Electron diffraction patterns show that (001) of mackinawite is parallel to (001) of greigite, and [110] of mackinawite is parallel to [100] of greigite. This orientation relationship confirms that the cubic closepacked S array in mackinawite is retained in greigite and implies that oxidation of some Fe2+ in mackinawite drives rearrangement of Fe to form the new phase. Small regions of the crystallites show Moiré fringes resulting from the lattice mismatch between mackinawite and greigite. Electron diffraction patterns of mackinawite subjected to prolonged exposure to the atmosphere also show faint spots corresponding to greigite. We propose that in these experiments surplus Fe is accommodated by reaction with either adsorbed O2 or H2O to form amorphous nanophase Fe-O(H). Because greigite is so easily formed by oxidation from mackinawite, greigite should be an important precursor for pyrite nucleation, although any orientation relationship between greigite and pyrite remains to be determined.

The importance of melt composition in controlling trace-element behaviour: an experimental study of Mn and Zn partitioning between forsterite and silicate melts

Abstract Partition coefficients for Mn2+ and Zn2+ between forsterite and melts in the system Fo-Ab-An at temperatures of 1150–1400°C and 1-bar pressure have been determined experimentally. For a given temperature the values of the Nernst partition coefficients (DMn and DZn) depend on the composition, and hence structure, of the melt phase, however, for a given degree of melt polymerization (expressed as the number of non-bridging oxygens per tetrahedral cation, NBO/T), the temperature dependence of the partition coefficients is less than the error in measurement. In these systems, melt composition is therefore more important than temperature in controlling trace-element behaviour. The exchange partition coefficients K D( Mn Mg ) and K D( Zn Mg ) are also more dependent on NBO/T than on temperature. Previous attempts to extract the ΔS and ΔH for the exchange reaction by regressing ln KD vs. 1 T are not valid as they ignore the changes in KD controlled by melt composition. Our data imply that for this system ΔH is small and ΔS varies with melt composition. In deploymerized melts the Nernst partition coefficients are ∼0.8 for Zn2+ and ∼0.6 for Mn2+ whereas in highly polymerized melts, values up to 4.5 for Zn2+ and 3.0 for Mn2+ have been found. In these highly polymerized systems small changes in the degree of polymerization are accompanied by large changes in the partition coefficients. This implies that fixed values of mineral/melt partition coefficients are unlikely to be adequate in trace-element modelling studies of highly differentiated igneous rocks.

Neutron powder diffraction study of the scintillator material ZnWO4

With a view to the technological applications of ZnWO4 crystals, we have performed Rietveld profile refinement of medium-resolution, time-of-flight, neutron powder diffraction data of synthetic ZnWO4 to improve the quality of the current crystal structure derived from single crystal X-ray techniques that produced a final solution with an R factorof only 10%. The new structural data for ZnWO4, monoclinic space group P2/c, lattice parameters of 0.469263(5), 0.572129(7), 0.492805(5) nm for a, b and c respectively, and a β angle of 90.6321(9) ° with two formula units per unit cell. The ZnO6 octahedra contain 3 pairs of Zn-O bonds of 0.2026(2), 0.2090(2) and 0.2227(3) nm and the WO6 octahedra contain 3 pairs of W-O bonds 0.1789(2), 0.1914(2) and 0.2133(3) nm. These new data confirm the basic structure of ZnWO4 and provide accurate off-centring magnitudes for the Zn and W cations.

Neutron diffraction at simultaneous high temperatures and pressures, with measurement of temperature by neutron radiography

Abstract The commissioning and operation of apparatus for neutron diffraction at simultaneous high temperatures and pressures is reported. The basic design is based on the Paris-Edinburgh cell using opposed anvils, with internal heating. Temperature is measured using neutron radiography. The apparatus has been shown in both on-line and off-line tests to operate to a pressure of 7 GPa and temperature of 1700°C. The apparatus has been used in a neutron diffraction study of the crystal structure of deuterated brucite, and results for 520°C and 5.15 GPa are presented. The diffraction data that can be obtained from the apparatus are of comparable quality to previous high-pressure studies at ambient temperatures, and are clearly good enough for Rietveld refinement analysis to give structural data of reasonable quality.

X-ray absorption studies of metal complexes in aqueous solution at elevated temperatures

Abstract Experimental methods for performing X-ray absorption spectroscopy (XAS) on metal complexes in dilute aqueous solutions at temperatures up to 573 K are presented. The solutions are contained in silica tubes for study by X-rays of energies greater than 16 keV. For experiments using X-rays of energies in the range 6.5–16 keV, we have developed a titanium cell with Kapton windows. These cells can be heated in an aluminium oven up to 573 K. Using these methods we have studied the tetrahedral: octahedral equilibrium for cobalt(II) in aqueous chloride solutions, Na 2 MoO 4 · 2H 2 O in aqueous solution and cadmium(II) in aqueous chloride solutions. For cobalt(II) in chloride concentrations between 5 and 7 m we have found evidence that [CoCl 4 ] 2− is not the only important tetrahedral species. In molybdate(VI) solutions, [MoO 4 ] 2− is the predominant species over the 1 temperature range 298–573 K. Cadmium(II) in aqueous chloride solutions shows two major cadmium complexes [Cd(OH 2 ) 6 ] 2+ and [CdCl 4 ] 2− . Bond distances for the principal species in these solutions are presented and are found to be in general agreement with those obtained from X-ray crystallographic studies on related solids.

Distortion Characteristics Across the Structural Phase Transition in (Cu1−xZnx)WO4

Rietveld analysis of neutron powder diffraction data on the sanmartinite (ZnWO4)-cuproscheelite (CuWO4) solid solution has enabled the comparison of microscopic and macroscopic order parameters associated with the P2/c-P1 structural phase transition. The macroscopic spontaneous strain, calculated from the lattice parameters, conforms well with a second-order Landau model. Furthermore, this is also true of the symmetry-related atomistic M--O order parameter and the quadratic elongation of the MO6 octahedra. It is clear that the Jahn-Teller effect, associated with the divalent Cu cation, is the driving force for the phase transition and the excess elongation evident in the M--O(2) bond giving rise to the non-symmetry predicted strain element e22. The existence of a large region of order parameter saturation at the copper-rich end of the solid solution is also associated with the MO 6 elongation and the breakdown of the homogeneous strain field of the zinc solute atoms.

Accurate quantification of the modal mineralogy of rocks when image analysis is difficult

Abstract The volume proportions of the mineral phases in two strongly deformed olivine-orthopyroxene rocks have been quantified by whole-pattern stripping of fixed geometry X-ray powder diffraction data. The results were compared with the phase proportions as determined by Rietveld refinement of time-of-flight neutron powder diffraction data, and were shown to be in excellent agreement. The X-ray technique not only provides a very rapid and cost-effective method of determining phase proportions, but it also circumvents several of the problems associated with obtaining this information by image analysis. Moreover, the technique is particularly advantageous in strongly textured rocks or in rocks that contain significant residual strains. As such it offers a powerful technique for analysing the mineralogical composition of fine-grained and/or deformed experimental run products, which makes it of considerable potential for monitoring in situ the progress of mineral reactions during laboratory experiments.

Characterization and identification of mixed-metal phosphates in soils: the application of Raman spectroscopy

Abstract The development of protocols for the identification of metal phosphates in phosphate-treated, metalcontaminated soils is a necessary yet problematical step in the validation of remediation schemes involving immobilization of metals as phosphate phases. The potential for Raman spectroscopy to be applied to the identification of these phosphates in soils has yet to be fully explored. With this in mind, a range of synthetic mixed-metal hydroxylapatites has been characterized and added to soils at known concentrations for analysis using both bulk X-ray powder diffraction (XRD) and Raman spectroscopy. Mixed-metal hydroxylapatites in the binary series Ca -Cd, Ca -Pb, Ca -Sr and Cd -Pb synthesized in the presence of acetate and carbonate ions, were characterized using a range of analytical techniques including XRD, analytical scanning electron microscopy (SEM), infrared spectroscopy (IR), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and Raman spectroscopy. Only the Ca -Cd series displays complete solid solution, although under the synthesis conditions of this study the Cd5(PO4)3OH end member could not be synthesized as a pure phase. Within the Ca -Cd series the cell parameters, IR active modes and Raman active bands vary linearly as a function of Cd content. X-ray diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) suggest that the Cd is distributed across both the Ca(1) and Ca(2) sites, even at low Cd concentrations. In order to explore the likely detection limits for mixed-metal phosphates in soils for XRD and Raman spectroscopy, soils doped with mixed-metal hydroxylapatites at concentrations of 5, 1 and 0.5 wt.% were then studied. X-ray diffraction could not confirm unambiguously the presence or identity of mixed-metal phosphates in soils at concentrations below 5 wt.%. Raman spectroscopy proved a far more sensitive method for the identification of mixed-metal hydroxylapatites in soils, which could positively identify the presence of such phases in soils at all the dopant concentrations used in this study. Moreover, Raman spectroscopy could also provide an accurate assessment of the degree of chemical substitution in the hydroxylapatites even when present in soils at concentrations as low as 0.1%.

2p X-ray Absorption Spectroscopy in the Earth Sciences.

A complete knowledge of 3d transition-metal valencies, site occupancies and site symmetries is essential for a full understanding of mineral/melt energetics and behaviour. Over the last few years, significant advances in both instrumentation and theory associated with synchrotron radiation sources and experiments have enabled the development of 2p X-ray absorption spectroscopy as a sensitive, element-specific site and valency probe. The potential of this technique in the Earth sciences is discussed in this paper with examples reflecting the variety of problems set by 3d transition metals in natural systems.