‘Soft’ phonon modes,structured diffuse scattering and the crystal chemistry of Fe-bearing sphalerites

Electron diffraction has been used to carefully investigate the reciprocal lattices of a range of iron-bearing sphalerites looking for evidence of Fe clustering and/or Fe/Zn ordering in the form of either additional satellite reflections or a structured diffuse intensity distribution accompanying the strong Bragg reflections of the underlying sphalerite-type average structure. While a highly structured diffuse intensity distribution in the form of transverse polarized f110g sheets of diffuse intensity has been detected and found to be characteristic of all compositions,it does not appear to arise from Fe clustering and/or Fe/Zn ordering. Rather inherently low frequency,and therefore strongly thermally excited,phonon modes propagating along reciprocal space directions perpendicular to each of the six /110S real space directions of the average structure are suggested to be responsible for these f110g sheets of diffuse intensity. Monte Carlo simulation (for a range of Zn–S,Zn–Zn and S–S interaction strengths) and subsequent Fourier transformation is used to confirm the existence of these low-frequency phonon modes of distortion as well as to show that they are an intrinsic,predictable property of the corner-connected tetrahedral structure of sphalerite. The low-frequency phonon modes involve coupled (Zn,Fe) and S motion in one-dimensional strings along /110S real space directions

A flow-through hydrothermal cell for in situ neutron diffraction studies of phase transformations

A flow-through hydrothermal cell for the in situ neutron diffraction study of crystallisation and phase transitions has been developed. It can be used for kinetic studies on materials that exhibit structural transformations under hydrothermal conditions. It is specifically designed for use on the medium-resolution powder diffractometer (MRPD) at ANSTO, Lucas Heights, Sydney. But it is planned to adapt the design for the Polaris beamline at ISIS and the new high-intensity powder diffractometer (Wombat) at the new Australian reactor Opal. The cell will operate in a flow-through mode over the temperature range from 25–300 1C and up to pressures of 100 bar. The first results of a successful transformation of pentlandite (Fe,Ni)9S8 to violarite (Fe,Ni)3S4 under mild conditions (pH4) at 120 1C and 3 bar using in situ neutron diffraction measurements are presented

Composition-induced structural phase transitions in the (Ba1xLax)2In2O5+x (0pxp0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba1xLax)2In2O5+x, 0pxp0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x 0:2: The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with firstorder character for the tetragonal-orthorhombic transition, while the cubic-tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken

Mechanism and kinetics of hydrothermal replacement of magnetite by hematite

© 2018 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).The replacement of magnetite by hematite was studied through a series of experiments under mild hydrothermal conditions (140–220 °C, vapour saturated pressures) to quantify the kinetics of the transformation and the relative effects of redox and non-redox processes on the transformation. The results indicate that oxygen is not an essential factor in the replacement reaction of magnetite by hematite, but the addition of excess oxidant does trigger the oxidation reaction, and increases the kinetics of the transformation. However, even under high O2(aq) environments, some of the replacement still occurred via Fe2+ leaching from magnetite. The kinetics of the replacement reaction depends upon temperature and solution parameters such as pH and the concentrations of ligands, all of which are factors that control the solubility of magnetite and affect the transport of Fe2+ (and the oxidant) to and from the reaction front. Reaction rates are fast at ∼200 °C, and in nature transport properties of Fe and, in the case of the redox-controlled replacement, the oxidant will be the rate-limiting control on the reaction progress. Using an Avrami treatment of the kinetic data and the Arrhenius equation, the activation energy for the transformation under non-redox conditions was calculated to be 26 ± 6 kJ mol−1. This value is in agreement with the reported activation energy for the dissolution of magnetite, which is the rate-limiting process for the transformation under non-redox conditions

A Review of the Classification of Opal with Reference to Recent New Localities

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Our examination of over 230 worldwide opal samples shows that X-ray diffraction (XRD) remains the best primary method for delineation and classification of opal-A, opal-CT and opal-C, though we found that mid-range infra-red spectroscopy provides an acceptable alternative. Raman, infra-red and nuclear magnetic resonance spectroscopy may also provide additional information to assist in classification and provenance. The corpus of results indicated that the opal-CT group covers a range of structural states and will benefit from further multi-technique analysis. At the one end are the opal-CTs that provide a simple XRD pattern (“simple” opal-CT) that includes Ethiopian play-of-colour samples, which are not opal-A. At the other end of the range are those opal-CTs that give a complex XRD pattern (“complex” opal-CT). The majority of opal-CT samples fall at this end of the range, though some show play-of-colour. Raman spectra provide some correlation. Specimens from new opal finds were examined. Those from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey all proved to be opal-CT. Of the three specimens examined from Indonesian localities, one proved to be opal-A, while a second sample and the play-of-colour opal from West Java was a “simple” Opal-CT. Evidence for two transitional types having characteristics of opal-A and opal-CT, and “simple” opal-CT and opal-C are presented

A LA-ICP-MS sulphide calibration standard based on a chalcogenide glass

International audienceThe accurate measurement of trace element concentrations in natural sulphides by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been limited by the lack of matrix-matched calibration standards. The synthesis of a standard, IMER-1, by incorporating four minor and 34 trace elements into a chalcogenide glass matrix Ge28Sb12S60 is reported here. Chemical analysis by electron probe microanalysis (EPMA), LA-ICP-MS, solution ICP-MS, and inductively coupled plasma-optical emission spectroscopy (ICP-OES) confirmed the excellent homogeneity of major elements (1-σ relative standard deviation (RSD) <1% for S, Sb and Ge) and acceptable homogeneity of most trace elements (1-σ RSD <10%). The standard was validated by analysing trace-elements concentrations in three geological pyrite specimens using IMER-1 as the calibration standard and comparing the results to previously reported values also determined by LA-ICP-MS but using a different calibration standard, STDGL2b-2. The results suggest that IMER-1 may be an appropriate calibration standard for LA-ICP-MS analysis of trace elements in natural sulphides

Novel application of X-ray fluorescence microscopy (XFM) for the non-destructive micro-elemental analysis of natural mineral pigments on Aboriginal Australian objects

Open Access Article. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.This manuscript presents the first non-destructive synchrotron micro-X-ray fluorescence study of natural mineral pigments on Aboriginal Australian objects. Our results demonstrate the advantage of XFM (X-ray fluorescence microscopy) of Aboriginal Australian objects for optimum sensitivity, elemental analysis, micron-resolution mapping of pigment areas and the method also has the advantage of being non-destructive to the cultural heritage objects. Estimates of pigment thickness can be calculated. In addition, based on the elemental maps of the pigments, further conclusions can be drawn on the composition and mixtures and uses of natural mineral pigments and whether the objects were made using traditional or modern methods and materials. This manuscript highlights the results of this first application of XFM to investigate complex mineral pigments used on Aboriginal Australian objects

A thermosyphon-driven hydrothermal flow-through cell for in situ and time-resolved neutron diffraction studies

A flow-through cell for hydrothermal phase transformation studies by in situ and time-resolved neutron diffraction has been designed and constructed. The cell has a large internal volume of 320 ml and can operate at temperatures up to 573 K under autogenous vapor pressures (ca 8.5 106 Pa). The fluid flow is driven by a thermosyphon, which is achieved by the proper design of temperature difference around the closed loop. The main body of the cell is made of stainless steel (316 type), but the sample compartment is constructed from non-scattering Ti–Zr alloy. The cell has been successfully commissioned on Australia’s new high-intensity powder diffractometer WOMBAT at the Australian Nuclear Science and Technology Organization, using two simple phase transformation reactions from KAlSi2O6 (leucite) to NaAlSi2O6H2O (analcime) and then back from NaAlSi2O6H2O to KAlSi2O6 as examples. The demonstration proved that the cell is an excellent tool for probing hydrothermal crystallization. By collecting diffraction data every 5 min, it was clearly seen that KAlSi2O6 was progressively transformed to NaAlSi2O6H2O in a sodium chloride solution, and the produced NaAlSi2O6H2O was progressively transformed back to KAlSi2O6 in a potassium carbonate solution