Determination of zeolite-group mineral compositions by electron probe microanalysis

A new protocol for the quantitative determination of zeolite-group mineral compositions by electron probe microanalysis (wavelength-dispersive spectrometry) under ambient conditions, is presented. The method overcomes the most serious challenges for this mineral group, including new confidence in the fundamentally important Si-Al ratio. Development tests were undertaken on a set of natural zeolite candidate reference samples, representing the compositional extremes of Na, K, Cs, Mg, Ca, Sr and Ba zeolites, to demonstrate and assess the extent of beam interaction effects on each oxide component for each mineral. These tests highlight the variability and impact of component mobility due to beam interaction, and show that it can be minimized with recommended operating conditions of 15 kV, 2 nA, a defocused, 20 μm spot size, and element prioritizing with the spectrometer configuration. The protocol represents a pragmatic solution that works, but provides scope for additional optimization where required. Vital to the determination of high-quality results is the attention to careful preparations and the employment of strict criteria for data reduction and quality control, including the monitoring and removal of non-zeolitic contaminants from the data (mainly Fe and clay phases). Essential quality criteria include the zeolite-specific parameters of R value (Si/(Si + Al + Fe3+), the ‘E%’ charge-balance calculation, and the weight percent of non-hydrous total oxides. When these criteria are applied in conjunction with the recommended analytical operating conditions, excellent inter-batch reproducibility is demonstrated. Application of the method to zeolites with complex solid-solution compositions is effective, enabling more precise geochemical discrimination for occurrence-composition studies. Phase validation for the reference set was conducted satisfactorily with the use of X-ray diffraction and laser-ablation inductively-coupled plasma mass spectroscopy

Fe K-EDGE X-RAY ABSORPTION SPECTROSCOPY OF SILICATE MINERALS AND GLASSES

Structural parameters determined for crystalline iron, fayalite and aegirine agree closely with X-ray crystallograhic data. A glass of NaFeSi2O6 composition has Fe predominantly present as Fe3+ in tetrahedral coordination i.e. as a network former. CaFeSiO4 and CaFeSi2O6 glasses have about 1/3 of the total Fe in octahedral coordination i.e. as a network modifier

Synchrotron X-ray powder diffraction study on synthetic Sr-Fresnoite

The Sr analogue of the mineral fresnoite (Sr2TiSi2O8) is of interest as a potential storage medium for radioactive Sr from nuclear waste. No high or low temperature crystal structure information is known on this phase. Therefore high-resolution synchrotron X-ray powder diffraction measurements have been done on a synthetic sample of Sr-fresnoite in the temperature range 87-1223K. This was done as a test experiment using the HRPD beamline P02.1 at PETRA-III, DESY. Synchrotron X-ray wavelengths of 0.2067(3)Å (293K and 573-1223K) and 0.2079(3)Å (87-499K) were used. Powder diffraction data were collected with a counting time of 30s using a PerkinElmer XRD 1621 flat panel image plate detector. CeO2 was included as an internal standard to calibrate the sample to detector distance. The P4bm tetragonal crystal structure of fresnoite (Ba2TiSi2O8) was used as a starting model for Sr-fresnoite. Small amounts of SrTiO3 and SrSiO3 were also found as impurities in this sample; therefore four-phase Rietveld refinements were done. The P4bm fresnoite structure is retained over the temperature range 87-1223K

An XAS Study of the Semi-Conducting Sulfides M2S3 (M = As, Sb, Bi)

XANES has been used to probe the low-lying vacant states in the Period 15 sulfide semi-conductors M2S3 (M = As, Sb, Bi). The As K- and L3-, Sb K- and L3-, and Bi L3- and L1-edges are related to the S K-edge XANES in terms of bands of mixed orbital character. In the K-edge spectra transitions from the 1s state to states of some p character can be seen in the region between 5 eV before the edge and 15 eV after it. The S spectra are alike showing the similar nature of the electronic structure of these compounds. The white line intensity decreases down the period showing the density of empty S 2p states at the Fermi level is also decreasing. The Period 15 spectra are related to theoretical band structure calculations for As2S3

A structural basis for ionic-diffusion in oxide glasses

The relationships between the environments of cations in alkali silicates measured by X-ray absorption fine structure (XAFS) and magic angle spinning nuclear magnetic resonance (MASNMR) are considered. Both are consistent with the modified random network for glass structure in which modifiers form channels percolating through the network. It is proposed that the mechanisms determining the distribution of bridging and non-bridging oxygen atoms at the glass transition are the same as those that promote ionic transport at lower temperatures in the glass. In particular the results of XAFS and MASNMR can be used to predict the activation energy for ionic transport and the magnitude of the electrical conductivity. Values of these parameters for alkali disilicates are in good agreement with those measured directly from transport properties