Determination of zeolite-group mineral compositions by electron probe microanalysis

Abstract

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 + Fe 3+ ), 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.

An experimental study of symmetry lowering of analcime

“…losses can be minimized with the use of carefully selected operating conditions. In the present study, to prevent the loss of NaKα X-ray intensity during electron-beam irradiation, measurements were performed under the experimental conditions based on the recommendation of London (1996, 2005) and on the recent study of natural zeolites (Campbell et al 2016). In the study, the X-ray spectra were thus measured using a beam diameter of 20 μm, an accelerating voltage of 15 kV, and a beam current of 2 nA with an acquisition time of 30 s on both the peak and background.…”

“…Therefore, even if the Na polyhedron is lost from the cage, the water molecule can remain around the center of cage with the hydrogen bond. The Na deficiency is fairly common in analcime (e.g., Pechar 1988, Line et al 1995, Cruciani and Gualtieri 1999, Neuhoff et al 2004, Gatta et al 2006, Campbell et al 2016), but the number of H2O molecules never decreases with decreasing the Na content.…”

A Further Investigation of NH4+ Removal Mechanisms by Using Natural and Synthetic Zeolites in Different Concentrations and Temperatures

“…All of the samples revealed good crystal forms and crystallinity. Chemical analyses were performed by way of Electron Probe Microanalyses (EPMA, JEOL, model JXA-8900 R, Tokyo, Japan) as a method for analyzing each grain of zeolite [18]. The results of the chemical composition analyses of the zeolites are shown in Table 1.…”

Aqua Traiana, a Roman Infrastructure Embedded in the Present: The Mineralogical Perspective

“…These crystals were very fine, with a thickness lower than 5 µm, therefore only one measurement could be performed on each crystal. However, numerous (more than three for each sample) point analyses were performed to evaluate the chemical composition of zeolites, according to [49]. By EMPA, these acicular crystals were identified as Mg-enriched chabazite (Table 4).…”