Micro-XRF

KEYWORDS sulfide alteration, Acid Mine Drainage, micro-XRD, micro-XRF, micro-XANES. 

Micro-XRF transects evidenced a significant mineralogical control on the mobility of several elements released during sulfide and gangue minerals alteration (Fig. 3) in particular goethite-rich layers are enriched in Ni and Zn, whereas hematite-rich layers selectively concentrate As, Se, Mo, and Cu. 

Use of micro-XRF in the characterization of hydrothermal alteration application to the subaqueous felsic dome-flow complex of the Cap d Ours segment of the Glenwood rhyolite, Rouyn-Noranda, Quebec... 

Keywords hydrothermal alteration, Micro-XRF, volcanogenic mineralization, felsic complex,... 

Geochemical analyses were conducted on unpolished thin sections 30 pm thick using micro-XRF - EDAC Eagle III mapping at the I Universite du Quebec a Chicoutimi (UQAC). Use of the micro-XRF permits analyses of major element compositions with a relatively fast, nondestructive, in situ method through points or maps. The parameters (Table 1) were selected in order to optimize the speed and quality of the results on the basis of micro-probe analyses of chlorite. 

In addition to micro-XRF analyses, 13 samples were analyzed by X-Ray Fluorescence (XRF) (ALS Chemex) to compare major element results. Although some discrepancies were observed, the difference between XRF and micro-XRF was relatively consistent and it is therefore possible to establish a correction factor for each element. 

The range of possible uses for micro-XRF in application to the analysis of hydrothermal alteration is significant elemental variations in minerals (e.g., Fe and Mg in chlorite), calculation of... 

Application of this method within the Cap d Ours segment of the Glenwood rhyolite has identified distinct distributions of alteration associated with a low-temperature hydrothermal system. The ability of this method to detect diffuse alteration will make micro-XRF analyses a powerful tool in exploration. 

Coupled Micro-XRF elemental mapping and LA-ICP-MS geochemistry of pyrites to decipher the cause of gold precipitation in quartz-sulfide gold-bearing veins, Poderosa-Pataz district, Peru... 

Fig. 1. Left side microphotographs of visible gold occurrences in relation to pyrite crystals. Right side related micro-XRF elemental mapping of As and spot location of LA-ICP-MS analyses with As values (ppm). The exact location of the micro-XRF map is indicated by the white square on the microphotographs (left side). On right side images, the light grey pixels indicate a relative enrichment of As in pyrites. For reference, the white contorted lines are the pyrite crystal borders. White and black spots are As-rich and As-poor portions of pyrites respectively. Note the discordant aspect of the As-enrichment corridors relative to the pyrite crystals and the sharp transition from As-poor and As-rich zones indicated by the LA-ICP-MS analytical As values.

Quantitative LA-ICP-MS analyses were performed according to the subdivision between As-rich and As-poor portions of pyrite grains as revealed by micro-XRF maps. Suites of elements can be distinguished relative to As-enriched zones in pyrites (Fig. 2) 1) Au, Ag, Hg, and Sb are enriched by a factor of about 10 relative to As-poor zones 2) Pb, Bi, and Co are slightly enriched 3) Ni, Se, Mo, Ge, Te and Ga remain constant and 4) Zn, Cu, Cd, In and Sn are depleted. Spatially, the transition from As-enriched to As-poor zones in pyrite is very sharp (Fig. 1). In these examples, the bimodal... 

Micro-XRF uses a monochromatic X-ray beam focused to a few microns in diameter. The principle is the same as for the XRF system discussed earlier. A typical analytical mode is to raster the sample under the X-ray beam to produce element maps of the samples. This technique is used to determine bulk compositions of IDPs and the material in the Stardust tracks. 

Frisia, S., Borsato, A., Fairchild, I.J. Susini, J. (2005). Variations in atmospheric sulphate recorded in stalagmites Variations in atmospheric sulphate recorded in stalagmites by synchrotron micro-XRF and XANES analyses. Earth and Planetary Science Letters 235, 729-740. 

So far this technique has not been used for the analysis of zeolites, but rather for libraries of sol-gel derived materials [15], or beads obtained from a splil-and-pool protocol [16,17]. It makes use of different micro-XRF analyzers which focus the X-ray beam by a multi capillary system to a spot of around 50 pm diameter, as for instance in the Eagle II pprobe from Roentgenanalytik [15,16], or the ThermoNORAN Omicron system [17], One can expect that analysis of zeolites will be possible with about the same level of precision as reached for the sol-gel derived libraries. However, this is not sufficient to determine Si/AI-ralios reliably, especially since it is typically not known whether the sample is fully crystalline when analyzed by XRD in high-throughput mode. Nevertheless, some initial information on elemental composition can be achieved by micro-XRF. 

Examples of applications of X-ray spectrometric analytical techniques to elemental determinations in a variety of materials are presented in Table 2.12. Some recent applications papers may be mentioned. Total reflection XRF has been applied by Xie et al. (1998) to the multielement analysis of Chinese tea (Camellia sinensis), and by Pet-tersson and Olsson (1998) to the trace element analysis of milligram amounts of plankton and periphyton. The review by Morita etal. (1998) on the determination of mercury species in environmental and biological samples includes XRF methods. Alvarez et al. (2000) determined heavy metals in rainwaters by APDC precipitation and energy dispersive X-ray fluorescence. Other papers report on the trace element content of colostrum milk in Brazil by XRF (da Costa etal. 2002) and on the micro-heterogeneity study of trace elements in uses, MPI-DING and NIST glass reference materials by means of synchrotron micro-XRF (Kempenaers etal. 2003). 

Kempenaers L, Janssens K, Jochum KP, Vincze L, Vekemans B, Somogyi A, Drakopoulus M and Adams F (2003) Micro-heterogeneity study of trace elements in USGS, MPI-DING and NIST glass reference materials by means of synchrotron micro-XRF. ] Anal Atom Spectrom 18 350-357. 

MW molecular weight MXRF micro X-ray fluorescence spectrometry (also micro-XRF, JrXRF) mycorrhiza fungi fungi living in soil in close contact with plant roots (symbiosis, nutrient exchange)... 

Figure 8. Micro-XRF image of Pu sorbed onto manganese oxide coatings on smectite grains in Yucca Mountain tuff reacted with a Pu(V)-containing aqueous solution. Image was collected on the insertion device beam line 13- - at GSECARS (Advanced Photon Source), (after Duff et al. 1999a,b)...

Canadian urban single family dwelling. This sample is characterized by a potentially hazardous concentration of total Pb (1,670 mg kg ), of which 95% is bioaccessible. Health Canada required information about the solid sample Pb speciation to provide information about potential sourcefs) for the elevated Pb, and the reason for the high metal bioaccessibility. This was achieved using a combination of bulk XAFS, micro-XRF and micro-XRD analyses. 

Fig. 9 Micro-XRF Pb La, Ca Ka, Cr Ka, Pe Ka, Mn Ka and Zn Ka maps collected from a surface of 1.64 mm x 1.18 mm of the dust samples at a step size of 20 pm. Pb and Fe maps show a small correlation in some areas between Pb and Fe. The bright spectral areas are regions with high concentration of the element of interest (E = 18,000 eV count rate = 1 s/step)...

Wavelength-dispersive XRF instramentation is almost exclusively used for (highly reliable and routine) bulk-analysis of materials, e. g., in industrial quality control laboratories. In the field of energy-dispersive XRF instrumentation, next to the equipment suitable for bulk analysis, several important variants have evolved in the last 20 years. Both total reflection XRF (TXRF) and micro-XRF are based on the spatial confinement of the primary X-ray beam so that only a Hmited part of the sample (+ support) is irradiated. This is realized in practice by the use of dedicated X-ray sources. X-ray optics, and irradiation geometries. 

Fm in certain nnits, capillary optics are used. Typical direct excitation units are all handheld units, isotope-based nnits, the majority of low-power and medium-power benchtop EDXRF systems, as well as the micro-XRF and dedicated plating thickness XRF units. 

Classically, XRF has been considered a bulk analysis technique because standard EDXRF and WDXRF systems have analysis spot sizes with a diameter in the mm-cm range, depending on the system. This requires a relative large volume of sample, with inhomogeneous materials requiring a great deal of sample preparation, discussed in Section 8.2.7. Developments in X-ray optics now permit the analysis of discrete microscopic particles, and the creation of elemental maps of a sample with high spatial resolution. The systems are variously called micro-XRF spectrometers, p-XRF... 

Figure 8.46 A micro-XRF system with a large sample chamber, the SPECTRO MIDEX. This is used for analysis of large objects, such as the complete circuit board shown to the left of the analyst. ( AMETEK, Inc. www.ametek.com. Used with permission.)...

Many systems have proprietary mapping software and a charge-coupled device (CCD) camera for visual observation of the area being measured. Applications of micro-XRF spectroscopy will be discussed in Section 8.2.8. 

WDXRF units usually operate primarily under vacuum and use a helium atmosphere for loose powder samples and liquid samples. Benchtop EDXRF and micro-XRF systems can operate under vacuum, helium, or air, based on the configuration. Handheld or portable units generally operate in air at atmospheric pressure. 

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