instrumental techniques applied to mineralogy and geochemistry

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X-ray Absorption Spectroscopy in Mineralogy and in the Earth and Environmental Sciences 51 two orders of magnitude. Consequently, S/N is enhanced and XANES spectra on even very diluted heavy atoms can be measured proficiently. v) Finally, it should be noted that XAS experiments can be performed on a highly flexible sample environment. In this way, XAS experiments can be performed under extreme conditions of temperature, different gas atmospheres, and even under highapplied pressure on solid, liquid and gaseosous samples. These characteristics make XAS an incomparable tool to face different problems in Mineralogy and in the Earth and Environmental Sciences. Indeed, the flexible sample environment allows approach the temperature and pressure conditions of the Earth’s interior where phase transitions in minerals under high pressure are known to govern some of the major geophysical properties. In a similar way, XAS provides an unique insight into the accurate knowledge of trace-element behaviour in rock-forming and accessory minerals that results fundamental to understand and model petrologic and geochemical processes. These rock-forming minerals are always complex solid solutions and, therefore, the only approach capable of characterising the local coordination of the individual substituents is the use of element-selective spectroscopic methods like XAS. This is for example the case of the characterization of trace Nd and Ce site preference and coordination in natural melanites. The identification of the crystalchemical parameters controlling trace-element partitioning between minerals and silicate melts is a topic of considerable interest. Garnets represent an important group of rockforming minerals in this regard, since they are stable over a wide range of physicochemical conditions and are able to incorporate several trace elements commonly used in geochemical modeling. For these reasons, aluminosilicate garnets (X 3 Al 2 Si 3 O 12 , X = Fe 2+ , Mn 2 +, Mg, Ca) have received much attention, regarding their complex crystal chemistry, their thermodynamic properties or trace-element behaviour. Despite the importance of the geochemical behavior of REE in garnets, little work has been done to decipher their site preference and local coordination in trace or minor amounts. Indeed, there are no examples in the literature of direct structural investigations on the site location and geometry of REE in natural garnets, possibly due to their low concentrations. A recent work by Quartieri et al. has reported the XAS study of the site location and geometry of trace amounts of neodymium (from 176 to 1074 ppm) and
52 Jesús Chaboy cerium (791 ppm) in natural garnets. The comparison of the experimental spectra recorded at high energy ( Nd K-edge, 43569 eV; Ce K-edge 40443 eV) with theoretical ab-initio computations demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site and not in in extended structural defects or interstitial sites. The local geometry around the two rare-earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAS at high energy. FIGURE 8. Schematic view of the crystal structure of X 3 Y 2 (ZO 4 ) 3 garnets and the comparison between the experimental Nd K- edge XANES signal of a natural garnet (A204) and those calculated on the basis of the FMS theory assuming Nd in the dodecahedral (X) or in the octahedral (Y) site of garnet structure. This garnet contains traces of Nd (1125 ppm) and Ce(830 ppm) being its composition (Ca 2.9 Mg 0.06 Mn 0.02 Na 0.02 ) (Mg 0.08 Ti 0.89 Zr 0.04 Al 0.28 Fe 3+ 0.71) (Si 2.33 Fe 3+ 0.62). The capability of XAS to detect and characterize even trace amounts of atoms that are dispersed within minerals has been recently exploited also by Earth and Environmental scientists working on the understanding of the mineralogical and biogeochemical processes and interactions associated with environmental pollution. Within this scenario XAS is the main experimental weapon supplying information on the speciation of toxic materials. Understanding the transport and deposition of metals is crucial to both predicting toxic metal bioavailability and the accumulation of economic concentrations of metals to form mineral deposits. Toxic metals at the Earth's surface are
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