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X-ray absorption spectroscopy at the Mg K-edge is used to obtain information on magnesium environment in minerals, silicate and alumino-silicate glasses. First-principles
XANES calculations are performed for minerals using a plane-wave density functional formalism with core-hole effects treated
in a supercell approach. The good agreement obtained b...
Contexts in source publication
Context 1
... the case of forsterite, farringtonite and enstatite, the theoretical Mg K-edge XANES spectrum is the average of the indi- vidual contributions obtained with the absorbing atom located at each non equivalent crystallographic site. Calculations were carried out from the experimental crys- tal structures listed in Table 3. In the case of forsterite, pyrope, enstatite, diopside and spinel, calculations were performed from theoretical chemical compositions, that do not contain any impurity or trace element. ...
Context 2
... [6] Mg, the quadratic elongation (QE) and the angular variance (AV) are given inorder to evaluate the degree of distortion of the octahedron. The structural parameters, hMg-Oi, QE and AV are determined from the crystallographic structures given in Table 3 and in Hochella et al. (1979), Sharp et al. (1987) and Perdikatsis and Burzlaff (1981) for cordierite, monticellite and talc, respectively Phys Chem Minerals [4] Mg, [6] Mg and [8] Mg. However, [5] Mg is difficult to distinguish from [4] Mg and [6] Mg. ...
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Citations
... Hence, the change in peak (b) position is most likely associated with the distortion of the MgO 6 octahedron. 47 The degree of distortion in MgO 6 ...
Although the concentration of uranium (U) in seawater is extremely low (3.3 μg L⁻¹), the total amount of U in Earth's oceans is more than one thousand times greater than the amount in terrestrial ores. To extract useable quantities of U from seawater, highly selective adsorbent materials are needed since competing elements (e.g. sodium, calcium) are present at orders of magnitude higher concentrations. Layered double hydroxide (LDH) materials are an intriguing adsorbent material for U seawater extraction, as they are simple to prepare and can incorporate multiple metal chemistries to modulate structure and properties. Herein, X-ray absorption spectroscopy (XAS) is used to provide fundamental insight into the adsorption mechanism of U extraction from seawater and how doping of trivalent lanthanides into MgAl LDH materials can enhance their selectivity. It is revealed that the mechanism of U sorption from U-spiked seawater is primarily surface complexation with abundant Mg/Al–OH sites, along with the ion exchange mechanism whereby interlayer nitrate is replaced by anionic species (carbonate, hydroxyl and uranyl carbonate). Further, lanthanide doping increased the ionic character of bonding within the LDH and hence the selectivity for binding U via surface sorption. Neodymium doped LDH exhibited superior U selectivity to state-of-the-art amidoxime functionalised polymers, as well as adsorption capacity and kinetics comparable to these state-of-the-art adsorbents. These findings indicate that dopant engineering of LDHs provides a simple, effective method for controlling selectivity and producing adsorbents capable of challenging separations such as U extraction from seawater.
... Some of these techniques include secondary ion mass spectrometry (SIMS) [27,75,76], atomic force microscopy (AFM) [27,77,78,79,80], scanning electron microscopy in conjunction with the energy or wavelength dispersive X-ray spectrometry (SEM/EDX or SEM/WDX) [81], gammaray spectrometry or proton-induced X-ray (PIGE or PIXE) [82], as well as nuclear reaction analysis (NRA) or Rutherford backscattering spectrometry (RBS) [77,83,84]. In a few cases also X-ray photoelectron spectroscopy (XPS) and elastic recoil detection analysis (ERDA) was utilized for the characterization of the altered glazing surfaces [77,85,86,87]. Finally, the non-destructive spectroscopic ellipsometry (SE) [88,89] and Auger electron spectroscopy (AES) [90] has received considerable interest in describing glazing surface morphology, including the early phase of the corrosion process, particularly glass weathering. ...
Several factors, including incentives associated with aesthetics, transparency, high chemical, and mechanical durability, and its excellent corrosion resistance, have rapidly accelerated the interest and use of glass as windows, façades, or load-bearing elements in structural applications. Nonetheless, the glass is chemically attacked when subjected to certain environmental conditions and its chemistry, structure, as well as its optical and mechanical properties, are altered by the different weathering processes throughout its service life. Several techniques exist for evaluating the performance of weathered glass. These include both natural and artificial ageing techniques. However, little correlation has been shown to exist between natural and artificial ageing, especially the comprehensive comparison between the naturally aged and artificially weathered glazing systems have yet to be examined. In this review paper, the weathering of structural glass systems when exposed to environmental conditions is presented. Emphasis in the literature has been placed chiefly on the different types of glazing in the construction industry and their resistance to three main weathering agents: humidity, temperature, and soiling. Main optical and mechanical tests reported in the literature are summarized, and the properties described in each of them are examined, providing evidence of current challenges, limitations, and insight on future prospects.
... Mg K-edge XANES spectra of the slag core and reacted surface of the granulated slag MG-1.2-Ti are presented in Fig. 9. The XANES spectrum of the slag core was composed of four main features, located at about 1306.1 eV for a pre-edge peak P, 1309.2 eV and 1314.2 eV for two main features A and B, and 1326.0 eV for a broader peak C. Previous studies on Mg-bearing minerals and silicate glasses have attributed these features to various Mg environments [51][52][53][54][55]. The pre-edge peak P was assigned to a highly asymmetric environment of the first coordination shell. ...
... eV) and D (1330 eV), in addition to the two main features A and B. No pre-edge peak P was present. The features C' and D were related to multiple scattering phenomena occurring in the outermost coordination shells around Mg [51,52]. The higher intensity of the features C' and D in the surface layer indicates a higher long-range order around Mg atoms than in the slag. ...
... The energy position of peak A (1310.9 eV) suggests a coordination >6 fold for Mg. However, polyhedral distortions in the first coordination shell of Mg, or multiple scattering phenomena, can increase the peak A position of [6] Mg-bearing minerals above 1310 eV [51,52]. Altogether, the features A, B, C' and D give a XANES spectrum very close to that of hydrotalcite [56,57]. ...
Small TiO 2 contents in the slag-glass significantly reduce their cementitious reactivity. We investigated the role of Ti in the glass network and its influence on slag-glass dissolution. XANES and EPR analysis showed that 67% of Ti was present as Ti(IV) with about 70% in [5] Ti and 30% in [4] Ti coordination, and 33% as Ti(III). The presence of Ti had only a minor impact on initial dissolution rates at pH 11 (R ≈ 10 − 6 mol glass .s − 1 .m − 2). During dissolution , Ti accumulated in an amorphous layer at the glass-surface. Ti-coordination in this layer was 50% [5] Ti and 50% [6] Ti. At pH 11, the layer was mainly composed of TiO 2 (66 wt%), but intermixed with a hydrotalcite-like phase at pH 13. The loss of cementitious reactivity in the presence of Ti appears to be not only due to stabilization of the glass structure but also to the formation of a Ti-rich surface-layer, that may become passivating.
... In Figure 5, a dotted straight line is drawn between SiO 2 and MgO with Mg in 6-fold coordination, and it is clearly observed that the V m of Mg-silicate glasses are below this line, which suggests that the CN of Mg is significantly less than 6 in amorphous magnesium silicates. Such an interpretation agrees with 25 Mg Nuclear Magnetic Resonance (NMR) spectroscopy that shows that Mg is essentially in four-fold coordination in silicate glasses (Fiske and Stebbins 1994;Georges and Stebbins 1998;Kroeker and Stebbins 2000;Shimoda et al. 2007a,b), as well as with XANES at the Mg K-edge (Trcera et al. 2009) while Ca is essentially in six fold coordination in silicate glasses as shown by XANES at the Ca K-edge Neuville et al. 2004b;Ispa et al. 2010;Cicconi et al. 2016). These gentle variations may indicate that Ca and Mg remain almost in the same coordination number, CN, in silicate glasses, this CN being slowly affected by the SiO 2 content or by the variation in the number of bridging oxygens around Si. 4 Cation sizes are given in Whittaker and Muntus (1970) and molar volumes are from Robie et al. (1979). ...
... i.) that the coordination number of Mg is almost constant in this system and is not really affected by the Mg/Al substitution, an idea in good agreement with XANES at the Mg K-edge (Trcera et al. 2009); ii) that the molar volume depends on the proportion of Al in five-fold coordination. For the Ca, Sr and Ba aluminosilicate glasses the changes observed in the slopes near 25 mol% of Al 2 O 3 correspond to the tectosilicate line (R = M 2+ O/Al 2 O 3 = 1), characterized by an increase in the proportion of [5] Al in the glass Novikov et al. 2017). ...
... However, the network is more perturbed by the presence of Mg than by Ca because of the higher ionic field strength of Mg Roy and Navrotsky 1985) and the Ca/Mg coordination number should play an important role. The coordination number of Mg is well characterized (Fiske and Stebbins 1994;Kroeker and Stebbins 2000;Trcera et al. 2009): it is 4 in silicate glasses, and increases up to 5-6 in aluminosilicate glasses, in agreement with glass molar volumes variations (Fig. 15). Turning to Ca, in silicate glasses it is mainly located in distorted sites with 6-7 oxygen neighbors (Cormier et al. 2003;Neuville et al. 2004bCicconi et al. 2016). ...
... This overestimation of Ca-O distance is likely due to the Guillot force field [50] used here, where a similar overestimation has been previously reported in the literature for a comparable force field (e.g., Matsui [64]) [33]. The Ca cations in the CMAS glasses are seen to have an average CN of ~6.7-6.8 (Table 3), and the CN distributions in Figure which is also consistent with literature data on Mg coordination in Mg-containing silicate glasses [62,70] (a brief summary has been given in ref. [33]). Overall, the results in Table 3 suggest that the MD-generated structural representations are able to capture the local atomic structure of CMAS glasses. ...
In this investigation, force field-based molecular dynamics (MD) simulations have been employed to generate detailed structural representations for a range of amorphous quaternary CaO-MgO-Al2O3-SiO2 (CMAS) and ternary CaO-Al2O3-SiO2 (CAS) glasses. Comparison of the simulation results with select experimental X-ray and neutron total scattering and literature data reveals that the MD-generated structures have captured the key structural features of these CMAS and CAS glasses. Based on the MD-generated structural representations, we have developed two structural descriptors, specifically (i) average metal oxide dissociation energy (AMODE) and (ii) average self-diffusion coefficient (ASDC) of all the atoms at melting. Both structural descriptors are seen to more accurately predict the relative glass reactivity than the commonly used degree of depolymerization parameter, especially for the eight synthetic CAS glasses that span a wide compositional range. Hence these descriptors hold great promise for predicting CMAS and CAS glass reactivity in alkaline environments from compositional information.
... All three spectra have similar overall shapes in the pre-and post-edge regions. Previous studies have revealed that the near-edge region between 1306 and 1321 eV can be divided into three main components (assigned to A, B, and C) [24][25][26][27] . The near-edge region contributed by the three components showed a main peak centered at 1315 eV (marked B) with shoulders at 1312 and 1320 eV (marked A and C, respectively) (Fig. 5). ...
Mineral trapping through the precipitation of carbonate minerals is a potential approach to reduce CO2 accumulation in the atmosphere. The temperature dependence of amorphous magnesium carbonate (AMC), a precursor of crystalline magnesium carbonate hydrates, was investigated using synchrotron X-ray scattering experiments with atomic pair distribution function (PDF) and X-ray absorption fine structure analysis. PDF analysis revealed that there were no substantial structural differences among the AMC samples synthesized at 20, 60, and 80 °C. In addition, the medium-range order of all three AMC samples was very similar to that of hydromagnesite. Stirring in aqueous solution at room temperature caused the AMC sample to hydrate immediately and form a three-dimensional hydrogen-bonding network. Consequently, it crystallized with the long-range structural order of nesquehonite. The Mg K-edge X-ray absorption near-edge structure spectrum of AMC prepared at 20 °C was very similar to that of nesquehonite, implying that the electronic structure and coordination geometry of Mg atoms in AMC synthesized at 20 °C are highly similar to those in nesquehonite. Therefore, the short-range order (coordination environment) around the Mg atoms was slightly modified with temperature, but the medium-range order of AMC remained unchanged between 20 and 80 °C.
... The MD-RMC model of the CMAS glass shows an average Mg-O distance of 2 Å and a coordination number (CN) of Mg of 4.53 with a large predominance of [4] Mg ( Fig. 5 and S5). This CN is smaller than in a 1.5CaO-0.5MgO-2SiO 2 glass, where neutron scattering and RMC modeling yield a CN of 5 [35], but the coexistence of [4]and [5] Mg in the CMAS model glass is in line with the coordination values found in the glass literature [54]. An important outcome is that there is no evidence of an important fraction of 6-coordinated Mg (less than 10% [6] Mg in the MD+RMC model glass). ...
This study presents a description of the structure of amorphous slags, with particular attention to the Ca local environment and the medium range arrangement of Ca-sites. Using Ca K-edge X -ray absorption spectroscopy (XANES and EXAFS) and pair distribution function (PDF) as well as numerical modeling combining Molecular Dynamics and Reverse Monte-Carlo, we demonstrate that Ca occurs in a complex environment with a distribution between 6- and 7-coordinated sites. Ca atoms are not distributed randomly within the glass structure: the Ca sites are linked by edges and form a complex percolating sub-network of network-modifying cations. The aluminosilicate network in slags is highly depolymerized with, on average, Q2.06 arrangements for Si, indicating the predominance of chain-like structural units and Q2.67 arrangements for Al in more polymerized units cross-linking the Ca-domains. The Ca percolating sub-network forms clusters that may enhance the early dissolution rate of slags in water.
... This overestimation of Ca-O distance is likely due to the Guillot force field [50] used here, where a similar overestimation has been previously reported in the literature for a comparable force field (e.g., Matsui [64]) [33]. The Ca cations in the CMAS glasses are seen to have an average CN of ~6.7-6.8 (Table 3), and the CN distributions in Figure which is also consistent with literature data on Mg coordination in Mg-containing silicate glasses [62,70] (a brief summary has been given in ref. [33]). Overall, the results in Table 3 suggest that the MD-generated structural representations are able to capture the local atomic structure of CMAS glasses. ...
In this investigation, force field-based molecular dynamics (MD) simulations have been employed to generate detailed structural representations for a range of amorphous quaternary CaO-MgO-Al2O3-SiO2 (CMAS) and ternary CaO-Al2O3-SiO2 (CAS) glasses. Comparison of the simulation results with select experimental X-ray and neutron total scattering and literature data reveals that the MD-generated structures have captured the key structural features of these CMAS and CAS glasses. Based on the MD-generated structural representations, we have developed two structural descriptors, specifically (i) average metal oxide dissociation energy (AMODE) and (ii) average self-diffusion coefficient (ASDC) of all the atoms at melting. Both structural descriptors are seen to more accurately predict the relative glass reactivity than the commonly used degree of depolymerization parameter, especially for the eight synthetic CAS glasses that span a wide compositional range. Hence these descriptors hold great promise for predicting CMAS and CAS glass reactivity in alkaline environments from compositional information.
... In contrast with the evident cutoff distances for the average CNs of Si and Al atoms seen in Fig. 5(a), the CNs for Mg and Ca atoms (with oxygen) are highly dependent on the selected cutoff distance, which might contribute to the different oxygen CNs reported in the literature for Mg (∼4-7 [76,[90][91][92]) and Ca (∼5-9 [10,16,34]) atoms in silicate glasses from simulations in comparison with experimental data. By using cutoff distances corresponding to the first minimum after the main peak of the partial PDFs, we see in Table II (Table II). ...
... Even at these fixed cutoff distances (Table II), both Mg and Ca atoms have a distribution of oxygen CNs, as illustrated in Fig. S9b and Table S1 in the Supplemental Material. Figure S9b and Table S1 show that the Mg environment in the CMAS glass consists of four-, five-, six-, and sevenfold coordinated sites, with fivefold dominating as confirmed using x-ray absorption near-edge structure (XANES) [76,90] x-ray/neutron diffraction coupled with RMC refinement [76], and MD simulations [51]. Nevertheless, previous NMR ( 25 Mg) studies on MAS and CMAS glasses show Mg is mainly in six coordination [86,92]. ...
... For an X-Y atom-atom pair, the coordination number of X, averaged over the ten DFT-optimized structural representations, is given, along with one standard variation shown in the brackets. Literature data on different types of silicate glasses (e.g., CAS [6,16], MAS [76,90] MS [92], CMAS [86], and NCAS [47]) are also given for comparison. Table II), we get a (Ca CN)/(Mg CN) ratio of ∼7.4 and ∼7.5 around Si and Al atoms, respectively. ...
Quaternary CaO−MgO−Al2O3−SiO2 (CMAS) glasses are important constituents of the Earth's lower crust and mantle, and they also have important industrial applications such as in metallurgical processes, concrete production, and emerging low-CO2 cement technologies. In particular, these applications rely heavily on the composition-structure-reactivity relationships for CMAS glasses, which are not yet well established. In this study, we combined force-field molecular dynamics (MD) simulations and density functional theory (DFT) calculations to generate detailed structural representations for a CMAS glass. The generated structures are not only thermodynamically favorable (according to DFT calculations) but also agree with experiments (including our x-ray and neutron total scattering data as well as literature data). Detailed analysis of the final structure (including partial pair distribution functions, coordination number, and oxygen environment) enabled existing discrepancies in the literature to be reconciled and has revealed important structural information on the CMAS glass, specifically (i) the unambiguous assignment of medium-range atomic ordering, (ii) the preferential role of Ca atoms as charge compensators and Mg atoms as network modifiers, (iii) the proximity of Mg atoms to free oxygen sites, and (iv) clustering of Mg atoms. Electronic property calculations suggest higher reactivity for Ca atoms as compared with Mg atoms, and that the reactivity of oxygen atoms varies considerably depending on their local bonding environment. Overall, this information may enhance our mechanistic understanding on CMAS glass dissolution behavior in the future, including dissolution-related mechanisms occurring during the formation of low-CO2 cements.
... The tetrahedral four-fold Mg sites tend to be regular whereas the pentahedral and octahedral sites are rather distorted, as inferred from NMR (Kroeker and Stebbins, 2000). A detailed analysis of Mg in a wide variety of local environments using Mg K-edge XANES may be used as a basis for future studies (Li et al., 1999;Trcera et al., 2009). CN Mg is important as it appears to strongly affect T g across the SiO 2 -MgO join, where T g of invert glasses increases with increasing CN Mg (Wilding et al., 2004a) (Figs. 5 and 6). ...
... The substitution of Mg by Ca is ubiquitous in AE silicate systems that is important for basic rocks as well as for potential biomedical applications. The diopside composition, CaMgSi 2 O 6 , glass has been studied using numerous techniques: 25 Mg (George and Stebbins, 1998;Kroeker and Stebbins, 2000;Shimoda et al., 2008), 29 Si and 17 O NMR (Brandriss and Stebbins, 1988;Nasikas et al., 2012), Ca K-, Mg K-, Si K-, and O K-edge XANES (Binsted et al., 1985;Ildefonse et al., 1995;Li et al., 1999;Moulton et al., 2016b;Trcera et al., 2009), Raman (Franz and Mysen, 1995;Moulton et al., 2016a;Nasikas et al., 2011;Retsinas et al., 2014), X-ray diffraction (XRD) and neutron diffraction (ND) (Cormier and Cuello, 2013;Shimoda et al., 2005;Taniguchi et al., 1997), as well as by theoretical simulations (Sun et al., 2011;Zhang et al., 2010). Finally, Hung et al. (2016) provide direct evidence for O 2À in near orthosilicate composition (Ca,Mg)-silicate glasses. ...
Alkali- and alkaline-earth-bearing silicate glasses form the base models for many more complex glasses and glass-ceramics. When alkali or alkaline-earth cations are mixed into the silica network it is weakened by the formation of non-bridging oxygens from bridging oxygens which link adjacent tetrahedra. This depolymerization has a profound effect on the properties of the resulting glass, melt, or glass-ceramic. Each of the alkali- and alkaline-earth-silica binary glass systems are reviewed. This discussion focuses on the Mixed Alkali Effect and phase separation. A broad overview of industrial applications is provided by key contributions where additives enhance the intrinsic glass properties.
