François Farges

Coordination chemistry of Ti(IV) in silicate glasses and melts: I. XAFS study of titanium coordination in oxide model compounds

Abstract The coordination environment of Ti(IV) in a variety of Ti-bearing crystalline oxide and silicate model compounds has been studied using Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at ambient temperature and pressure in order to provide a quantitative basis for interpreting Ti K-edge XAFS spectra of silicate glasses and melts (Parts II, III, and IV) Farges and Brown, 1996; Farges et al., 1996a,b). Pre-edge features of Ti K-edge XAFS spectra can be used to derive accurate information on the local coordination environment of Ti only if both pre-edge position and heights are considered. Using these features, it is also possible to distinguish between one coordination environment vs. a mixture of several others (e.g., [5]Ti vs. [4]Ti + [6]Ti). Quantitative analysis of the Ti x-ray absorption near edge structure (XANES) spectra, based on ab-initio multiple-scattering calculations for a variety of Ti-containing clusters and anharmonic analysis of the normalized XAFS oscillations, show that O first neighbors and alkali/alkaline-earth second neighbors around Ti contribute to the XAFS spectra. However, second neighbors are more prominent in the XANES region because the effects of disorder associated with these contributions are less important in this region than in the extended XAFS (EXAFS) region. Therefore, XANES spectra can be used to probe the degree of disorder in the medium-range structural environment around Ti in crystalline and amorphous materials, including Ti-bearing aperiodic structures, such as metamict, glassy, and molten compounds.

Structural environments of incompatible elements in silicate glass/melt systems: I. Zirconium at trace levels

The structural environments of trace levels (2000 ppm) of Zr4+ in several silicate glasses were examined as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Glass compositions investigated were albite (NaAlSi3O8: AB) and a peralkaline composition (Na3.3AlSi7O17: PR)- Zirconium was added to the oxide-carbonate mix prior to melting in the form of ZrO2 (baddeleyite). A second set of Zr-silicate glasses containing 2000 ppm Zr and 1.0 to 2.4 wt% halogens (F as NaF and Cl as NaCl) was also synthesized. These included the Zr-AB and Zr-PR base-glass compositions as well as Zr-sodium trisilicate composition (Na2Si3O7: TS). In all glasses studied, Zr is mainly 6-coordinated by oxygen atoms (d[Zr-O]

Structural environments of incompatible elements in silicate glass/melt systems: II. UIV, UV, and UVI

2.07 ± 0.01 A). In the most polymerized glass (AB), a small but significant amount of Zr was also found to occur in 8-coordinated sites (d[Zr-O]

Coordination chemistry of Ti(IV) in silicate glasses and melts: II. Glasses at ambient temperature and pressure

2.22 A). No clear evidence for F or Cl complexes of Zr was observed in any of the halogen-containing glasses. The regularity of the Zr site increases in the series AB < TS

COORDINATION CHEMISTRY OF TI(IV) IN SILICATE GLASSES AND MELTS : III. GLASSES AND MELTS FROM AMBIENT TO HIGH TEMPERATURES

PR. We attribute this change to an increase in the number of non-bridging oxygens in the first-coordination sphere of Zr related to the depolymerizing effects of halogens and/or sodium. Minor but significant interactions of Zr with the tetrahedral network were observed (d[Zr-{Si, Al}]

Continuous Cauchy wavelet transform analyses of EXAFS spectra: A qualitative approach

3.65–3.71 A ± 0.03 A), which are consistent with Zr-O-{Si, Al} angles close to 160–170°, as in catapleiite (Na2ZrSi3O9 · 2H2O). Intermediaterange order, as reflected by the presence and number of second-neighbor {Si, Al} around Zr, increases significantly with increasing melt polymerization. The local environment around Zr is more strongly influenced by bonding requirements than by the network topology of the melt. Stabilization of zirconium in 6-coordinated sites in relatively depolymerized melts should act to decrease the crystal-melt partition coefficients of Zr and may explain the normally incompatible character of Zr during magmatic differentiation. The presence of Zr in sites of higher coordination (ZrO8) in highly polymerized melts could be a precursor to the crystallization of zircon from such melts and thus may explain why Zr becomes a more compatible element, especially in the latest stages of magmatic differentiation.

Determination of the iron oxidation state in Earth materials using XANES pre-edge information

Abstract The structural environments of trace to minor levels (≈2000 ppm to ≈3.0 wt%) of U in several silicate glasses were examined as a function of oxygen fugacity, melt composition, and melt polymerization using X-ray (XANES and EXAFS) and optical absorption spectroscopies. Glass compositions were diopside (CaMgSi2O6: DI), anorthite (CaAlSi2O8: AN), albite (NaAlSi3O8: AB), sodium trisilicate (Na2Si3O7: TS), a peralkaline composition (Na3.3AlSi7O17: PR, approximately halfway between AB and TS), and a calc-alkaline rhyolite composition (RH). A second set of silicate glasses of the same base compositions containing ≈2000 ppm to ≈3.0 wt% U and ≈0.6 to 2.5 wt% F or Cl was also synthesized. In the glasses synthesized under oxidizing conditions (in air), UVI occurs as uranyl groups with two axial oxygens at ≈ 1.77–1.85 ± 0.02 A and four to five equatorial oxygens at ≈2.21–2.25 ± 0.03 A . In glasses synthesized under more reducing conditions (fO2 ≈ 10−3−10−7 atm), UV occurs in moderately distorted 6-coordinated polyhedra [ d(U V -O) ≈ 2.19–2.24 ± 0.03 A ], which may co-exist with smaller numbers of UVI species and/or UVI species. Under the most reducing conditions used (fO2 ≈ 10−8−10−12 atm), UIV occurs in less distorted octahedra [ d(U IV -O) ≈ 2.26–2.29 ± 0.02 A ]. No clear evidence for U-F or U-Cl bonds was found for any of the halogen-containing glasses, suggesting that U-halogen “complexes” are not present. In addition, no U-U (second-neighbor) interactions were detected, indicating that no significant clustering of U atoms is present in any of the glasses studied. Bond strength-bond length calculations and constraints placed on local bonding by Paulings second rule suggest that UIV and UV in 6-coordinated sites in silicate melts will preferentially bond to nonbridging oxygens (NBOs) rather than bridging oxygens (BOs). The unusually low 6-fold coordination of UIV and UV in relatively depolymerized silicate melts (e.g., peralkaline and halogen-rich melts) results in a high U-O bond strength in the melt that is not observed in crystalline U-bearing minerals. This difference in bond strength is partially responsible for the small crystal-melt partition coefficients of UIV. In addition, the common silicate minerals comprising igneous rocks lack appropriate crystallographic sites which can stably accommodate this large and highly charged cation. These factors help explain the normally incompatible character of UIV during magmatic differentiation. In contrast, the low solubility of UIV and UV in more polymerized silicate melts, such as those produced during the late stages of magmatic differentiation, can be explained by a shortage of NBOs. Increasing amounts of 8-fold coordinated U should favor the incorporation of both UIV and UV in accessory minerals like zircon, thorite, titanite, apatite, uranium oxides, etc., thus its more compatible behavior in the latest stages of magmatic differentiation.

Transition elements in water-bearing silicate glasses/melts. part I. a high-resolution and anharmonic analysis of Ni coordination environments in crystals, glasses, and melts

Abstract The coordination environment of Ti(IV) in a number of Ti-silicate and Ti-aluminosilicate glasses has been determined by x-ray absorption fine structure (XAFS) spectroscopy at the Ti K-edge at ambient temperature and pressure. These glasses contain 2.7–30.5 wt% TiO2 and varying amounts of Na2O, K2O, or CaO (5.0–38.7 wt%) and Al2O3 (0–11.9 wt%), and have NBO/T ratios ranging from 0.07–0.81. Quantitative analysis of the Ti XANES spectra, based on ab initio multiple-scattering calculations for a variety of Ti-containing clusters, and anharmonic analysis of the normalized XAFS oscillations suggest the presence of three types of atoms around Ti: O first neighbors, (Si, Ti)-second neighbors, and alkali third neighbors. Five-coordinated Ti, [5]Ti, is the dominant Ti species in the glasses most concentrated in Ti (> 16 wt% TiO2) and is located in distorted square pyramids ([5]TiO)O4), with one short Ti O titanyl distance (1.67–1.70 ± 0.03A) and four long Ti O distances (1.94–1.95 ± 0.02A). In addition, minor amounts of [4]Ti were detected, the proportion of [4]Ti increasing in the order: Na glasses The presence of Ti-(Si, Ti) correlations near 3.2–3.4 ± 0.1A, as in crystalline Na2([5]TiO)SiO4, is consistent with [5]TiO5 and SiO4/TiO5 polyhedra sharing corners in these glasses, with Ti O-(Si, Ti) angles of ≈120°–130° ± 10°. Quantitative analysis of the Ti K-edge XANES for the K-bearing glasses suggests the presence K around Ti, in good agreement with bond-valence predictions, which indicate that [5]Ti is most likely to bond to both nonbridging oxygens (one O in short Ti O titanyl bonds) and bridging oxygens (four O in long Ti O bonds), thus can act as a new type of Q4 specie with one additional nonbridging oxygen. Then, we propose [5]Ti to behave simultaneously a network former and a network modifier, with the network former role dominant. Bond valence models explain why the relative proportions of [4]Ti and [5]Ti change when the type of low field strength cation or the type of network-forming cation (Si vs. P) changes in oxide glasses. These models also provide a structural basis for the study of glasses and melts at higher temperatures (see Part III of this study).

Coordination of Ti in crystalline and glassy fresnoites: A high-resolution XANES spectroscopy study at the Ti K-edge

Abstract The local structural environment of Ti in five Na-, K-, and Ca-titanosilicate glass/melts with TiO 2 concentrations ranging from 2.7–30.5 wt% has been determined by in situ Ti K-edge x-ray absorption fine structure (XAFS) spectroscopy at temperatures ranging from 293–1650 K. In parallel, two Ti-model compounds (Ni 2.6 Ti 0.7 O 4 spinel and TiO 2 rutile) were studied under the same conditions to better understand the effects of temperature (anharmonicity) on the XAFS spectra. Temperature-induced anharmonicity was found to vary, largely as a function of the Ti-coordination, and increases significantly around Ti with increasing temperature when present as [6] Ti. In contrast, anharmonicity appears negligible around [4] Ti at temperatures below 1200 K. We predict that anharmonicity should be weak around [5] Ti as well. No clear evidence was found for a significant change in the average nearest-neighbor coordination environment of Ti in the Na- and K-titanosilicate glasses and melts that exhibit anomalous heat capacities variations just above their glass transition temperatures, T g (860–930 K). The small (predicted and measured) linear thermal expansion of the ( [5] TiO 2+ ) O bond in these systems at high temperature is expected to have an insignificant effect on the local environment of [5] Ti during the glass-to-supercooled liquid transition. In the most dilute Ti-glass studied (KS1; 2.7 wt% TiO 2 ), the local environment around [4] Ti (especially the second-neighbor alkalis) is relatively ordered at ambient temperature, but this order decreases dramatically above T g . Lower quench rates appear to favor [4] Ti over [5] Ti. The origin of the observed anomalous positive variations in heat capacities of these melts may be related to significant changes in the medium-range environment around Ti above T g including the disappearance of percolation domains involving interfaces between alkali-rich and network-former rich regions during structural relaxation at T g ; these percolation domains are related to the dual structural role of Ti in silicate glass/melts (acting simultaneously as network former and network modifier).

Coordination chemistry of titanium (IV) in silicate glasses and melts: IV. XANES studies of synthetic and natural volcanic glasses and tektites at ambient temperature and pressure

Abstract To better understand the extended X-ray absorption fine structure (EXAFS) spectroscopic information obtained for complex materials such as those encountered in Earth materials, we propose to use the Continuous Cauchy Wavelet Transform (CCWT). Thanks to this method, EXAFS spectra can be visualized in three dimensions: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R’), and the CCWT modulus (corresponding to the continuous decomposition of the EXAFS amplitude terms). Consequently, more straightforward qualitative interpretations of EXAFS spectra can be performed, even when spectral artifacts are present, such as multiple-scattering features, multi-electronic excitations, or noise. More particularly, this method provides important information concerning the k range of each EXAFS contribution, such as next nearest-neighbors identification. To illustrate the potential of CCWT analyses applied to EXAFS spectra, we present experimental and theoretical spectra obtained for thorite and zircon at the Th LII and Zr K edges, respectively. Then, we present CCWT analyses of EXAFS spectra collected for amorphous materials of geochemical and environmental interest, including sodium trisilicate glass and an aqueous chloride solution, at the Mo K and Au LIII edges, respectively. Further studies based on CCWT phase terms are underway, in order to quantitatively characterize anharmonic information from EXAFS contributions.