XAS determination of the Fe local environment and oxidation state in phonolite glasses

Abstract

The Fe oxidation state, coordination geometry, and (Fe-O) distances have been determined by Fe K-edge XANES and EXAFS for a set of silicate glasses of phonolite composition produced at different oxygen fugacity conditions with the aim of determining the effect of iron oxidation state and local structural environment on the viscosity of the corresponding melts. Comparison of the pre-edge peak data with those of Fe model compounds with known oxidation state and coordination number allowed for determination of the Fe oxidation state and coordination number for all the glasses analyzed. The Fe3+/(Fe 3++Fe2+) ratio varies from 0.44 to 0.93 (±0.05) in the glasses studied. The determined values are in excellent agreement (within 0.03 difference) with those independently measured by the titration method. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination, whereas Fe2+ exists both in fourfold and fivefold coordination for this phonolitic composition, although the presence of minor amounts of sixfold-coordinated Fe cannot be ruled out by XANES data alone. EXAFS data of the most oxidized sample indicate that Fe3+ is in tetrahedral coordination with (Fe-O) = 1.85 A (±0.01). This value compares well with literature data for [4]Fe3+ (e.g., in tetra-ferriphlogopite or rodolicoite). Calculated NBO/T ratios decrease with Fe oxidation (from 0.23 to 0.19). For phonolitic glasses of this study, going from reducing to oxidizing conditions results in a higher fraction of network-forming Fe, thus increasing the polymerization of the tetrahedral network and producing shorter (and stronger) (Fe-O) bond distances. Both the polymerization increase and the structural variations in the Fe local environment can qualitatively explain the strong increase in melt viscosity observed at higher oxygen fugacity

The rheology of peralkaline rhyolites from Pantelleria Island

“…Chemical analyses are reported in Table 1. Iron oxidation state (Fe 2+ /Fe 3+ ratio) was determined by redox titration using dichromate (K 2 Cr 2 O 7 ) for FeO at LMU-Munich (Giuli et al, 2011) for both anhydrous (Table 1) and selected hydrous glasses (Table 2). Water-bearing samples, containing up to 3.55 wt.%, were synthesized using a rapid-quench internally-heated pressure vessel (IHPV, Hannover University).…”

Redox kinetics of iron in alkali silicate glasses exposed to ionizing beams: Examples with the electron microprobe

“…[26]. As other trivalent cations, Fe 3+ is generally accommodated in natural glasses as a network former [27,28]. The changes in the oxidation states of iron upon ionizing radiations have been the subject of many studies, e.g.…”

The effect of the [Na/(Na+K)] ratio on Fe speciation in phonolitic glasses

“…Le Losq and Neuville (2013) have recently studied the effect of Na/K ratio on the structure and rheology of silica-rich glasses, proposing the occurrence of two sub-networks in mixed alkali tectosilicate glasses. In previous experiments on phonolite glasses (with constant alkali content), we determined that the oxidation of Fe produces an increase in polymerization as well as shorter (and stronger) Fe-O bonds (Giuli et al 2011).…”

“…The common Fe species interpreted from investigations of silicate melts and glasses include [4] Fe 2+ , [5] Fe 2+ , [4] Fe 3+ , and [5] Fe 3+ (see Wilke et al 2001;Giuli et al 2002Giuli et al , 2011Giuli et al , 2012aJackson et al 2005;Métrich et al 2006;Rossano et al 2008). The presence of [6] Fe 2+ (e.g., Calas and Petiau 1983;Virgo and Mysen 1985;Dingwell and Virgo 1987) and a minor presence of [6] Fe 3+ (Wilke et al 2007) have been reported.…”