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This study focuses on analysis of local structural arrangements of ferric and ferrous iron in binary alkali-silicate networks, and the effect of changing Fe2+/Fe3+ ratio on the glass structure, investigated by Raman spectroscopy. Three alkali trisilicate glasses, Li2Si3O7, Na2Si3O7 and K2Si3O7, were synthesized with 4.4–6.0 wt% iron oxide, to form three series with the same nominal composition but changing Fe2+/Fe3+ ratio. Structural analyses of the high-wavenumber envelope in Raman spectra of Fe-free alkali-trisilicate analogues have been useful in identifying structural species in the Fe-bearing glasses. Peak fitting of the high-wavenumber region between ca. 820 and 1200 cm− 1 indicates that the increase in ferric iron content is associated with a considerable increase in the Q2 species while the abundance of Q4- and Q3-decreases. However, the most prominent change with the increase in the ferric content is the rise of the peak centred at ca. 980 cm− 1 that we identify as related to the vibration of Fe3+-O-Si linkage, regardless of the coordination geometry around ferric cation. We have observed a roughly linear trend in the change of the Fe3+-related Raman peak area with the increase in the ferric iron content for all three alkali trisilicate series, with increasing slope from K to Li. Nonetheless, there are discrete differences in the dependence due to composition of the glass. Based on the results of this study, the obtained linear calibration trend can be used only for an approximate determination and not as a routine quantification of ferric content.

The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na + K) ratio. Low temperature (708–1000 K) viscosities were determined by the micropenetration technique in the 10 9.6 to 10 13.6 Pa s range. The effect of Fe 2 + /Fe tot , from 0.15 to 0.86, was explored for [Na/(Na + K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe 2 + /Fe tot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe 2 + /Fe tot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe 3 + contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction.

We determined the viscosity and heat capacity of a series of two basaltic liquids containing H 2 O, F, H 2 O + CO 2 , H 2 O + F, and H 2 O + CO 2 + F. One was a natural calc-alkaline basalt from Fuego volcano, Guatemala, and the other was an Fe-free synthetic analog. The viscosity measurements were performed in the low-temperature, high-viscosity range (~ 10 9 –10 12 Pa s) just above the glass transition, where the kinetics of volatile exsolution are slow. Differential scanning calorimetry measurements were performed at atmospheric pressure from room temperature up to ~ 100 K above the glass transition. The water contents ranged from nominally anhydrous to 3 wt.% H 2 O, with F contents up to 2 wt.%, and CO 2 contents up to 0.2 wt.%. Volatiles do not noticeably affect the heat capacity of glasses. The glass transition temperatures obtained from calorimetry and viscometry are in good agreement. Water has a strong viscosity-reducing effect on basaltic melts. F has a measurable viscosity-reducing effect in basaltic melts, but it is significantly smaller than that of water. The combined effects of H 2 O and F on viscosity appear to be additive on a wt.% basis. Both the effects of H 2 O and F on basaltic melts are smaller than those for more polymerized melts. Small quantities of CO 2 do not measurably affect basaltic melt viscosity, at least in the presence of > 1 wt.% water. Future viscosity models incorporating fluorine need to account for the compositional dependence of its effects on dry and hydrous melts.

In this study the effect of oxygen fugacity (fO2) on the oxidation state and coordination of Fe was investigated in different alkali trisilicate glasses (Rb2Si3O7 = RFS, K2Si3O7 = KFS; Na2Si3O7 = NFS; Li2Si3O7 = LFS) doped with ~ 5 wt.% of Fe2O3 with main focus on K- and Na-bearing compositions. Most of the experiments were conducted at ambient pressure in a gas mixing furnace at 1250 °C with controlled redox conditions (log fO2/bar: − 0.68 to − 16.18). The quenched glasses were analyzed using several methods. Analyses by a colorimetric wet chemistry method revealed a continuous increase in Fe2 +/Fetotal towards more reducing conditions without reaching 100% Fe2 + even at extremely reducing conditions (range of Fe2 +/Fetotal: from 0.08 in air to 0.93 in H2 atmosphere). X-ray absorption near edge structure (XANES) spectroscopy shows an increase of Fe coordination with decreasing ionic radius of the coexisting alkali, while the average coordination number seems to be independent on the oxidation state of iron aside from the largest studied alkali Rb, which seems to support lower coordinated Fe (tetrahedral) at more oxidizing conditions. The Fe2 +/Fetotal ratios inferred by XANES, using an intensity ratio based calibration of Wilke et al. (2004), are systematically higher by 10% compared to the wet chemistry results of this study, which may be due to the different external Fe2 +/Fetotal determination method (Mossbauer spectroscopy) used in their calibration. A new calibration curve based on wet chemistry and centroid positions is proposed for alkali silicate glasses. In optical spectroscopy, the position of the main Fe2 +-related peak shifts to lower wavenumbers with increasing ionic radius of the incorporated alkali and with increasing abundance of ferrous iron. Absorption coefficients eFe(II) and eFe(III) were calculated for the absorbance band at ~ 9000 and ~ 26,000 cm− 1, respectively. A decrease in eFe(II) was detected with decreasing ionic radius of the incorporated alkalis (eFe(II)KFS = 31.8 ± 2.6 L·mol− 1·cm− 1, eFe(II)NFS = 30.7 ± 2.3 L·mol− 1·cm− 1 and eFe(II)LFS = 23.6 ± 1.7 L·mol− 1·cm− 1). Finally, the results of this study are compared with recent models, which predict fO2 based on the knowledge of the Fe2 +/Fetotal ratio. All models overestimate Fe2 +/Fetotal in alkali silicate melts at very reducing conditions probably due to an unanticipated stabilization of Fe3 + by adjacent Fe2 +.

Glasses produced from decompression experiments conducted by Fiege et al. (2014a) were used to investigate the fractionation of sulfur isotopes between fluid and andesitic melt upon magma degassing. Starting materials were synthetic glasses with a composition close to a Krakatau dacitic andesite. The glasses contained 4.55–7.95 wt% H2O, ∼140 to 2700 ppm sulfur (S), and 0–1000 ppm chlorine (Cl). The experiments were carried out in internally heated pressure vessels (IHPV) at 1030 °C and oxygen fugacities (fO2) ranging from QFM+0.8 log units up to QFM+4.2 log units (QFM: quartz–fayalite–magnetite buffer). The decompression experiments were conducted by releasing pressure (P) continuously from ∼400 MPa to final P of 150, 100, 70 and 30 MPa. The decompression rate (r) ranged from 0.01 to 0.17 MPa/s. The samples were annealed for 0–72 h (annealing time, tA) at the final P and quenched rapidly from 1030 °C to room temperature (T). The decompression led to the formation of a S-bearing aqueous fluid phase due to the relatively large fluid–melt partitioning coefficients of S. Secondary ion mass spectrometry (SIMS) was used to determine the isotopic composition of the glasses before and after decompression. Mass balance calculations were applied to estimate the gas–melt S isotope fractionation factor αg-m. No detectable effect of r and tA on αg-m was observed. However, SIMS data revealed a remarkable increase of αg-m from ∼0.9985 ± 0.0007 at >QFM+3 to ∼1.0042 ± 0.0042 at ∼QFM+1. Noteworthy, the isotopic fractionation at reducing conditions was about an order of magnitude larger than predicted by previous works. Based on our experimental results and on previous findings for S speciation in fluid and silicate melt a new model predicting the effect of fO2 on αg-m (or Δ34Sg–m) in andesitic systems at 1030 °C is proposed. Our experimental results as well as our modeling are of high importance for the interpretation of S isotope signatures in natural samples (e.g., melt inclusions or volcanic gases).

The individual influences of F, B and P on viscosity of hydrous pegmatite forming melts have been determined experimentally. A starting glass composition (68.01 wt.% SiO 2 , 20.14 wt.% Al 2 O 3 , 7.73 wt.% Na 2 O and 4.26 wt.% K 2 O, Al / (Na + K) = 1.16) was doped with different amounts of F (up to 4.81 wt.%), B 2 O 3 (0.93 wt.%) and P 2 O 5 (up to 2.98 wt.%). The viscosity of melts containing 0.08 to 6.15 wt.% H 2 O was determined in the high and low viscosity range using the micropenetration technique and the falling sphere method, respectively. Falling sphere experiments were carried out at 200 to 650 MPa and 1173 to 1530 K. Micropenetration measurements were performed in the temperature range of 586 to 1124 K at ambient pressure. For all compositions a large decrease of viscosity upon hydration was observed, consistent with previous findings. The results also confirm that the viscosity decreases with the addition of F at all investigated temperatures. This decrease is more pronounced at low temperature and at low water content. According to our data, P and B do not play a major role on viscous flow in water-rich systems. However, the depolymerizing effect of H 2 O and F is not sufficient to explain very low viscosities of complex highly fractionated melts containing H 2 O, F, B, P and Li (Bartels et al., 2011). Thus, although we confirm that F is clearly a fluxing agent, Li must play a crucial role in lowering the viscosity of natural pegmatite forming melts and combined effects between different constituents need to be taken under consideration.