Chlorine-bearing basaltic glasses

6.2.1. Influence of chlorine on the viscosity of basaltic melts

The addition of 2.53 mol% chlorine to the present basaltic melt increases the viscosity due to a polymerisation of the structure. But the further addition of chlorine (>2.53 mol%) results in a decrease in viscosity by 0.5 log units (2.82 mol% chlorine) due to a depolymerisation of the network structure (figure 44). Webb et al. (2014) shows that the addition of 0.57 mol% Cl₂O_-1 (1.14 mol% Cl^-) and 1.41 mol% Cl₂O_-1 (2.82 mol% Cl^-) to a basaltic composition results in a decrease in viscosity by 0.5 log units (pink line). The first addition of 1.14 mol% Cl^- to X24 deviates from the present chlorine-bearing basaltic trend (blue line), whereas the further addition of 2.82 mol% Cl^- to X24 correlates with the present melt. The iron-free basaltic melt (HX24 - Webb et al. (2014)) follows the similar trend of the present basaltic melt (blue line) which indicates an effect of FeO_total content on the viscosity. The present basaltic melt has a lower FeOtotal content (~ 5 wt%), compared to the X24 (~ 10 wt%). Dingwell and Mysen (1985) investigated albite melts with a FeO_total content ~ 5 wt% and showed a small decrease in viscosity by the addition of chlorine (turquoise line). Therefore, the FeOtotal content is not the only effect on the viscosity. Furthermore, Baasner et al. (2013a) suggested that the addition of Al2O3 to silicate melts strongly increases the viscosity, but the Al2O3 content of the

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present basaltic melt similar to X24 (~9 mol%). Therefore, some other elements like Na₂O and CaO also have an effect on the viscosity. Webb et al. (2014) suggested the hypothesis that the presence of chlorine in basaltic melt increases the Fe²⁺/Fetotal, whereby the number of network-modifiers (Fe²⁺) increases (figure 45). This results in a decrease in viscosity due to a depolymerisation effect. Figure 45 shows that the addition of chlorine to the present basaltic melts does not result in an increase in Fe²⁺/Fe_total compared to X24 basalt.

Therefore, the suggestion from Webb et al. (2014) cannot be proven by the present data in figure 45. The different viscosity trends resulted of the diverse incorporation mechanisms of chlorine in the basaltic melts. For the exact determination, the chlorine environment has to be measure with NMR spectroscopy.

Figure 44: The change in viscosity due to addition of chlorine at the same temperature as the original melt has a viscosity of 10¹² Pa s. Present chlorine-bearing basaltic melt (blue circles) - Literature data: Baasner et al.

(2013a)- Na2O-CaO-Al2O3-SiO2 melts (NACS – grey open rectangles) and Na2O-CaO-SiO2 melts (NCS – grey close rectangles); Dingwell and Hess (1998)- albite melt (turquoise line); Webb et al. (2014) – basaltic melt (pink circles) and haplo-basaltic melt (pink rectangles). The error bars are smaller than the circles.

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Figure 45: Fe²⁺/Fetotal iron ratio as a function of chlorine for the present basaltic glass compared to X24 basaltic glass (Webb et al. 2014).

6.2.2. Influence of fluorine on the heat capacity of basaltic melts

Comparison of configurational heat capacity (C_p^conf)as function of chlorine content of diverse silicate melts is demonstrated in figure 46. The presence of 2.53 mol% Cl^- in the present basaltic melt results in a decrease in c_p^conf from 18.86 to 17.58 J mol^-1 K^-1, whereas the

further addition of chlorine (> 2.53 mol%) results in an increase in C_p^conf from 17.58 to 19.18 J mol^-1 K^-1. Webb et al. (2014) shows that the addition of 1.41 mol% Cl2O-1 (2.82 mol%

Cl^-) to a basaltic composition results in a decrease in C_p^conf from 20.66 to 18.35 J mol^-1 K^-1 (pink line). In comparison to the present basaltic melt, Webb et al. (2014) showed a similar trend of the iron-free basaltic melt (HX24 – open pink rectangles). Therefore, the higher C_p^conf values (X24) result by higher FeOtotal contents around ~ 10 wt%. Generally, the melt composition shows a larger effect on the C_p^conf than the FeO_total contents, which is shown by the data from Baasner et al. (2013a). The authors demonstrated the lowest Cpconf

values due to the simple melt composition. Furthermore, Baasner et al. (2013a) suggested that the aluminium effect on the c_p^conf can be neglected.

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Figure 46: Comparison of configurational heat capacity Cpconf

as function of chlorine content of silicate melts.

Literature data: Webb et al. (2014) – basaltic melts (pink open rectangles) and haplo-basaltic melts (pink close rectangles); Baasner et al. (2013a)- Na2O-CaO-Al2O3-SiO2 melts (NACS – grey open rectangles) and Na2 O-CaO-SiO2 melts (NCS – grey close rectangles).

6.2.3. Effect of chlorine on the structure of peralkaline melt

The addition of chlorine to peralkaline melts modifies the network structure due to two different structural behaviours. Based on diverse melting systems, the incorporation mechanisms of chlorine in peralkaline melts are discussed in the literature. Evans et al.

(2008) conducted XANES analysis and indicated that Cl^- prefers the bonding to divalent cations like Ca²⁺ and Mg²⁺ over monovalent ones such as Na⁺ and K⁺ in multi component alumina-silicate melts. Therefore, Baasner et al. (2013a) suggested that the increasing viscosity results in the formation of Na-Cl and Ca-Cl₂ complexes in peralkaline melts (NACS).

These new bondings result in a decrease in NBO and an increase in BO due to the alkali loss of the structure. The formation of complexes by the addition of > 2.53 mol% chlorine explains the increasing viscosity. The presence of < 2.53 mol% Cl^- results in a decrease in viscosity due to the replacement of Si-O-Si bonds by Si-Cl bonds (Giordano et al. 2004). The chlorine disrupts the network structure, which leads to a depolymerisation. Due to that, the presence of chlorine in the basaltic glass results in a decrease in density by 2.69 to 2.66 g cm^-³.

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The present glasses were analysed by Raman spectroscopy, whereby the structure of peralkaline melts can be discussed. The Raman spectra are sensitive for the variation of the iron speciation, which was caused by the addition of halogens to the present glasses. During the synthesis, the redox conditions are constant for all glasses at 1 atm. The results of the addition of halogens to the basaltic glasses are represented in figure 19. A difference is not visible between the halogens except for the iron speciation. In general the HF region shows an asymmetric peak near 935 cm^-1, which is enhanced by the addition of halogens accompanied by the iron speciation. This region indicates the interconnected three-dimensionally structure with Al and Si. The band near 935 cm^-1 could reflect the Si-Cl stretch band similar to the Si-F bonding (Mysen and Virgo 1985b). Furthermore, the LF region reflects an increase in intensity of a shoulder around 530 cm^-1 accompanied by the addition of halogens.

6.3. Comparison with recent models