of oxygen (bridging, non‐bridging, and so‐called free oxygen) whose mixing determines the polymerization and the thermodynamic properties of the melt mixture as a function of composition (Toop and Samis, 1962a,b; Allanore, 2013, 2015 and references therein; Moretti, 2020 and references therein). Nevertheless, silicate melts still lack a fully developed acid–base framework formalizing the thermodynamic properties of reactive species formed during the solvolysis, as the solvent itself changes its polymerization properties upon introduction of other oxide components, which are highly soluble contrary to what observed for salts in aqueous solutions. The most general thermodynamic approaches postulate mineral‐like molecular structures to interpolate existing data.
In molten silicates the electric charge is primarily transported by cations, whose contribution increases with concentration of network modifiers, hence basicity. The major element, oxygen in its three forms and particularly O2– and O‐based anionic complexes do not contribute substantially to the charge transport (Dickson and Dismukes, 1962; Dancy and Derge, 1966; Cook and Cooper, 1990, 2000; Cooper et al., 1996a,b; Magnien et al., 2006, 2008; Cochain et al., 2012, 2013; Le Losq et al. 2020). This is a striking difference when comparing silicate melts to previously described electrolytes, particularly with the aqueous electrolytes, in which the concentration of hydroxide ions or water is always large enough to sustain high current densities (Allanore, 2013). From a practical standpoint the anode reaction producing oxygen (the inverse of Reaction 1.6) is strongly impacted by the low transport of free oxide ions, that is free oxygen, which is at very low concentration. Besides, the anode design and technical performance would greatly benefit of the precise knowledge of oxygen physical chemistry in silicate melts, which for the moment is still too limited to narrow compositional ranges that have been investigated spectroscopically (Allanore, 2015).
Nevertheless, because of their nature, silicate melts can dissolve important amounts of metals. Besides, they exhibit a large range of thermal stability, with high temperature conditions that favor fast kinetics of redox exchanges. Upper temperature limits for electrochemical applications are given by the formation of gaseous silicon monoxide or by alkali oxide thermal decomposition in alkaline systems or by high vapor pressure for Mn‐bearing systems (Allanore, 2015). In terms of transport properties, silicate melts are a solvent with high viscosity, a fact that is however compensated by the high diffusivity of the metal cation (Allanore, 2013), i.e., the cathode reactant.
Many measurements have however been carried out on melts of geological interest, also fostered by the interest in silicate electrolysis to produce on site metals and particularly molecular oxygen for terraformation of extraterrestrial planets (e.g., Haskin et al., 1992). Electrochemical series were then established in binary SiO2‐MO systems (e.g., Schreiber, 1987) but also in ternary joins such as the diopside one (Semkow and Haskin 1985; Colson et al., 1990) for redox exchanges of the type:
(1.39)
in which half‐reactions of the type
(1.40)
combine with Half‐reaction 1.7. Nevertheless, such series do not consider the effect of the solvent, the melt and its structure, in determining the speciation state (e.g., anionic or cationic) following the definition at Reaction 1.27. The effect of the solvent also includes the amphoteric behaviour of some dissolved oxides such as Fe2O3 or Eu2O3, which can behave either as acids, yielding FeO2– (i.e., FeO45– tetrahedral units) and EuO2– (i.e., EuO45–) or bases, yielding Fe3+ and Eu3+ cations (Fraser, 1975; Ottonello et al., 2001; Moretti, 2005; Le Losq et al., 2020). The multiple speciation behaviours determined by pO2– can be summarized by the following reaction mechanism (e.g., Moretti, 2005; Pinet et al., 2006):
Predominance and stability diagrams (e.g., E‐pH, E‐pO2–, E‐logfO2, or logfO2 vs. the log‐fugacity of pH or other gaseous species in the system such as SO2, CO2) depend on the availability of good thermodynamic data and especially a well‐established testament of acid‐base properties of the investigated system and its solvent(s). For silicate melts and glasses, such a testament is represented by the oxobasicity scale from the Lux definition (Reaction 1.27). Electrochemical experiments should then be envisioned to complete and validate the database in order to ensure predictions about forming species and measure their activities.
1.2. OXYGEN FUGACITY: THE CENTRALITY OF AN ELUSIVE PARAMETER
Voltage E and oxygen fugacity (fO2) are both measures of oxidation state. The relation between fO2 and E for a given electrolytic medium can be established by the anode reaction where oxygen is produced. In the case of aqueous solutions, conversion is provided by half‐reaction 1.15 and Equation 1.18. We can then replace E‐pH diagrams with analogous logfO2‐pH diagrams. In this treatment, the actual speciation state of solutions is still the key to investigate the system, but half‐reactions are not considered, and the equilibrium values of overall reactions are used, same as for activity plots. As for E‐pH plots, boundaries will shift by varying the total amount of soluble elements in the electrolytic solution, hence the activity of dissolved ionic species or the corresponding gas fugacity (e.g. when carbonates or sulfides and sulfates are present).
Figure 1.4 shows that in logfO2‐pH diagrams phase, boundaries that in E‐pH diagrams were dependent on pH only (and not on the concentration of dissolved ions) are horizontal. Moreover, a quick comparison between figures 2 (Eh‐pH diagram) and 4 (logfO2‐pH diagram) also shows that on the logfO2 vs. pH representation the pyrite–magnetite boundary, which appears only for pH > 7, maintains the positive slope. At the T conditions of Figure 1.4 this boundary is represented by the overall reaction:
(1.42)
Besides, at 145°C (water saturated conditions) the pyrite–pyrrhotine boundary is defined at pH > 7 and is positive because:
(1.43)
whereas for pH < 7 the boundary is horizontal, given by the proton‐free reaction:
As we have already seen, when considering high‐temperature non‐aqueous (oxide) systems in the inner Earth geospheres, there is no acid–base framework and anchoring fO2 or E to pH makes no sense in absence of the solvent liquid water.
To
