Magma Redox Geochemistry. Группа авторов

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СКАЧАТЬ of redox exchanges in higher temperature geological systems, geoscientists turned their attention to molecular oxygen transfer among molecular components such as oxides or mineral‐like macromolecular entities.

      The practice between geoscientists becomes to assess criteria for fO₂ (or aO₂) estimations disconnected from the formal description of the acid–base character of magmas. In particular, techniques were established involving mineral phases coexisting in igneous rock to establish thermodynamic or empiric laws and trends from quenched glasses via indirect measurements, most often of spectroscopic nature (e.g., Neuville et al., 2020). This change of perspective reflects the obvious consideration that geoscientists deal with samples (solidified rocks) made accessible at Earth’s surface and which represent the final snapshots at the end of a long thermal and chemical evolution, whose a posteriori reconstruction is the objective of the geochemical (lato sensu) investigation.

      We may then say that for practical reasons geoscientists remained anchored to the original Lavoisier‐like definition of oxidation occurring in combustion processes, related to the exchange of oxygen molecules. The fact that most of the chemical analyses were from techniques in which oxygen was not directly determined but allowed to give oxides has also further favored these approaches.

      In this framework, a mutual exchange of knowledge has always characterized the field of geochemistry and petrology on one side and that of metal extraction in metallurgy in the other. Relations of the type

(1.45)

with v the charge (positive) of the cation of the metal M in the corresponding oxide. Reaction 1.45 is the main target of extractive metallurgy (see also Reaction 1.34), but also sketches the ensemble of processes that occurred since early Earth’s evolution to segregate the metallic core.

Ellingham diagrams (Ellingham 1944; Figure 1.5) are used in metallurgy to evaluate the ease of reduction of metal oxides, as well as chlorides, sulfides, and sulfates. The diagram shows the variation of the standard Gibbs free energy of formation, ΔG⁰, with temperature for selected oxides and is used to predict the equilibrium temperature for reactions of the type of Reaction 1.45 and particularly the oxygen fugacity under which ore will be reduced to its metal. The standard Gibbs energy change of formation of a compound (the Gibbs energy change when one mole of a compound is formed from elements at P = 1 bar) is given by:

      (1.46)

      with R the universal gas constant and A and B constants.

      To compare the relative stabilities of the various oxides, the Ellingham diagram is prepared for oxidation reactions involving one mole of oxygen. For the oxidation of a metal, ΔG⁰ represents the chemical affinity of the metal for oxygen. When the magnitude of ΔG⁰ is negative, the oxide phase is stable over the metal and oxygen gas. Furthermore, the more negative the value, the more stable the oxide is. The Ellingham diagram also indicates which element will reduce which metal oxide. The similarity between the electromotive force series (E⁰) and the Ellingham diagram, which rates the tendency of metals to oxidize, should be easily recognized.

      When both Me and Me^ν+_2/νO in Reaction 1.45 are in their standard states, the equilibrium constant, K₄₅, corresponding to this reaction can be expressed as:

(1.47)

where fO₂⁰ is the pure gas component gas fugacity at standard state (in this case 1 bar and T of interest). If, at any temperature, the acting oxygen fugacity is greater than the calculated value from Equation 1.47, spontaneous oxidation of metal M occurs, while oxide Me^ν+_2/νO_(s) decomposes to metal Me and gaseous oxygen at the oxygen partial pressure less than the equilibrium value. In other words, an element is unstable, and its oxide is stable at higher oxygen potentials than its ΔG_f⁰–T line on the Ellingham diagram. Therefore, the larger negative value for ΔG_f⁰ an oxide has, the more stable it is. In the Ellingham diagram of Figure 1.4, it can be seen, for example, that the reduction of Cr₂O₃ by carbon is possible (from the thermodynamic standpoint) at temperature above 1250°C and at each reported temperature by aluminum. It is worth noting that the Ellingham line for the formation of carbon monoxide (CO) has a negative slope, while those of all other oxides have positive slopes. As a result, at sufficiently high temperatures, carbon will reduce even the most stable oxides.

      

Figure 1.5 (a) Ellingham diagram for the main components of the melt/slag (solid lines) and possible reducing agents (dotted lines). The slopes of the lines representing ΔG_f⁰–T relations change at the temperature in which the phase transformations of reactants or products occur. Modified from Zhang et al. (2014). (b) Full Ellingham diagram for some relevant oxides including CO₂ equilibria and normographic scales for oxygen fugacity and related quantities via CO/CO₂ and H₂/H₂O ratios at P_tot = 1 bar. The scale of /_O ratio is used in the same manner as that of P_CO/P_CO2 ratio, except that the point H on the ordinate at T= 0 K is used instead of point C.

      Modified from Hasegawa (2014). All reactions’ components are considered in their pure stable phase t 1 bar and T of interest.

      Reaction 1.45 illustrates in fact how pairs of metals and their oxides, both having unitary activity, can be used as redox buffers, such that fO₂ values can be easily fixed at any temperature. Even in the presence of a third phase, such as silicate melts or any other liquid, gas–solid assemblages allow a straightforward application of Equation 1.47 to impose fO₂, unless solid phases are not refractory, and dissolve other components exchanges with the coexisting liquid. Reaction 1.1, involving metal iron and wustite, is a typical example (so‐called IW buffer) of one of these gas–solid equilibria fixing fO₂.

      In order to illustrate the effect of the fluid phase, refined versions of the Ellingham diagrams СКАЧАТЬ