Oxygen Fugacity

Used to express the idealized partial pressure of a gas, in this case oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only, in a nonideal mixture. Oxygen fugacity (ƒO₂) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form.  , Earth’s magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava., an asteroid’s regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).  , etc.) and corrected for its nonideal behavior, like when at high pressure and/or temperature, or even when bound within a mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous.

One may think of oxygen fugacity as a way to determine the amount of oxygen required as a gas (O₂) under conditions where gaseous oxygen is not available like for example in a magma. In a magma, oxygen is available as a moleculeCollection of atoms held together by chemical bonds into a discrete, finite structure. One way molecules are represented is by a chemical formula where symbols for the elements are used to indicate the types of atoms present and subscripts are used to indicate the relative numbers of atoms. For example, like SiO₃, an oxygen ionAtom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more., and even perhaps as O₂ gas. Oxygen fugacity allows complex systems where oxygen is present in many forms to be described simply as an ideal gas thus simplifying discussions of chemical reactions.

Fugacity, as a general term, is a thermodynamic term for a substance’s potential to transfer from one environmental systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with to another. When the fugacity of chemical species (can be same or different) in two different systems are in unequal contact, the chemical species will be redistributed between the systems with net transfer of the chemical species from the higher fugacity system to the lower fugacity system. When the fugacities are equal, the net transfer is zero and equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static exists.¹

The concept of fugacity was introduced by Lewis in 1908 and later accepted after publication of the textbook written by Lewis and Randall in 1923. In 1957, Eugster was the first to introduce the concept of gas fugacity in petrologyScience dealing with the origin, history, occurrence, chemical composition, structure and classification of rocks.. He experimented with the externally controlled oxygen fugacity and observed the influence of the oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants potential on reactions giving rise to specific mineral assemblages, and designated the oxidation potential of oxygen as the partial oxygen pressure, p(O₂), later renamed as the oxygen fugacity by Eugster and Wones in 1962.¹