Phase equilibria

The mineral assemblages in most igneous and metamorphic rocks preserve a record of the chemical equilibrium related to the initial rock-forming process. Phase equilibria studies attempt to determine quantitatively the pressure and temperature conditions of rock formations from these mineral assemblages.

Chemical Equilibrium

The traditional methods of studying rock bodies are descriptive in nature and involve mapping large-scale outcrop features in the field and detailed microscopic observations of rock textures and mineralogy in the laboratory. These methods, which are often successful by themselves, are supplemented by a second, more theoretical approach wherein rocks are treated as chemical systems and the principles of phase equilibria are applied to determine the conditions of their origin. A full and complete description of a rock body is still required, but that is no longer the main goal of petrologic study. The principles of phase equilibria are simply the laws that govern attainment of equilibrium of chemical reactions such as A + B = C + D, where A and B are known as reactants, and C and D are known as products.

Before exploring how these principles cast light on rock-forming processes, the concept of chemical equilibrium must first be developed. By analogy with gravitational potential, there must exist a similar tendency in chemical systems to lower their energy state through chemical reactions. Reasoning along these lines, J. Willard Gibbs introduced the term “chemical potential” to describe the flow of chemical components from one site (of high potential) to another (of lower potential) during reactions that lead a chemical system toward its lowest energy state. The total energy available to drive a chemical reaction must therefore be the sum of the chemical potentials of each component in the system multiplied by the number of moles of each component. The usual definition of chemical potential of a phase (or pure substance) is “the molar free energy” (or free energy per mole). This simple statement leads to a workable, three-part definition of chemical equilibrium, which is central to the understanding of phase equilibria.

First, if the chemical potential of a component is the same on either side of a reaction equation, the component can have no tendency to participate in the reaction. Second, in a multicomponent system consisting of several phases under uniform temperature (T) and pressure (P), equilibrium must prevail when the chemical potential of each component is the same in all phases in which the component is present. Third, the condition of equilibrium is one of maximum chemical stability. The second part of the definition is equivalent to saying that, for a given chemical reaction, the free energy of the reactants must equal the free energy of the products if a condition of chemical equilibrium prevails under fixed conditions of T and P. If either T or P changes, the system is no longer in equilibrium, but it will immediately adjust itself in such a way as to “moderate” the effect of the disturbing factors. The last statement is known as the moderation theorem, or Le Châtelier's principle. If the free energy on the product side of an equation is less than the free energy on the reactant side, the reaction will be spontaneous. If the opposite is true, no reaction is possible. Scientists are thus able to predict the result of any chemical reaction if the free energy of the reactants and products under the reaction conditions (T, P) is known.

Equilibrium State of a System

The most significant processes of rock formation—magmatism, metamorphism, and sedimentation—all involve large-scale flow of energy and movement of matter that produce an uneven distribution of chemical potential. Inevitably, the result must be chemical reactions tending to restore these natural systems to a state of equilibrium. The equilibrium state of a system is governed by its bulk composition (X), temperature (T), and pressure (P). For most geological processes, T and P change slowly relative to the rates of most chemical reactions, which means that most rock-forming reactions may be considered to take place under constant T and P and, if X also remains constant, most such reactions should easily attain chemical equilibrium.

In geology, the major concern is not so much with achievement of equilibrium but rather with preservation of equilibrium mineral assemblages through hundreds of millions of years, which must follow before deep-seated rocks are finally exposed at the surface. Rocks formed at depth must clearly undergo significant reductions in T and P prior to exposure at the surface, and there are several mechanisms that may induce changes in X during this lengthy period. Geologists are acutely aware of the implications of the moderation theorem: Retrograde metamorphism, mineralogical inversions and exsolutions, hydrothermal alteration, and weathering are but a few of the processes that could trigger reequilibration in rock bodies before they are exposed for study. Fortunately, microscopic studies coupled with Gibbs's pioneering work in phase equilibria provide the means to discern whether a given mineralogical assemblage preserves a former equilibrium.

Gibbs Phase Rule

The “phase rule,” also called the Gibbs phase rule, was initially derived by Gibbs in 1878 from the mathematical formulas of thermodynamics. The phase rule, which is fully applicable to all chemical systems, expresses the relationship between the governing variables T, P, and X, and the number of phases that may coexist in a state of equilibrium. Usually the phase rule is expressed in equation form as P + F = C + 2, where P = number of phases, C = number of chemical components, and F = degrees of freedom possessed by the system (normally T, P, X). Phases are chemically pure, physically separable subparts of the system and may be gases, liquids, or solids. In the formal sense, C is the minimum number of chemical entities needed to define completely the composition of each reactant and product phase in a given reaction.

Although the objective of phase rule applications is to determine F for major rock-forming reactions, a far simpler situation that could be experimentally verified in any high school laboratory or even in an ordinary kitchen may be considered. Pure water (H₂O) boils at T = 100 degrees Celsius at sea level (P = 1 bar; atmospheric pressure). The effect of dissolving common salt (NaCl) in water is to raise the boiling temperature approximately 0.8 degree Celsius for each mole percent of NaCl in the liquid phase. The steam given off by boiling is pure H₂O and, therefore, the salt concentration in the remaining liquid must progressively increase with temperature during boiling.

To apply the phase rule to this simple system, one first must tally up the participating phases: there is steam and there is liquid salt solution, and one must conclude that P = 2. Both pure water and salt are required to form these coexisting phases and, therefore, C = 2. The phase rule for this process (boiling), under conditions of fixed pressure (P = 1 bar), tells one that

F=C-P+2=2-2+2=2

Therefore, the system has two degrees of freedom and is said to be “divariant,” which means that, because one degree of freedom is utilized by fixing P = 1 bar, only one additional variable need be known to specify completely the state of the system. That may be either T (boiling temperature) or X (composition of the boiling solution); in other words, T and X are dependent variables at constant P. This T-X compositional dependence is easily determined for P = 1 bar by direct experiment. If the resulting T-X data were graphically plotted, the diagram would indicate, for example, that the boiling temperature for an 8 mole percent solution is close to 106 degrees Celsius. Conversely, if it were known only that the boiling temperature of a salt solution were 106 degrees Celsius, that would necessitate a solution concentration of 8 mole percent at P = 1 bar.

Application of the Phase Rule

To apply the phase rule to a reaction that has some geological significance, consider the appearance of diopside in siliceous dolomite during contact metamorphism by the reaction

CaMg(CO3)2(dolomite)+2SiO2(quartz)=CaMgSi2O6(diopside)+2CO2(gas)

The reaction involves three mineral phases and a fugitive gas phase, which is necessarily lost from the rock if diopside appears; P = 4. Note that the Ca:Mg (calcium-to-magnesium) ratio is the same in the reactant phase (dolomite) and the product phase (diopside); consequently, the minimum number of components needed to define the compositions of the four phases in the reaction is three. Therefore, C = 3. Substituting these values into the phrase rule, one obtains

F=C−P+2=3−4+2=1

and the reaction has one degree of freedom and is said to be univariant. For any given reaction pressure, the phases appearing in that reaction can coexist in equilibrium at one—and only one—temperature.

Univariant reactions are of great interest to petrologists because it is frequently possible to estimate the depth of a rock-forming reaction and, therefore, P, from field relationships. It is then a simple matter to estimate the reaction temperature from the P-T diagram for the univariant reaction involved. For example, suppose that field relationships lead to the conclusion that diopside-bearing contact metamorphic rocks formed by the reaction above at an estimated depth of 4 kilometers. The pressure equivalent of this depth is about 1,000 bars. The experimentally derived P-T diagram indicates a reaction temperature of about 450 degrees Celsius.

Mineralogical Phase Rule

The principles of phase equilibria were first applied to rocks in 1911 by Swiss geochemist V. M. Goldschmidt in his classic account of contact metamorphism in the Oslo area in Norway. Countless similar attempts have followed, with most authors concluding, as did Goldschmidt, that rocks, in general, record a state of chemical equilibrium governed by the temperature and pressure prevailing at their time of origin. The major generalization emerging from eighty years of such studies is that rocks with a large number of mineral phases tend to have a low number of degrees of freedom. Goldschmidt recognized that, at Oslo, metamorphism must be controlled mainly by T and P and that divariant equilibrium (F = 2) is the general case for isochemical rock-forming reactions. For F = 2, the phase rule reduces to P = C. Dubbed “the mineralogical phase rule,” this equation cannot be mathematically derived from thermodynamic principles. It simply reflects the common case in nature where rock-forming processes are approximately isochemical and their phase equilibria are controlled by both T and P operating as independent variables. It follows that rocks recording univariant equilibria are more restricted in occurrence than those recording divariant equilibria. Similarly, the rarity of rocks recording invariant equilibria (F = 0) can be understood, as the phase rule requires that both a unique T and a unique P be maintained during their formation. Rocks of complex mineralogy are occasionally encountered in which the number of phases, P, exceeds C + 2 and, consequently, the value of F is negative. Phase rule departures, indicated by negative F values, are a sure sign of disequilibrium and serve as a reminder that not all mineral assemblages can be treated by the methods of phase equilibria.

Petrogenetic Grids

The temperature and pressure range in which any mineral may exist is limited, and it is the task of experimental petrology to ascertain these limits for rock-forming minerals. The data resulting from this experimental work are utilized to construct “phase diagrams.” Such diagrams show the effects of changing P, T, X values on mineral stability fields and are, therefore, simply graphical expressions of the phase rule. In metamorphic petrology, the major use of phase equilibria data has been for construction of “petrogenetic grids.” A petrogenetic grid, initially conceived by Canadian petrologist N. L. Bowen in 1940, is a P-T diagram on which experimentally derived univariant reaction curves are plotted for a particular metamorphic rock type (for example, blueschists, marbles, calcsilicates, or pelites). The value of such “grids” lies in the fact that each natural equilibrium assemblage recognized in the field will fall within a definite P-T pigeonhole and thereby inform the field geologist immediately of the P-T conditions of the metamorphic terrane under study. This goal, so simple in concept, has proved elusive even after half a century of vigorous experimental, theoretical, and field effort. The problem lies in the lack of truly univariant reactions. For nearly seventy years, field geologists mapped isograds recording the “first appearance” of notable zone minerals such as biotite, garnet, staurolite, kyanite, and sillimanite under the impression that they represented the intersection of the ground surface with a plane of univariant equilibrium. Virtually all such “isograds” have proved to be the result of divariant equilibria and thus plot as a “band”—which may be rather wide—on a P-T diagram. This undesirable result has the effect of “smearing” grid boundaries and rendering them less useful as metamorphic indicators.

The general absence of univariant reactions in metamorphic rocks was eventually recognized because of theoretical and experimental advances in phase equilibria studies. The problem stems from the fact that most mineral phases participating in metamorphic reactions are solid solutions of variable composition and the most common reactions lead to the release of a fluid, the composition of which may vary with time. Each of these effects introduces an additional degree of freedom in phase rule terms, and, as a result, virtually all important reactions are divariant.

In spite of these difficulties, petrogenetic grids, based on divariant and quasi-univariant equilibria, have gradually evolved for all major metamorphic rock types. These are not the simple, quantitative grids envisioned by Bowen, but they do provide quick, reliable, and fairly narrow estimates of P-T conditions for common metamorphic mineral assemblages. Modern grids, continually subject to refinement, are phase equilibria's greatest contribution to metamorphic petrology.

Application to Igneous Petrology

In the area of igneous petrology, phase equilibria methods and data have become indispensable. Natural rocks, spanning the compositional spectrum, are melted under strictly controlled laboratory conditions to determine solidus and liquidus temperatures at pressures ranging from 1 to 35,000 bars. The results of such experiments place tight constraints on depths and temperatures of magma generation. They also permit the experimentalist to explore P-T effects on partial melting (anatexis) in terms of melt composition and refractory solid phases. The resulting phase diagrams, like metamorphic grids, permit petrologists to “see” deep into the crust and upper mantle and to test hypotheses dealing with the origin of magma.

For nearly a century, igneous petrologists have studied crystal-melt equilibria of simplified, synthetic melts as models for complex, natural magmas. The objective is to reduce the number of equilibrium phases by elimination of minor components of real magmas. Studies of this type were introduced by Bowen at the Geophysical Laboratory of the Carnegie Institution in Washington, D.C. Through its many subsequent researchers, this laboratory published hundreds of phase diagrams and earned a reputation for meticulous and exhaustive experimental work.

Phase equilibrium studies have provided a rather complete understanding of two fundamentally different modes of magma crystallization. Equilibrium crystallization occurs when P-T-X conditions change so slowly that chemical reactions within the melt are able to maintain the state of chemical equilibrium. Conversely, fractional crystallization results when changes in P-T-X conditions outpace the compensating reactions. This disequilibrium process greatly influences the behavior of natural magmas and extends the range of melt compositions that can be derived from a given parent magma. This latter type of behavior, recognized through the early phase equilibria studies of the Geophysical Laboratory, is the major factor in explaining the compositional diversity of igneous rocks.

The relatively simple phase diagrams of synthetic systems unraveled the complexities of sequential crystallization, cast light on the mechanics of crystal nucleation, and exposed the crucial role that water plays in magmatic processes. Collectively, these diagrams are the foundation of modern igneous petrology.

Goal of Phase Equilibria Studies

The refined symbolic notation and elegant mathematical derivations of thermodynamics are likely to remain unappreciated by the majority of laypersons and geologists alike. It is precisely these formalisms, however, that place phase equilibria on a quantitative footing and permit calculation of mineral stability fields from compositional data. Future development in the area of phase equilibria will follow this theoretical line.

The qualitative form of phase equilibria is expressed in phase diagrams rather than equations. Such diagrams have been a major part of petrology since the 1950's. Historically, emphasis in phase equilibria studies has been on high-temperature igneous and metamorphic rocks, which are most likely to preserve former equilibrium mineral assemblages. This preservation is the fundamental prerequisite for any application of phase equilibria methods. For this reason, the phase equilibria approach has generally not been applied to sedimentary rocks, except for saline deposits formed by intense evaporation of seawater and record chemical equilibrium.

The phase diagrams and sophisticated calculations utilized in phase equilibria studies are often imposing, but that merely reflects the compositional complexity of natural rocks and minerals. What must be appreciated is that the goal of such studies is both simple and practical: to determine how rocks form. All processes taking place on or within the earth (as well as all other solar system bodies) involve the flow of energy and mass. If scientists wish to advance beyond simply describing these processes—that is, to understand the chemical nature of the world—the phase equilibria approach must be employed.

Principal Terms

degree of freedom: the variance of a system; the least number of variables that must be fixed to define the state of a system in equilibrium, generally symbolized by F in the phase rule (P + F = C + 2), where P is the number of phases and C is the number of chemical components

equilibrium: the condition of a system at its lowest energy state compatible with the composition (X), temperature (T), and pressure (P) of the system; the smallest change in T, P, or X induces a state of disequilibrium that the system attempts to rectify

isochemical processes: processes that leave rock compositions unchanged; in thermodynamic terms, a system in which X remains constant even if T and P change

mole: the amount of pure substance that contains as many elementary units as there are atoms in 12 grams of the isotope carbon-12

phase: any part of a system—solid, liquid, or gaseous—that is physically distinct and mechanically separable from other parts of the system; a boundary surface separates adjacent phases

phase diagrams: graphical devices that show the stability limits of rocks or minerals in terms of the variables T, P, and X; the simplest and most widely used are P-T diagrams (X = constant) and T-X diagrams (P = constant)

system: any part of the universe (for example, a crystal, a given volume of rock, or an entire lithospheric plate) that is set aside for thermodynamic analysis; open systems permit energy and mass to enter and leave, while closed systems do not

thermodynamics: the science that treats transformations of heat into mechanical work and the flow of energy and mass from one system to another, based on the assumption that energy can neither be created nor destroyed (the first law of thermodynamics)

Bibliography

Aharonov, Einat. Solid-Fluid Interactions in Porous Media: Processes That Form Rocks. Woods Hole: Massachusetts Institute of Technology, 1996. Aharonov examines the processes involved in rock formation. This is a technical book at times, it but can be understood by the careful reader.

Angrist, Stanley W., and Loren G. Hepler. Order and Chaos: Laws of Energy and Entropy. New York: Basic Books, 1967. An elementary treatment of the basic thermodynamic concepts that lie behind phase equilibria studies. One of very few books that present this topic in nonmathematical terms. Aimed at high school readers.

Best, Myron G. Igneous and Metamorphic Petrology. 2d ed. Malden, MA: Blackwell Science, 2003. A widely used text for undergraduate geology majors. The treatment skillfully balances the traditional and phase equilibria approaches to petrology. Chapter 1 introduces basic thermodynamic concepts from the geological perspective. Chapters 8 and 14 summarize phase equilibria applications to igneous and metamorphic processes, respectively. For college-level readers with some background in geology and chemistry.

Blatt, Harvey, and Robert J. Tracy. Petrology: Igneous, Sedimentary, and Metamorphic. 3rd ed. New York: W. H. Freeman, 2005. Undergraduate text in elementary petrology for readers with some familiarity with minerals and chemistry. Thorough, readable discussion of most aspects of phase equilibria. Abundant illustrations and diagrams, good bibliography, and thorough indices.

Bowen, Norman L. The Evolution of the Igneous Rocks. Mineola, N. Y.: Dover, 1956. A reprint of the 1928 classic that first brought the phase equilibria approach to American geologists. Still valuable as an introduction to igneous processes, though some parts are dated. Suitable for college-level readers who have had a course in physical geology.

Bowen, Robert. Isotopes and Climates. London: Elsevier, 1991. Bowen examines the role of isotopes in geochemical phases and processes. This text does require some background in chemistry or the earth sciences but will provide some useful information about isotopes and geochemistry for someone without prior knowledge in those fields. Charts and diagrams help clarify difficult concepts.

Carey, Van P. Liquid Vapor Phase Change Phenomena. 2d ed. Flourence, Ky.: Taylor & Francis, 2007. A well-written text that thoroughly covers two phase flow. Easy to understand with some science background.

Ciccioli, Andrea, and Leslie Glasser. “Complexities of One-Component Phase Diagrams.” Journal of Chemical Education 88 (2011): 586-591. An easily accessible article for a reader with little chemistry background. Many figures and good descriptions complement the text. A vital read for anyone striving to learn more about phase change than what is provided in the average textbook.

Ehlers, Ernest G. The Interpretation of Geological Phase Diagrams. San Francisco: W. H. Freeman, 1972. Aptly titled, this book is a “must” for those interested in the practical side of phase equilibria. A well-indexed guide to most chemical systems relevant to igneous petrology. Theoretical aspects of the subject are omitted, except for a terse presentation of the phase rule in Chapter 1. For college-level readers with a knowledge of mineralogy.

Emsley, John. The Elements. 3rd ed. Oxford: Oxford University Press, 1998. Emsley discusses the properties of elements and minerals, as well as their distribution in the earth. Although some background in chemistry would be helpful, the book is easily understood by the high school student.

Fermi, Enrico. Thermodynamics. Mineola, N.Y.: Dover, 1956. A reprint of the 1937 work by one of the most prominent physicists of the twentieth century. Concepts are presented simply and without regard to applications. For college-level readers with some background in calculus; others may still find the text valuable as the emphasis on mathematical derivation is minimal compared with modern books on the subject.

Fyfe, W. S., F. J. Turner, and J. Verhoogan. Metamorphic Reactions and Metamorphic Facies. New York: Geological Society of America, 1958. This monumental book established the thermodynamic approach in metamorphic petrology, and its influence is seen in all subsequent texts on the subject. A major reference for any serious student of metamorphism. Will be found in all university libraries.

Greenwood, Norman Neill, and Allen Earnshaw. Chemistry of Elements. 2d ed. Oxford: Butterworth-Heinemann, 1997. An excellent resource for a complete description of the elements and their properties. The book is filled with charts and diagrams to illustrate chemical processes and concepts. Bibliography and index.

Grotzinger, John, et al. Understanding Earth. 5th ed. New York: W. H. Freeman, 2006. An excellent general text on all aspects of geology, including the formation of igneous and metamorphic rocks. Contains some discussion of the structure and composition of the common rock-forming minerals. The relationship of igneous and metamorphic petrology to the general principles that form the basis of modern plate tectonics theory is discussed. Suitable for advanced high school and college students.

Hillert, Mats. Phase Equilibria, Phase Diagrams and Phase Transformations. 2d ed. New York: Cambridge University Press, 2008. Extremely theoretical and technical. Suited for a graduate-level student. The writing is dense and exhaustive.

Powell, Roger. Equilibrium Thermodynamics in Petrology: An Introduction. New York: Harper & Row, 1978. This text, as well as many others of recent vintage, treats phase equilibria as a problem-solving methodology rather than as a body of knowledge derived through field and laboratory work. Written for advanced students of geology, but Chapters 1 and 2 are suitable for general readers seeking an introduction to equilibrium concepts and phase diagrams.

Van Ness, H. C. Understanding Thermodynamics. Dover Publications, 1983. Discusses the abstract properties of thermodynamics, making them more understandable. A short book of only 100 pages that covers topics quickly.