Phase changes

Phase changes among liquids, solids, or gases are important in many geologic processes. The formation of ice from water, of minerals from magma, of gases bubbling out of magma, and of halite (sodium chloride) precipitating out of a lake are examples of phase changes in nature. Many of these phase changes aid in the understanding of deposits that are of economic importance.

Polymorphs

A phase is a physically distinct and mechanically separable portion of a mixture. Different phases in a mixture may be of differing chemical composition, or they may be identical in composition. Thus, the rock, granite, containing the minerals quartz, feldspar, and mica, is made up of three physically distinct components or three phases. Also, water, ice, and vapor are three distinct phases even though they share the same chemical composition.

Three different minerals in a rock—for example, quartz, plagioclase, and alkali feldspar—constitute three separate phases. The number of separate mineral grains is not the same, however, as the number of phases. There might be 231 grains of quartz, 257 grains of alkali feldspar, and 199 grains of plagioclase in a given rock, but the rock does not contain this many different phases. Instead, there are only three phases in the rock, corresponding to the three different minerals with the same composition and physical properties. Minerals of the same composition with different crystal structures are called polymorphs. Calcite and aragonite are examples of polymorphs of calcium carbonate. Water, ice, and water vapor are polymorphs of dihydrogen oxide, and the spectacularly diverse minerals graphite and diamond are polymorphs of carbon. Ice may exist as several different phases depending upon the temperature and pressure.

Liquid Phases

There are also many liquid phases. Water is a liquid with the same composition as ice. Ice cubes and the water in which they float may be considered as two separate phases, since the ice and water have different physical properties (ice is lighter or less dense than water, for example) and are separated by boundaries. Melted rocks form liquid rock material called magma. The magma may be considered one phase, while any minerals suspended in it are considered separate phases.

Two or more liquids may coexist as separate phases if they do not mix. Oil and water form separate layers with a boundary between them, so they are separate phases. Being less dense than the water, the oil floats on top of the water. Similarly, carbonate-rich magmas may not mix with many silicate-rich magmas, and they may form separate liquid phases. Water and ethyl alcohol, in contrast, mix in all proportions and thus form only one homogeneous phase with no boundary surfaces. In a similar fashion, two silicate magmas of somewhat different composition may mix and form a homogeneous magma of an intermediate composition.

Ice changes to water at 0 degrees Celsius; at one atmosphere pressure and 100 degrees Celsius, water changes to steam. Such phase changes may differ with changes in atmospheric pressure. At about 200 times atmospheric pressure, the boiling point of water is more than 300 degrees Celsius, and the freezing point is less than 0. At a pressure of less than 0.006 atmosphere, liquid water is not stable; rather, ice changes directly to water vapor at less than 0 degrees Celsius without any intervening water phase. There is even one temperature (0.1 degree Celsius) and pressure (0.006 atmosphere), called the triple point, at which ice, water, and steam coexist.

The phase relations of water have direct application to understanding the formation of certain features on Mars. Some features appear to have been formed by a running fluid such as water. The atmospheric pressure of Mars is currently too low for the planet to have any running water. Billions of years ago, however, Mars's atmospheric pressure might have been high enough to permit stabilized water to exist there. Thus, water could have been an erosional agent on Mars early in that planet's history.

Solid and Gas Mixing

Though it may seem difficult to visualize, some solids of different composition may be able to mix partially or completely in all proportions. The silicate mineral olivine, for example, can accommodate any ratio of magnesium to iron into its composition; the magnesium end member is said to have a complete solid solution with the iron end member. Gases, in contrast to solids and liquids, mix in all proportions. The earth's atmosphere, for example, is a fairly homogeneous mixture of nitrogen (the predominant gas) and oxygen. There are also small amounts of other gases, such as water vapor and carbon dioxide.

Phase Changes in Metamorphic Rocks

Important phase changes occur among solids in metamorphic rocks. Metamorphic rocks were formed from other rocks by chemical reactions in the solid state because of differing temperatures and pressures. The minerals kyanite, andalusite, and sillimanite are different aluminum silicate minerals with the same composition occurring in metamorphic rocks. Phase changes among these three solids depend on temperature and pressure, as in the ice-water-steam system. No liquid or gas, however, is involved in the aluminum silicate minerals. The triple point of the aluminum silicate minerals is at about 600 degrees Celsius and nearly 6,000 times atmospheric pressure (6 kilobars), so changes among these minerals take place only deep within the earth. Sillimanite is stable from about 600 degrees Celsius and 5-6 kilobars up to more than 800 degrees and 1-11 kilobars. In contrast, andalusite is stable at pressures up to only 6 kilobars over a wide range of temperatures (200 to 800 degrees). Kyanite is also stable over a wide temperature range but at a higher pressure for a given temperature than is the case for either andalusite or sillimanite. For a geologist, then, knowing which of these aluminum silicate minerals is present in a rock helps to show the range of temperature and pressure at which the rock formed. There are also solid-to-liquid phase changes in sedimentary systems. A variety of minerals may crystallize or precipitate from water to form sediments.

Phase Changes in Igneous Rocks

In igneous rocks, too, there are many phase changes between solids and liquids. Igneous rocks form from the crystallization of minerals from magma or melted rock, usually of silicate composition. As magma slowly cools within the earth, it forms minerals that gradually either sink or float in the magma, depending on their density (weight in relation to volume). The minerals that are heavier or denser than the magma gradually sink, and the lighter or less-dense minerals gradually float upward. The magma composition gradually changes as the minerals are extracted, because the minerals' compositions are different from that of the magma. Magma forms by the melting of solid rock in the lower crust or upper mantle of the earth. The magma's composition will depend on the composition of the rock melted, the pressure, and the degree of melting. Also, the magma composition will be different from that of the solid. The melting of a typical rock in the upper mantle, for example, will produce a basaltic magma. A basaltic magma will produce a dark, fine-grained rock of low silica content, called basalt, when extruded at the surface of the earth. The melting of a silica-rich rock in the continental crust is more likely to produce magma with a high silica content. These high-silica magmas will crystallize to light-colored rocks called dacite or rhyolite when extruded at the surface.

Saturation Points

The maximum amount of a mineral that may be dissolved in water is called its saturation point. Different minerals have different saturation points in water. Considerably less calcite (calcium carbonate mineral) may dissolve in water than gypsum (calcium sulfate mineral). Even more halite (sodium chloride) may dissolve in water than gypsum. If the saturation points for these minerals are exceeded, the minerals will begin to crystallize or precipitate and sink to the bottom of the water. Saturation points of minerals may be exceeded when water evaporates or when the temperature changes. If seawater is present in a bay in which evaporation exceeds the influx of new seawater, calcite, gypsum, and halite may precipitate, in that order, as the water gradually evaporates. Vast amounts of salt deposits of halite and gypsum are believed to have formed in this fashion during the geologic period called the Permian (about 250 million years ago) in Kansas and Oklahoma. Such salt deposits are not nearly as common as are limestones. Limestones are sedimentary rocks composed of mostly calcite. The calcium carbonate is believed to have precipitated in warm, shallow seas either by inorganic precipitation or by organisms forming calcite or aragonite. The precipitation of calcite or aragonite is aided by the evaporation of seawater in shallow seas and by warming of the water.

Experiments at Atmospheric Pressure and Temperature

A variety of techniques are used to study phase changes. The technique selected to study phase changes depends on the pressure, temperature, and types of phases. The easiest phase changes to study are those involving precipitation of minerals from water solutions at atmospheric pressure and temperature. One of the intriguing problems in the study of sedimentary rocks, for example, is why among ancient rocks so much limestone that is made up of calcite and dolostone is composed of dolomite (a calcium/magnesium carbonate mineral), as modern sediments seem to be forming mostly aragonite and calcite. Little dolomite is apparently forming today. Experiments in the laboratory have helped geologists to explain such observations. The precipitation of calcite and aragonite in the laboratory is temperature-dependent. A temperature of about 35 degrees Celsius, for example, favors precipitation of needlelike crystals of aragonite. In contrast, a lower temperature of 20 degrees favors precipitation of mostly stubbier crystals of calcite.

To identify the minerals, scientists observe them under a microscope or by X-ray diffraction. X-ray wavelengths and the distances between atoms in the minerals are about the same, so the X-rays will be reflected off planes of atoms in the mineral. The angle of reflection depends on the distance between the atoms and the wavelength of the X-rays. Since every mineral has different spacings between atoms, the reflections of different minerals have different angles and serve as “fingerprints” for the minerals. Thus, calcite and aragonite can easily be distinguished. However, it is difficult to produce dolomite under any conditions in the laboratory. Such laboratory observations are consistent with the observed abundance of aragonite needles in warm, shallow seas and with the greater abundance of calcite forming from cooler waters. They are also consistent with the lack of observed dolomite formation.

The only dolomite that can form in the laboratory is produced by the conversion of calcite to dolomite in contact with concentrated waters with a high magnesium-to-calcium ratio. Thus, only under special geologic conditions below the land surface will calcite convert—slowly—into dolomite. Waters high in magnesium moving through calcite-rich rocks below the surface will convert the calcite into dolomite over long periods of time. This process may be occurring in certain places presently, though it simply cannot be observed.

Experiments at Higher Temperatures and Pressures

Other experiments in furnaces at high pressure tell geologists that aragonite is in reality stable only at a pressure much higher than atmospheric pressure. Aragonite is unstable at atmospheric pressure, so any aragonite forming presently should slowly revert to the more stable calcite with time. This fact explains why there is no aragonite in ancient rocks.

Experiments involving phase changes at higher pressures and temperatures are more difficult to carry out because of the problem of controlling and measuring the temperature and pressure. In some experiments, a cylindrical container or hydrothermal vessel composed of a special steel alloy is hollowed out in the center so that a sample container may be placed inside of it. The sample container is composed of pure gold or platinum and is sealed at one end. The sample and some water are placed in the container, and the other end is sealed. The container is placed inside the hydrothermal vessel, and water is pumped into the container and heated to the desired temperature. As the temperature gradually rises, the water vapor must periodically be released so that the pressure does not rise too high and rupture the hydrothermal vessel. The gold or platinum container distorts easily and transmits the pressure to the sample inside the container. After the experiment has continued for the desired length of time, the container is suddenly cooled so that the sample is frozen in the state it had reached at the higher temperature and pressure.

Suppose the experimenter is studying the melting of rocks. After the sudden cooling, he may find that some of the sample is glass with embedded crystals of one or more minerals. Presumably the glass represents liquid that was quickly “frozen.” The minerals may be identified by observing them under a microscope or by using X-ray diffraction. The exact mineral and glass composition may be determined through the use of an electron microprobe. In this technique, a narrow electron beam is focused on a part of the material to be analyzed, causing electrons of various elements to be removed from the atoms. Other electrons take the places of the removed electrons. X-rays of certain specific energies are then emitted; because this radiation is characteristic of a given element, that element may now be identified. The number of gamma rays or their intensity depends on the amount of the element in the sample; thus, the concentration of the element may be determined.

The mineral and glass composition at a series of temperatures and pressures may be determined during the gradual solidification of a silicate liquid—for example, to understand how the crystallization of the minerals may change the composition of the liquid. These liquid changes in the experiment may then be related to the changing composition of a series of natural lavas to see whether they might have formed by a similar process.

Occurrence in Familiar Processes

Phase changes occur in familiar processes every day. The phase changes from ice to water to water vapor are familiar to most people. Ice is less dense than water, so the ice takes up more space than does the original water. A drink placed in a freezer may explode as a result of this phase change. This effect is avoided in automobile cooling systems when ethylene glycol is mixed with the water; the freezing point of this mixture is much lower than that of water. Ice floats in water because of its lower density. What if ice were denser than water? Then ice would surely sink to the bottom of lakes and oceans, and it might remain there the year round. Profoundly different oceanic, lake, and atmospheric circulation and very different climates and ecosystems would be the result. In fact, marine life would not exist in colder climates as lakes would freeze completely with no layer of ice at the top for insulation.

People cook with boiling water. It is generally known that the boiling point of water decreases with increased elevation as the pressure is reduced. Cooking time must thus be increased to compensate for this lowered boiling temperature at higher elevations. Alternately, salt could be added to the water to raise the boiling point.

Water vapor in the atmosphere can increase only up to a certain maximum point, called the saturation point. This saturation point varies with temperature. More water vapor may be contained in warmer air. Rainfall results when warm, saturated air rises and cools. The cooler air cannot hold as much moisture as can warmer air, so rain falls.

Role in Understanding Geological Processes

An understanding of phase changes is essential for an understanding of geological processes. The concentration of elements in geologic systems involves one or more phase changes. The so-called fractional crystallization process, for example, involves the precipitation of minerals from a slowly cooling magma. The minerals either sink or rise in the magma, depending on whether they are heavier or lighter than the same volume of the magma. Some elements are more concentrated in the minerals than in the magma; others are more concentrated in the magma than in the minerals. Some elements may boil out of magma with water vapor and become concentrated in hydrothermal deposits. Common table salt (sodium chloride) forms vast deposits where large, saline bodies of water evaporated slowly over long periods of geologic time, much as the Great Salt Lake in Utah is doing today. Animal matter may slowly change to petroleum or natural gas when buried gradually below the surface of the earth. As large swamps are gradually buried, they may be transformed into coal. Buried deep, some of this material may change to graphite (all carbon). The mineral diamond (also all carbon) may have existed as graphite before it was transformed to diamond at even greater pressure within the earth.

Quartz (silica or SiO₂) is a common mineral at the earth's surface. The silica polymorphs trydimite and cristobalite are found in volcanic rocks formed at high temperatures. Under extreme pressure, quartz is converted to high-density polymorphs known as stishovite and coesite, also called shocked quartz. The only known occurrence of pressure high enough to cause this change is the impact of meteors on the planetary surfaces. The presence of coesite is used as evidence that various suspect structures were caused by meteor impact. Thus, the different polymorphs of quartz provide evidence concerning the origin of the rock in which it is found.

Principal Terms

gas: a substance that can spontaneously fill its own container

igneous rock: a rock formed from molten rock material (magma or lava)

liquid: a substance that flows

magma: a liquid, usually composed of silicate material and suspended mineral crystals, that occurs below the earth's surface

metamorphic rock: a rock in which the minerals have formed in the solid state as a result of changing temperature or pressure

phase: that part of nature that has a more or less definite composition and thus homogeneous physical properties, with a definite boundary that separates it from other phases

precipitate: the process in which minerals form from water or magma and settle out of the liquid

sedimentary rock: a rock that has formed from the accumulation of sediment from water or air; the sediment might be fragments of rocks, minerals, organisms, or products of chemical reactions

solid: a substance that does not flow and has a definite shape

Bibliography

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.

Brownlow, Arthur H. Geochemistry. Englewood Cliffs, N.J.: Prentice-Hall, 1995. A variety of phase changes are discussed in this introductory text in geochemistry. Suitable for a college student who has taken introductory courses in geology and chemistry. Many illustrations.

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. An excellent and very detailed discussion of the principles used to interpret geological phase diagrams in geology. Suitable for college-level students with basic knowledge of mineralogy and chemistry.

Ernst, W. G. Earth Materials. Englewood Cliffs, N.J.: Prentice-Hall, 1969. This book is part of a series which supplements introductory textbooks in geology. It discusses mineralogy, igneous rocks, sedimentary rocks, and metamorphic rocks in more detail than do most introductory textbooks. Features good treatments of phase changes and phase diagrams in all the rock types. Accessible to the college-level student who has studied general geology.

‗‗‗‗‗‗‗‗‗‗. Petrologic Phase Equilibria. San Francisco: W. H. Freeman, 1976. A detailed treatment of phase diagrams in geologic processes. Appropriate for college students with background in chemistry and mineralogy. Illustrated.

Hamblin, William K., and Eric H. Christiansen. Earth's Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2003. This geology textbook offers an integrated view of the earth's interior not common in books of this type. The text is well organized into four easily accessible parts. The illustrations, diagrams, and charts are superb. Includes a glossary and laboratory guide. Suitable for high school readers.

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.

Krauskopf, Konrad B. Introduction to Geochemistry. 3rd ed. New York: McGraw-Hill, 2003. In this well-written introductory geochemistry text, a variety of phase changes are discussed in detail. College students who have taken geology and chemistry courses will find it helpful. Contains many figures.

Mason, Brian, and Carleton B. Moore. Principles of Geochemistry. 2d ed. New York: John Wiley & Sons, 1982. An introductory college-level text in geochemistry. Includes some discussion of phase changes. Illustrated with many figures.

Thompson, Graham R. An Introduction to Physical Geology. Fort Worth: Saunders College Publishing, 1998. This college text provides an easy-to-follow look at physical geology. Thompson walks the reader through each phase of the earth's geochemical processes. Illustrations, diagrams, and bibliography included.