Crystal Chemistry:Past, Present, and Futurer

American Mineralogist, Volume 70, pages 443454, 1985 Crystal chemistry:past, present, and futurer CrHRrns T. Pnrwrrr Departmentof Earth and SpaceSciences StateUniuersity of New York StonyBrook, New York 11794 Abstract Crystal chemistry is an important part of the scienceof mineralogy and describesthe relationships of mineral crystal structures with the correspondingphysical and chemical properties.Crystal chemistryhas evolved over a period of fifty years or more from a purely qualitative endeavorto one that is quantitative and that can provide essentialinsight to the behavior of minerals under varying conditions of pressure,temperature, and chemicalenvi- ronment.Increased interest in the chemicaland physicalproperties of mineralsand how these are related to important problemsin the geologicaland materialssciences makes the future look verypromising. Introduction and technologyas well. The subject of this paper, crystal chemistry,embodies aspects of all three areasin Figure 1 Role of crystal chemistry and, therefore,should be consideredto lie in the overlap The study of crystal chemistry is of fundamental impor- regionamong the differentcircles. tance to the science ofmineralogy, as well as to other fields Evans (1952)defined crystal chemistry as "the study of such as materials science and solid-state chemistry. How- the relationship of the internal structure of a body to its ever, it is my perception that mineralogy itself means differ- physical and chemical properties.It aims at interpreting ent things to different people, as do crystallography and the propertiesof any substancein terms of its crystal struc- petrology, disciplines that are also considered to be oflicial ture, and, conversely,at associatingwith any structural interests of the Mineralogical Society of America and the characteristica correspondingset of physicaland chemical Society often regulates its activities according to these cate- properties.Ideally crystal chemistry should enable us to gories. For example, awards are given each year to prom- predict and synthesizechemical compounds having any de- ising young scientists,but alternately to crystallographers siredcombination of propertieswhatsoever." one year and mineralogists/petrologists the next. I believe The early developmentof crystal chemistrywas the pri- that these labels are not really representative of the way in mary result of experimentsand interpretation of W. L. which we apportion our scientific efforts and that another Bragg and coworkers at Manchesterin the 1920'sand by kind of division is more realistic. This division is illustrated V. M. Goldschmidtand coworkersin Oslo in the 1930's. in Figure I and is characterized by three major parts, de- Their efforts were characterizedby an empirical approach scriptive mineralogy, mineralogical applications, and phys- to the understandingof crystal structure through simple ics and chemistry of minerals. Descriptive mineralogy, in- models basedon observedinteratomic distances.Later in volving identification and classification of minerals is really the 1930's,Pauling showedthat it was important to ac- classical mineralogy and describes virtually all mineral- count for chemicalvalence and introduced the conceptof ogical activity before the 1930's,and still is of great interest electronegativityto help account for varying degreesof to many scientists.Since the 1930's,applications of miner- covalencein chemical bonds. Subsequently,the approach alogical theory and experiment have become more impor- to crystal chemistry has developedalong essentiallytwo tant in the understanding of and application to petrology, tracks, one still largely empirical and based on observa- geochemistry, and even geophysics. This constitutes the tions of interatomic distancesand incorporating relatively second division in Figure I and is also the one in which simple ideas about valence,coordination, and electronic most people are involved today. The third category, the structure.The secondapproach is one basedon quantum study of physical and chemical properties of minerals, is mechanicsand has been used mostly for interpretation of one that has seen substantial growth in recent years and spectraor for treatment of molecular structuresor hypo- one that is becoming increasingly important, not only for thetical clustersthat simulate configurationsfound in real the earth sciences, but for many other aspects of science minerals. A major problem for crystal chemistsinterested in min- I Adapted from Presidential Address at the annual meeting of erals is that most of the important mineral structuresare the Mineralogical Society of America, November 6, 1984, Reno, relatively complex and are composedof infinite periodic Nevada. arraysof individualions rather than of molecules.Thus, it 0003-{)o4xl85/050@43$02.00 443 PREWITT: CRYSTAL CHEMISTRY: PAST, PRESEN? AND FUTURE Principles of crystal chemistry Empirical crystal chemistry is based on the concept of Oescriplrve periodic structures containing ions of specific size, charge, Minerology and cobrdination number, with one or two other parame- ters such as electronegativity helping to account for cova- lence or metal-metal bonding effects. The coordination groups are combined with each other, resulting in an over- all symmetry that can be defined through the unit cell and A ppI icotion Physicol8 space group. If the coordinations of the cations by anions ChemicoI are defined, then the coordinations ofanions by cations are fixed, as illustrated by the relation for compound PAoaBo *X., ,p:pp+ee PHYSICALA whereP and Q are the CN's of the cationsand R is the CN DESCRIPTIVE CHEMICAL of the anion. The size of an ion is an arbitrary value de- APPLICAT IO N MI NERALOGY PROPERTIES pending on what fraction of the spacingsbetween the ion Pelrology Identificolion Theory and its neighborsis allocatedto the central ion. In mineral- Geochemi slry Clossificoiion Modeling ogical crystal chemistry, it is most common to use the Geophysi c s Experimenl "hard sphere"model where eachion is consideredto be a Fig. 1. Circles representing the fields of mineralogy. Crystal sphereof a specificsize, although molecularchemists often chemistry falls in the area of overlap among the circles. assigna bonding radius and a van der Waal's radius to representa non-sphericalion. has been diflicult to apply the quantum mechanical theory The chargeassigned to ions is usually the formal valence, developed by physicists for simple cubic structures or although even high school chemistry studentsknow that quantum chemistry that has been applied to molecules by most bonds have a significantdegree of covalenceand the chemists. Because of this, the empirical approach of Bragg, absolutevalue of the effectivevalence in real crystalsis less Goldschmidt, and Pauling is still being used by many in- than the formal valence.Various schemeshave been pro- vestigators although there has been substantial progress posed for specifyingthe actual charge on an ion or the recently in applying quantum mechanical techniques to degree of ionic character of a bond. Probably the most minerals. A good summary of the latter was made by G. V. well-known of theseschemes is that of Pauling (1960)who Gibbs (1982) in his MSA Presidential Address and today related the degreeof ionicity with the electronegativitydif- Gibbs, J. A. Tossell, A. Lasaga, their students, and others ferencebetween the two atoms involved. For example,his are making good progress toward combining the cluster relation calculations with considerations of long-range forces in :23(xt - crystals. A(Si-O) xa)2 My purpose is to discuss some of the basic concepts of indicatesthat the Si*O bond is about 50% i<rnic.Even so, the empirical approach to crystal chemistry, to show how it is surprising that the ionic concept works as well as it this is being applied to problems in mineralogy and materi- does.Burdett and Mclarnan (1984)emphasized this point als science,and to make observations about how the work in a reinterpretation of Pauling's rules based on band of crystal chemists will evolve during the rest of the 1980,s. structure calculations.These relatively simple rules have It is apparent that there is an enormous amount of pub- passedthe test of time and can still provide valuable in- lished information about crystal chemistry as defined sight to crystal chemistry;the application of modern quan- above that has not been compiled or organized in any tum mechanicalmethods can build on this foundation and significant way. The available textbooks are either outda- carry us still farther toward the understandingof crys- ted or emphasize only a small portion of the field. Fur- talline behavior. thermore, it is clear that treatments of crystal chemistry cannot stop at the boundary between earth science and Effective ionic radii materials science--I have often noticed a provincialism By far, the most important determinants of crystal among some scientists who only want to work on minerals chemicalbehavior are sizeand chargeofions. The concept or who are not interested unless a compound has impor- of ionic radius has beenan important part of crystal chem- tant physical properties that can be sold to a customer. I istry since Bragg (1920) first introduced a set of atomic think this is a bit short-sighted becausewe all have much radii that reproducedinteratomic distancesin various crys- to learn about the science of crystal chemistry that en- tals with a variation of about +0.064. Since then, many compasses the entire periodic table and it is important

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