F.W. Clarke Award

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F. W. CLARKE AWARD Introduction of David M. Sherman for the 1991 F. W. Clarke Award ROGER G. BURNS*

cobalt are strongly enriched in certain marine ferromanganese concretions and crusts, but characterizing the structural stability and crystal chemistry of the host Mn( IV) oxide minerals posed acute difficulties. The literature then abounded with popular reviews on the origins of color and pleochroism of minerals and gems, but unsatisfactory explanations were offered for causes of intense colors and opacity of many mixedvalence minerals, including blue sapphire, kyanite, and glaucophane, as well as black ilmenite, ilvaite, and riebeckite. Then, as now, speculations existed about the dense oxide phases likely to exist in the mantle and the electronic state of iron in the Earth’s interior. Exposure to ongoing research on these problems led David to pursue, on his own initiative, calculations of the electronic structures of manganese and iron cations in minerals, using the self consistent field-Xc+scattered wave molecular orbital theory (SCF-Xcu-SW MO theory) already pioneered at MIT by Jack Tossell, David Vaughan, and Bruce Loeffler for simple mineral structures. Those earlier MO calculations had been performed on high symmetry coordination clusters such as regular [Fe06] octahedra and [Fe!&] tetrahedra that approximated simple oxides and sulfides. David Sherman’s great contribution was to extend the quantitative MO calculations to low symmetry environments such as the [FeO,(OH),] octahedra in goethite and lepidocrocite, the trigonally distorted [Fe06] octahedron in hematite, and the edge-shared dimeric clusters that approximate edge-shared Fe’+ -Fe3+ octahedra in magnetite and face-shared Fe-Ti octahedra in sapphire. Sherman was the first to suggest that poorly crystalline, nanophase Fe(II1) oxides producing weak magnetic interactions between adjacent Fe3+ ions must exist on Mars in order to explain the visible-region reflectance spectrum profile of this planet. Sherman’s MO calculations also accounted for the diversity of Mn( IV) oxide polymorphs and their ability to bind other cations adjacent to Mn4+ vacancies in the chains or layers of edge-shared [ MnOd] octahedra. We now know why deep-sea manganese nodules are enriched in Co, Ni, and cu. More recently, David has turned his attention to calculating electronic structures, equations of state, and mechanisms of phase transitions in oxides and sulfides at high pressures. He has enlarged the dimensions of his cluster calculations to [ FeMg12014] groups modelling magnesiowtistite. His recent energy level calculations have led to interpretations of radiative heat flow and electrical conductivity in the Earth’s interior and predictions of spin-pairing transitions in Fe*+ ions in the deep mantle. He has also demonstrated that metallic iron containing interstitial oxygen may be stable in the core. Throughout his brief, but highly productive career, David

Mr. President, Officers and Members of the Geochemical Society, we are honoring today the achievements of a young scientist whose research has revolutionized our understanding of transition metal geochemistry. It was 1979, and the lingo of the Californian valley girls was in vogue. There arrived at MIT from the West Coast a recent graduate from the University of Santa Cruz. This chemist-turned-mineralogist brought with him several concepts that were to become buzz words in geochemical research during the 1980s. The corridors of MIT echoed with expressions such as Lewis acids, Hamiltonian operators, the HartreeFoch approach, Mulliken populations, Slater’s scattered wave theory, SCF-Xa-SW molecular orbital calculations, spinpairing in iron, and Mott transitions in the lower mantle. Quantum geochemistry was about to become more quantitative. Ladies and gentlemen, I introduce to you the disciple of these concepts in the earth and planetary sciences, David M. Sherman. We had a bumper crop of outstanding new graduate students at MIT as we entered the 1980s era. David was forced to share a communal office with a dozen other students two floors away from the Mineral Spectroscopy Laboratory. The isolation imposed on David only strengthened his independence and motivation to tackle new and difficult projects, such as performing computations during the graveyard shift in order to obtain the cheapest rates on the mainframe computer. Those were the days before the NSF provided free access to Cray computers. During his first two years in graduate school, David mastered several challenging courses in earth and planetary sciences, chemistry and materials science, including Keith Johnson’s course on quantum chemistry and physics of solids. Here, David saw the potential of using quantitative molecular orbital energy level calculations to explain chemical bonding in the solid state. This challenging course provided David with the background he needed to solve several controversial problems that were then current in transition element geochemistry. The Viking missions to Mars in 1977 had raised questions about the iron-bearing minerals in the martian regolith. The color and reflectance spectra of Mars’ bright regions are indicative of Fe3+ ions, but distinguishing between the several candidate ferric oxide, oxyhydroxide, hydroxysulfate, or clay silicate phases was ambiguous. By 1980, the Manganese Nodule Project had demonstrated that copper, nickel, and

* Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02 139-4307, USA. 1777

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Sherman has demonstrated remarkable ability to recognize and successfully tackle contemporary geochemical problems. The clarity of his published papers has done much to bridge the communication gap between quantum chemistry and experimental geochemistry. When Sherman writes or speaks,

his peers read and listen attentively. He is at the frontier of geochemical research. I am very proud to introduce Dr. David M. Sherman to the Geochemical Society. He is the worthy recipient of the 1991 F. W. Clarke Award.

Acceptance Speech for the 1991 F. W. Clarke Award DAVID M. SHERMAN* *+

First, let me say how grateful I am for this unexpected honor. My research on the electronic structures of transition-metal

oxides and silicates was originally started to understand the nature and mechanisms of electronic transitions in minerals. At the same time, however, I felt that an understanding of chemical bonding in minerals might, in turn, provide a deeper understanding of geochemistry and, ultimately, the chemical principles that govern the Earth’s formation and chemical differentiation. This award, particularly because it comes from the Geochemical Society, gives me tremendous encouragement to continue working in that direction. Indeed, I am optimistic that quantum chemistry will play an important role in the future of geochemistry. Before discussing these new research areas, however, I would like to briefly describe how I got started in my work and thank those who helped me along the way. As an undergraduate, I did a double major in Chemistry and Earth Sciences at the University of California, Santa Crux. I always wanted to connect the ideas I was learning in physical and inorganic chemistry classes to the materials and processes I was learning about in my geology classes. When I took the undergraduate quantum chemistry course, I was very excited by the molecular orbital theory of electronic structure and the chemical bond but assumed it could never be applied to something as complex as a mineral. It was about this time that I came upon a book, Mineralogical Applications of Crystal Field Theory, by Roger Bums. I thought it was a very interesting work that showed how the geochemical behavior of transition metals can be explained in terms of their electronic structures. After some correspondence and an application, Roger was willing to take me on as a graduate student at MIT. At the time, Roger’s group was interested in chargetransfer transitions in minerals and the optical spectra of planetary surfaces. Electronic transitions involving the d-orbitals of iron and other transition metals can occur in the visible and near-infrared. Hence, these transitions are important to optical spectra of minerals, photochemical processes, and radiative heat flow in planetary interiors. Although many aspects of the optical spectra of transition metal oxides and silicates can be well described in terms of crystal field theory, a number of important phenomena require us to think in terms of the more general (and realistic) molecular orbital or energy band formalisms. Charge-transfer and interband transitions are a good example. Ligand-to-metal charge transfer transitions show up in the optical spectra of iron( III) and manganese( IV) oxides and also govern the photochemical behavior of these minerals. Other interband transitions,

David M. Sherman

from the Fe( 3d) orbitals to the Fe( 4s,4p) conduction band, were believed to be important as a mechanism of chemical weathering on Mars and on the Earth’s Precambrian surface. Metal-metal charge transfer processes show up in the visible and near-infrared spectra of minerals and are important in the remotely sensed spectra of planetary surfaces. The real nature and mechanism of metal-metal charge transfer processes in minerals was not understood. It was not clear, moreover, whether some of the interband transitions in oxides and silicates were of sufficiently low energy to be induced by solar radiation and, hence, relevant to chemical processes on planetary surfaces. I told Roger that I wanted to try to do some molecular orbital calculations on Fe-Mn-Ti oxide clusters to try and address some of these problems. Because of J. C. Slater and his group, MIT was traditionally a center for computational quantum chemistry, particularly as applied to solids. For me, an important figure from that tradition was Keith Johnson, professor of Materials Science who developed the Self-Con-

* U.S. Geological Survey, Denver CO, USA. +Presentaddress: Molecular Science Research Center, Pacific Northwest Laboratory, Box 999, Richland, WA 99352, USA. 1779

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sistent Field Xa-Scattered Wave Method by adapting the “muffin-tin” approximation used in band theory to finite clusters. The SCF-X&-SW formalism allowed one to calculate the electronic structure of very large clusters containing transition metals and, as such, was a perfect tool for taking on some of the problems that Roger Bums had gotten me interested in. Keith was always a great inspiration and even the briefest meeting with him would fuel me for a week of grueling late-night computational work. Another person to whom I’m grateful is Jack Tossell. Aside from helping me get started, Jack pioneered the applications of the Xa-SW approach to mineralogical problems and over the years has often been a source of advice. Up to this time, there was still some confusion about how one relates the one-electron orbital energies to the energies of the multielectronic states observed in experimental optical spectra. Much of my dissertation consisted of discussions of multiplet theory and ways to relate the electronic transition energies between one-electron orbitals to the parameters of ligand field theory. The dissertation that resulted from all this, Electronic Structures of Iron and Manganese Oxides, with Applications to their Mineralogy, did not seem very geological and I was concerned that I might have a hard time getting a job as an Earth scientist. The U.S. Geological Survey, however, wanted to hire a mineralogist with a background in mineral spectroscopy to help support research in spectroscopic remote sensing. I am very grateful, therefore, to Steve Huebner and colleagues in the Igneous and Geothermal Processes branch who got me on board the U.S.G.S. to start my career. While at the Survey, I pursued some unexplored avenues left over from my dissertation and began to look at the optical spectra and crystal chemistry of clay minerals and other phases significant to spectroscopic remote sensing. I also put together a Mossbauer lab to look at site occupancies of iron in minerals. I was going to give up electronic structure calculations and focus on more conventional work that I could sell to my superiors. At the same time, however, I began to get interested in the physics and chemistry of the Earth’s deep interior. The central question for me was whether the pressures of the Earth’s mantle and core could induce electronic transitions that would affect the chemical behavior of major components. Pressure-induced electronic transitions underly the hypotheses that elements such as K and 0 are partitioned into the Earth’s outer core. Visible and nearinfrared electronic transitions are also significant insofar as

they can control the rate of thermal radiation across the core-mantle boundary. In recent years, moreover, the feasibility of applying abinitio electronic structure calculations to geochemical problems has increased enormously. Computers are now much faster and new computational methods have been developed and made accessible to the general scientific community. Until recently, one could only look at the electronic structures of finite clusters to investigate chemical bonding and electronic transitions in minerals. Now, however, it is feasible to do accurate band structure calculations on mineralogically interesting phases. This is largely thanks to the so-called linear modifications in band theory (i.e., the LAPW and LMTO methods) and recently developed extension of the HartreeFock LCAO approach to periodic systems. Using these methods and the faster computers that are now available, some large and geochemically interesting problems are now being addressed. In the next few years you will see many studies, by myself and colleagues in the mineral physics community, on the stabilities, equations of state and structural transitions of phases that may be present in the Earth’s lower mantle. From such work, quantum chemistry may provide important constraints for models of the Earth’s composition and the mechanism of its formation. Probably the most significant applications of quantum chemistry, however, will be to chemical processes that occur on mineral surfaces. Aside from their fundamental interest, such processes are central to the fate and mobility of hazardous substances in the environment. Already, funding agencies are interested in developing an understanding of geochemical processes at the molecular level. It is hoped that such fundamental science will lead to the technology needed to solve many pressing environmental problems. In short, I believe there are important opportunities ahead for “quantum geochemistry” and I hope the Earth Science community will embrace this emerging research. To conclude, I wish to thank Roger Bums, Keith Johnson, Steve Huebner, and other mentors and collegues, particularly at the U.S. Geological Survey, for their help in getting me started and giving me the freedom to pursue some uncharted areas of research. Most of all, however, I would like to thank my wife Anna for her constant support and patience. 1 am very grateful for this kind honor from the Geochemical Society and hope that my work in the future will live up to your expectations. Thank you.