Jonathan F. Stebbins
The 2009 Robert Wilhelm Bunsen Medal is awarded to Jonathan F. Stebbins in recognition of his outstanding work on the structure and physical properties of materials of geological interest.
We recommend Prof. Jonathan F. Stebbins, from Stanford University, as laureate of the 2009 Robert W. Bunsen medal for his pioneering investigations of the structure and physical properties of materials of geological interest.
Of particular importance are the connections established by Prof. Stebbins and his collaborators, through pioneering methods in Nuclear Magnetic Resonance (NMR) spectroscopy, between the atomic structure, at a microscopic level, and the macroscopic properties such as viscosity, density and heat capacity that are relevant to natural processes. In this respect, few people have been as consistent as Prof. Stebbins in considering structural studies not from a purely descriptive standpoint, but as a real tool to account for physical properties that often depend in a complex way on chemical composition, temperature and pressure.
Silicate melts have played a fundamental role throughout the Earth’s history because they are the most efficient agents to transfer matter and energy. Investigating the structure of melts is tricky, however, as the intrinsic problems resulting from the lack of long-range order are compounded by experimental difficulties caused by the elevated temperatures at which studies must be performed. The high-temperature NMR experiments pioneered by J.F. Stebbins have thus allowed him to perform a number of breakthroughs. Since viscous flow controls both heat and mass transfer, elucidation of its microscopic mechanisms in terms of the rate of silicon-oxygen bond exchange has been a fundamental result obtained with this technique.
A related feature has been the emphasis laid by Prof. Stebbins on the dynamic nature of melts, in which interatomic bonds are short-lived, structural changes take place very rapidly, and short-range order around atoms may be much more varied than in crystals. In fact, these effects are at the roots of the so-called configurational properties which are the hallmark of the liquid state and result in higher compressibility, thermal expansion and heat capacity for melts than for crystals. By deciphering some of the effects of temperature and pressure on structural and dynamical features of melts, Prof. Stebbins has obtained the first valuable information on these configurational changes.
In addition, the wide ranges of chemical composition relevant to geological processes makes it particularly important to understand the relationships between composition, structure and physical properties in the large temperature and pressures intervals relevant to earth’s processes. This basic problem has also been tackled successfully by Prof. Stebbins for both crystalline and molten phases thanks to the extensive use he has made of new multidimensional NMR techniques as applied to a number network-forming (O, Al, Si, B) or network-modifying (Li, Na, Mg) elements. Another interesting aspect of this endeavor has been the study of dynamical features that crystals can show, for instance during phase transitions.
As a matter of fact, this brief summary deals with only part of the topics investigated by Prof. Stebbins. But there is no doubt that the wealth of important results obtained, not to mention their relevance to the glass and ceramic industry or, even, to environmental problems, makes Prof. Stebbins ideally suited to be awarded the Bunsen medal for his major contribution to mineralogy, petrology, geochemistry and volcanology.