STRUCTURAL MINERALOGY ON SMALL LENGTH SCALES

Structural studies based on the interpretation of Bragg diffraction, in situ and time resolved, at ambient and non-ambient conditions, is the mainstay of our ability to understand, predict and modify the physical and chemical properties of materials, including minerals. Over 20 noble prizes related to “Bragg crystallography” are testimony to the enduring power and importance of a technique that unambiguously tells us “where the atoms are”. There are important earth materials where considerable structural disorder impedes the Bragg scattering approach. Derivation of testable structural models for these materials has lagged because of a lack of suitable sources to provide the counting statistics sufficient to measure elastic coherent diffuse scattering in the presence of dominant inelastic background and Bragg scattering. For proper normalization these data need to be measured to high Q to provide quantitative pair distribution functions (PDF) to compare to theoretical models. These PDFs also have to be of sufficient resolution to observe subtle deviations that might be crucial to distinguishing closely related structure models. With the increasing availability of high energy beams, particularly with focusing above 100 keV, and spallation source neutrons increasingly better quality PDFs are becoming available. This is already impacting studies of glasses, melts and nano-crystalline materials. Other "extreme conditions" such as kinetic studies of nano-crystalline materials growth in proteins may also be possible.

I acknowledge the considerable contributions to this work of L. Ehm, F.M. Michel, S.A. Antao, P.L. Lee, C.D. Martin, P. Chupas, K. Chapman, C. Tulk, D. Klug and B.C. Chakoumakos.