MSA Presidential Address. Making Crystallography Dynamic: Real-Time Studies of Mineral-Fluid Reactions Using X-Ray Diffraction

As a general rule, the time-scales that characterize mineral transformations decrease with depth, since the environmental changes that serve to displace mineral systems from their equilibrium states grow more pronounced with increasing proximity to Earth's surface. Thus, mineral phases that comprise the Earth's deep interior may not alter appreciably over millions of years, whereas the soil minerals that constitute the uppermost portion of the crust evolve over time-scales of days to years. The dynamical quality of the so-called Critical Zone is what underlies its geological importance; the rapidity of elemental cycling among the solid-fluid-gas reservoirs at the surface stabilizes the system against swings into radically different states. Ironically, capturing the shifts in soil mineralogy at the atomic scale has largely eluded the technique that first proved the existence of atoms. The static quality of conventional X-ray diffraction (XRD) is well-suited for imaging the crystalline structure of the deep Earth, but it cannot encompass the transience of the soil landscape.

Within the last decade, however, advances in XRD detector technology have combined with developments in environmental cells to open a new frontier in dynamical crystallography. Image plates, particularly in concert with the highly intense and monochromatic radiation at synchrotron sources, allow for the collection of high-quality diffraction patterns of powdered samples in minutes. The resulting increase in time resolution permits the determination of rate laws for mineral-fluid reactions that are coupled to atomic-scale changes in crystal structure. In this address, I will describe the extension of time-resolved X-ray diffraction techniques to four major soil processes: 1) cation exchange; 2) biomineralization; 3) stable isotope fractionation during redox reactions; and 4) nucleation and growth of oxyhydroxides.