Stack Group Research

In our research, we examine how mineral and crystal surfaces interact with their surroundings.  This is important because this interaction affects and even determines a lot of fundamental aspects of our lives, for example, drinking water quality is determined by what minerals are present in the watershed.  Mineral surfaces also strongly affect contaminant transport and reactions, and will affect the decision about what remediation steps are necessary for a contaminated site.  We even work on some applications that you would not think at first would be connected to minerals, like how anti-malarial drugs interfere with the malaria parasite, or the effects of aerosols on climate change.  Surprisingly, many of the mechanisms by which these reactions occur are not known.  This is true for reactions are usually considered as relatively simple, such as mineral weathering, or complex ones like the interaction of proteins from an organism and a biomineral.

The Stack group specializes in two different approaches, the first of which is scanning-probe microscopies, especially in situ electrochemical scanning tunneling microscopy (EC-STM) and atomic force microscopy (AFM).  EC-STM enables us to observe reactions as they progress at the atomic-, or near-atomic scale, but can only be used on semiconducting or conducting substrates, whereas AFM is more versatile but has a somewhat lower resolution.  We couple microscopic observations to wet-chemical experiments and spectroscopies, thus allowing us to make inferences about the reactivity of particular molecules and functional groups and relate that to macroscopic-scale measurements.  Shown below are the microscope head of one of our microscopes and an image we took of Shewanella oneidensis, an iron reducing bacteria (click to enlarge):

In addition to looking at bacteria, we look at mineral growth and dissolution.  On the left below is an 10 µm sized image of the soil mineral calcite just after we prepare a clean sample.  The jagged lines on the surface are steps where the bulk crystal structure has been broken into monomolecular layers.  If we zoom in and look at a step in detail, we something like the image in the middle (15 nm wide).  In addition to two steps in the middle portion of the image, we can see the atomic structure of the calcite surface, it looks like a striped pattern on the surface.  These rows are 5 Å apart, about the distance between rows of calcium or carbonates within the crystal lattice.  If we cut a slice through the step, the structure is something like the molecular model at right.  Click on the image to see a quicktime movie to see what happens when we expose that calcite sample to water.  The calcite dissolves through the nucleation of "etch-pits" (holes in the terraces), and then by retreat of steps.

Our other specialization is first principles calculations of mineral surfaces and reactions.  With the ongoing advances in computing power, it is becoming possible to simulate the large systems necessary to accurately model environmentally and geologically relevant processes.  We can now simulate the same reactions that are observable using the microscopies above.  Click on the image below for a quicktime movie of a water exchange reaction on a molecule called a keggin that has very similar functional groups to gibbsite, a common soil mineral: