In our group, 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 understanding reaction mechanisms and their rates. We couple microscopic observations to wet-chemical experiments, spectroscopies and computational models, thus allowing us to learn about the reactivity of particular molecules and functional groups on a mineral's surface. Shown below are the microscope head of one of our microscopes (an atomic force microscope) and an image we took of Shewanella oneidensis, an iron reducing bacteria that's commonly found in salt marshes and has been shown to change the oxidation state of a number of heavy metal and oxidize organic pollutants.(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.
We also do a lot of modeling 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. Here are a couple of images showing the results of a simulation of the {001} barite-water interface structure. Barite (BaSO4) is a mineral often found in the upper-water column in the oceans, but the saturation of these waters is such that it ought to dissolve so it's not really clear what it's doing there. One possibility is that some organisms are known to make their exoskeletons from it -- this process is what's known as biomineralization. There's a lot of interest right now in understanding just how biological organisms make inorganic minerals, and we're currently working on understanding some of the important reactions.
