Laura Cardon and Dr. Barry R. Bickmore, Geology
Clay minerals comprise a large portion of the abundant sediment that covers the earth’s surface. As such, they have a significant impact on society in regard to land development and also water and mineral resources. The purpose of this study was to help unravel the mysteries of clay mineral properties by focusing special attention on the effects of defects found in the clay mineral structure. These defects are believed to play a key role in the exchange of, oftentimes toxic, interlayer cations in the clay mineral. Better understanding of this exchange would lead to better water management and aid us in our abilities to ensure pure water sources.
The specific intent of this study was to determine to what extent, if any, defects played a role in clay mineral reactivity and the kinematic properties of these prolific sheet silicates. Using Atomic Force Microscopy, one can monitor the activity of particular isolated sheets, “islands,” found on the mineral surface of the clay. By varying the components and methods in the system we determine the resultant effects by comparison of each successive experiment.
We used several different methods to provide ample comparisons between samples. All samples were scanned in situ by an Atomic Force Microscope while in water or a solution of 4 M NaCl. Microscope tip pressures were varied dependent on the experiment set with some sets scanned in “soft” contact mode and others in “hard” contact mode. These scanning techniques were developed in this study after much trial and error using other techniques.
Our first set of experiments was used to provide rate standards for interlayer exchange on undisturbed mineral surfaces that have had long exposure to high concentrations of solution. One set of samples was scanned and then soaked in 4 M NaCl for either several hours or for many days. These samples were then scanned for mineral surface effects. By varying the soaking duration we could determine the relation between solution exposure and exchange rates. Another set of samples was soaked in 4 M NaCl solution and scanned while soaking. This set of samples also helped us to determine the standard time needed for exchange to occur on clay mineral surfaces without defects. Both sets of experiments led us to conclude that several hours to days are required before any mineral surface activity will manifest itself.
Other sets of experiments were used for control on rates by varying the tip pressure during scanning. Samples scanned in ”soft” contact mode under solution showed no surface changes after a day of scanning. On the other hand, approximately only one hour of scanning in “hard” contact mode was required to rip off small mineral sheets (“islands”) and to allow the microscope tip to begin to tear up the larger clay sheets below. This gave us a time constraint for the production of defects on the mineral surface before complete destruction of the surface proceeded.
With these time constraints we were then able to produce defects on our clay surfaces and then test and compare the exchangeability of defected surfaces to those surfaces without defects. This was done by scanning samples in water in “hard” contact mode for under an hour. This scanning duration was meant to produce defects in the mineral sheets without subsequently destroying the surface. Scanned samples were then injected with 4 M NaCl solution and scanned using the “soft” contact mode method. This “soft” method was used in order to observe the mineral surface activity without causing further structural damage of the sheets by means of the scanning tip. This method allowed us to observe reactivity in the interlayer of a defected mica in solution. Subsequent “soft” contact scanning of these defected micas imaged the development of frayed edge sites (popped up mica sheets) on the islands in the area previously scanned in “hard” contact mode in water. These sheets were then ripped off the mineral surface as continued disintegration occurred in the scanned section. This entire process was complete in 10-15 minutes with only the previously scanned portion of the surface being affected at such a rapid rate.
As expected, we were able to verify that defects do have a large effect on the chemical reactivity of a clay mineral. Increased ion exchange is apparent in the rapid development of frayed edge sites and the disintegration of the clay mineral surface. In addition, we were able to recognize the effect that Atomic Force Microscope tips can have to produce defects in the samples being scanned. Tip pressure should be carefully considered when conducting experiments using an AFM.
These results may mark the beginning of enhanced research in clay mineralogy. With the publication of a paper being written on this study, we will open the doors to a new way of thinking and observing clay mineral surfaces. We can also start to face the challenges brought into play when considering naturally defected minerals and possibly learn to work with such problems to produce better systems of land and water management.