CHEMICAL MAPPING OF INDIVIDUAL CLAY PARTICLES USING A JEM 2010

Variation in chemical composition in individual clay mineral particles is an important factor in understanding the physical properties of clay minerals such as sorption, dissolution, cation exchange, and rheology. Information on chemical composition is routinely obtained from energy dispersive spectroscopy (EDS) spot analyses using traditional TEM techniques. These analyses however do not reliably show spatial variation in chemical composition for individual clay particles. Spatial variation in chemical composition within individual particles may impact layer charge and cation exchange of clay minerals, and thus may explain variation in bulk physical properties.

The JEM 2010 scanning transmission electron microscope (STEM) can map spatial variation within individual particles using a rastering electron beam. Maps can routinely be made at 25,000x and 50,000x in 30 to 60 minutes using a 1 nm x 1 nm electron beam. Commonly, 16,000 EDS spectra are acquired over the area of interest producing a high resolution map.

Particles of illite, montmorillonite, and amorphous Fe-oxides clay particles were investigated using STEM techniques to ascertain chemical variability in these particles. Particles were also investigated using traditional TEM techniques. Results from the JEM 2010 concluded that illite and montmorillonite clay particles from bentonites were functionally homogenous in chemical composition however some examples of montmorillonite displayed enriched areas of potassium. Fe-oxides from spodic soils exhibited moderate heterogeneity with concentrated areas of potassium, aluminum, phosphorous, and iron.

Chemical mapping using the JEM 2010 is useful for assessing chemical variation within clay particles. However selected area electron diffraction and routine imaging of particles is best accomplished with standard TEM. The combination of TEM and STEM techniques appears be particularly useful for soil investigations and for development of clay mineral-based geotechnologies.