FEI Nova Nano SEM 430 field emission SEM
Installed in 2009 this instrument is configured for high-resolution (2nm) and analytical imaging of various materials. Secondary electron imaging can be performed on either coated or uncoated specimens. Through-the-lens “immersion mode” imaging allows resolutions to 3-4 nm under optimal conditions. Additional detection modes include back-scattered electron imaging, cathodoluminescence imaging (Gatan Chroma CL detector), and EDS X-ray mapping (dual Bruker 30 mm2 detectors).
Secondary electron (SE)
Secondary electrons vary in intensity largely in concert with surface tilt and thus provide a topographic image. SE imaging can be carried out at very low beam energies and with an extremely small diameter beam, allowing for high spatial resolutions. This imaging mode is used to examine the smallest pores in mudrocks. SE imaging allows close inspection of pores at the nanoscale. Such imaging has shown that many gas shales have pore systems that are located within organic matter. This fast opens the possibility of porosity prediction using an understanding of thermal maturation—a topic that is attracting much current investigation.
Back-scattered electron (BSE)
BSEs carry information on composition. Although BSE images are generally produced at higher beam energies (and lower magnifications) than SE images, they yield information on rock fabric with immensely greater clarity than that of light microscopy. BSE imaging carries information that relates to a very thin slice of the rock. In an Eagle Ford shale sample, BSE reveals the sizes and shapes of pyrite crystals, fossil debris and organic matter with great clarity.
The CL signal can be used to image variations in trace elements and crystal defects that are not readily visible in any other form of imaging. CL is especially useful for discriminating grains from cements in both silicate and carbonate systems. CL imaging in mudrocks can reveal microscale cementation that is an important control on mechanical rock properties.
CL images of zircons show high-resolution zoning information that are unable to be seen using SE imaging. The zircon below is imaged in CL and secondary electrons. Oval spots on the secondary electron image are regions were where the zircon was dated. CL helps in the interpretation of the age results.
X-ray Energy-Dispersive spectroscopy (EDS)
X-ray signals go beyond BSE imaging to directly display mineral distributions. Mineral species depicted in false color can be readily interpreted as to component type. Use of a field-emission beam, combined with the approach of summing signals from multiple detectors, allows X-ray mapping at some of the highest resolutions possible. X-ray maps of Eagle Ford Shale. The complex composition of the silt and clay-size grain assemblage. The image shows a foraminifera chamber filled with calcite cement, pyrite framboid and migrated bitumen.