Scanning Electron Microscopy (SEM)

A scanning electron microscope (SEM) is functionally very similar to an electron microprobe but is designed primarily for imaging rather than quantitative chemical analysis. The SEM excels at revealing the three-dimensional morphology of mineral grains at magnifications far exceeding those possible in a petrographic microscope, and its attached energy-dispersive spectrometer allows rapid identification of major elements in any grain of interest. ^[1]

Image Formation

SEM images are produced by systematically sweeping the focused electron beam back and forth across the surface of the sample in a raster pattern. At each point that will become a pixel in the final image, the detector records the intensity of the electron signal and the electronics package assigns a corresponding brightness to that pixel on the computer monitor. The result is a grayscale image built pixel by pixel from the detector output. ^[1]

Conventional SEM instruments produce usable images at magnifications ranging from 5× to 300,000×. Unlike a light microscope, the SEM has an exceptionally large depth of field at all magnifications, which reveals intricate three-dimensional surface detail that would be invisible in a conventional optical image. ^[1]

Detector Types and Signal Interpretation

When the focused electron beam strikes the sample, two distinct populations of electrons are emitted from the surface. Both are detected simultaneously by separate detectors, and each produces a different type of image.

Secondary electrons are low-energy electrons dislodged from the outermost atoms of the sample. The secondary electron detector is mounted to the side of the sample and is biased to attract these low-energy particles. Because the signal depends on how many secondary electrons can escape the surface and travel to the detector, surfaces angled toward the detector appear bright while recesses and holes appear dark. Secondary electron images are the standard tool for documenting surface topography and crystal morphology. ^[1]

Backscattered electrons are higher-energy electrons from the incident beam that have been deflected back out of the sample by their interaction with atomic nuclei. The backscatter electron detector is mounted above the sample. Because high-atomic-weight elements scatter electrons more efficiently, minerals with heavier constituent elements appear brighter in a backscatter image. This makes backscatter imaging ideal for quickly distinguishing different mineral phases in a polished section - ilmenite (FeTiO₃), for example, appears distinctly brighter than adjacent quartz (SiO₂) because of the higher atomic weights of iron and titanium. ^[1]

Sample Preparation

A small piece of mineral or rock is mounted on a sample holder. Because air interferes with the electron beam, the sample chamber is evacuated by a vacuum pump after samples are loaded. A thin coating of carbon or gold is applied to the sample surface to conduct the electrons from the electron beam away from the sample. Without this coating, electrons accumulate on the surface (charging), which distorts the image. For the highest resolution, some instruments use a cold field emission gun - a sharpened cathode tip from which electrons are pulled by a high voltage - in place of a heated filament. ^[1]

Chemical Analysis Capability

Most SEMs carry an energy-dispersive spectrometer (EDS), which captures the characteristic X-rays emitted when the electron beam strikes the sample. EDS provides semi-quantitative elemental data for the major elements present in a grain within seconds, making it extremely efficient for rapid mineral identification. However, EDS analyses from a standard SEM are less accurate than those from a dedicated electron microprobe because surface irregularities on SEM samples affect how much of the emitted characteristic X-ray spectrum actually reaches the detector. Some instruments are also equipped with wavelength-dispersive spectrometers to extend sensitivity to trace elements. ^[1]

Applications in Mineralogy

Because sample preparation for an SEM takes only a few minutes, it is a very efficient tool for examining a wide variety of samples quickly. The SEM is particularly well suited to fine-grained samples where it is impossible to obtain physical or optical properties by conventional methods. In such cases the combination of three-dimensional morphological imaging and rapid EDS chemistry allows preliminary identification without thin section preparation or X-ray diffraction. ^[1]

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