Scanning Electron Microscopy I: 579-586
In water samples examined under the electron microscope, halloysite and palygorskite (attapulgite) can have an appearance similar to chrysotile (thin fibers with apparent central canals). Individual fibers of the two clay minerals are difficult if not impossible to differentiate from chrysotile on the basis of morphology since the range of fiber diameters and central canal or apparent central canal widths overlap for all three minerals. Distinguishing between the three materials on the basis of viewing selected area electron diffraction patterns in the microscope is difficult because all three minerals give similar streaked line patterns. Both palygorskite and chrysotile exhibit patterns with approximately 5 Å spacing between lines while the spacing for halloysite is approximately 9 Å. Energy dispersive x-ray elemental analysis can be used in most cases to distinguish between the three materials on the basis of the relative amounts of magnesium, aluminum, and silicon. The high Al/Si ratio and low Mg/Si ratio is clearly characteristic of halloysite. Bundles and larger fibers can be classified correctly as palygorskite or chrysotile on the basis of Mg/Si and Al/Si ratios, but not all fibers less than 0.04 ?m in diameter could be correctly classified on the basis of chemistry.
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