FOR many industrial purposes the mineralogical composition of a product is of great importance, and as a diagnostic tool the electron microscope is often effective; for not only can individual particles be positively identified by electron diffraction but many of them can be recognized with a fair degree of confidence by their morphology.
Kaolinite can usually be recognized, for although it is rare to find all of the crystals with six sharp corners and no re-entrant angles, nearly all kaolins have particles possessing one or more sharp-angled corners in sufficient numbers to ensure identification. Halloysite is also very recognizable since it is in the form of troughs, tubes, or toroids. Nor can one often be in error when sepiolite or palygorskite laths are present; since one cannot by morphology alone distinguish between these minerals one may be forgiven for referring to the presence of a hormite. Muscovite and vermiculite may be indistinguishable, but may be referred to as mica. Illite cannot be recognized with confidence, but where the platelets are very thin and have irregular outlines, illite may be suspected. Some iron oxides are distinguishable, e.g., hematite with curved faces or goethite, which can either have its classical, elongated shape or may be so fine as to be seen as a pepper in the electron micrographs. Some of the hydrated aluminas are recognizable but others could be mistaken for illite. Of the smectites nontronite is the most easily recognized by its ribbons; but even one or two per cent of any of the smectites may be recognized by the way their single layers plate on to the carrier foil.