SILICA in fine particulate and mainly crystalline form has been studied by many different methods including both optical and electron microscopy and electron diffraction. Probably the most important reason for investigating silica in the micron and sub-micron range has been its importance as the causative factor in the lung disease known as silicosis. Optical methods have been used for identification, size measurement, and the study of surfaces. Some forms of amorphous silica have a recognizable shape and this may have some diagnostic value in identification, but crystalline silica appears to be either devoid of cleavage planes or to have insufficiently distinct weaknesses to break into anything other than randomly shaped particles. These cannot be identified by shape, and electron microscopy is of limited value in identification. This is partly due to the tendency for an amorphous or very disordered layer to form on particle surfaces, and to affect the structure to a depth of a similar order to that of the penetrating power of the electron beam. X-ray diffraction techniques for the quantitative determination of quartz must also take account of the effect of the amorphous layer on the quality and intensity of the diffraction pattern.
Since crystalline silica particles are of random shape, reasonably reliable statistical correlation can be established between particle sizes as measured by electron microscopy and other parameters such as the surface and mass as measured by other techniques. This can be of importance in respirable particles of less than 2 μm diameter, whose size can be reliably measured only by electron microscopy.