Polarized light optical microscopy limitations

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

The simplest method is optical microscopy, in which visible light (photons) is used to observe a sample. It has a resolution limit around 0.25-0.5 pm, which is on the order of 2/2, where 2 is the wavelength of incident light. From a strict colloid science point of view, it lies near the upper limit of colloid particle sizes and appears to be of limited utility. However, it is of great help in the identification of minerals, because it allows observation of crystal habits (the shape and size of crystals, which are determined by their internal symmetry). With experience, many minerals can be identified in a soil sample under a microscope, even from simple inspection. A unique feature of optical microscopy is the availability of polarized light, which is handy in distinguishing minerals or even different crystal types of the same compound (Bullock et al. 1985 Cady, Wilding, and Drees 2010). 

---