Abstract: Some investigators have used petrographic methods as the sole means of making clay mineral analyses, whereas others have considered petrographic methods to be useless for materials as fine grained as clays. The value of the method lies between these extremes. It has considerable value, but it has limitations and must be used with caution.
By the use of various auxiliary techniques it is usually possible to obtain fairly precise values for the indices of refraction and birefringence of the clay mineral components of a clay material. Frequently an interference figure can be obtained which yields further data on the optical characteristics. The presence of very fine nonclay minerals, such as quartz and carbonate, and organic material and iron oxide or hydroxide may prevent the making of satisfactory optical measurements.
It is necessary to decide if the optical data obtained are influenced by contamination with nonclay minerals, and if they indicate a single clay mineral or mixture of clay minerals. Fortunately the optical values themselves and the character of the particles on which they were determined give good clues regarding purity and monomineral characteristics.
Optical data in general will not permit the detection of relatively small amounts of a clay mineral in a clay mineral mixture. Also such data may be very misleading in studies of mixed-layer clay minerals.
In addition to optical data, petrographic studies of clay materials yield other important information. They are of value in determining the texture of the rock and the relative abundance and size of the nonclay minerals. In general microscopic studies are easy to perform and the probable value from them is so great, that they should be made in all clay material researches.