Old but indispensable

A polarization microscope can be used to identify the mineralogical composition of geological materials in order to help reveal their origin and evolution. Some of the properties and techniques used include: refractive index, birefringence, Michel-Lévy interference color chart, extinction angle, conoscopic interference pattern (interference figure), Becke line test, wave plate etc.

The microscopic investigation of mineral substances using polarized light, remains one of the classic, and to this day indispensable, mineralogical methods of analysis. Polarized light microscopy provides a nondestructive way to identify mineral substances. The limitations of this kind of microscopy are obvious where the chemical composition of a sample has to be determined. Depending on the specific objectives and the nature of the material to be investigated, a modem mineralogical study will therefore aim to combine polarized light microscopy with other modern complementary methods.

William Nicol, whose name is associated with the creation of the Nicol prism, seems to have been the first to prepare thin slices of mineral substances and his methods were applied by Henry Thronton Maire Witham (1831) to the study of plant petrifactions. A rock-section should be about 30 micrometers in thickness, and is relatively easy to make by expert hands. This method, of such far reaching importance in mineralogy, was not at once made use of for the systematic investigation of rocks, and it was not until 1858 that Henry Clifton Sorby pointed out its value.

Anisotropic, birefringent crystals present in ultra-thin mineral sections, when positioned between two crossed polarization filters, like is the case in the video, will in all cases appear black in certain positions as the stage (or table) is rotated. If we note these positions we may measure the angle between them and any cleavages, faces or other structures of the crystal. These angles are characteristic of the system to which the mineral belongs and often of the mineral species itself. The mineral sections when not extinguished are not only bright but are colored and the interference colors they show depend on several factors, the most important of which is the strength of the double refraction.

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