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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

A change, which cannot be called pleochroism, takes place when sections of the mineral Calcite are rotated over the polarizer of the petrographic polarizing microscope. Calcite is doubly refracting when evaluated under geological polarizing microscopes, and, consequently, most of its sections possess the two rectangular vibration directions for transmitted light. When the rotation takes place, the mineral shows, at one time, rough surface, well-defined borders, and conspicuous cleavages when closely examined with the aid of the polarizing microscope. At another time, smooth surface, faintly defined borders, and inconspicuous cleavages that are found more clearly visible under polarized microscopes. The change is from a mineral with a high refractive index to one with a low refractive index and this can be seen clearly exhibited when evaluated under polarizing microscopes for geologists. And these extremes of relief are exhibited when the two vibration directions of the calcite in section come to be parallel in turn with the vibration direction of the light emerging from the polarizer. This means that each of the two vibration directions has its own refractive index. A rapid rotation of the stage or of the polarizer beneath it produces a rapid change of relief, which cannot be better described than by the name of twinkling. Twinkling is quite different from pleochroism, which is essentially a change of color, whereas calcite remains quite colorless. Other mineral with a high double refraction shows the same effect, notably the other rhombohedral carbonates, but few other minerals show a conspicuous change of relief most especially when viewed under petrographic polarizing light microscopes. Though theoretically, al doubly refracting sections ought to show some such change. Only in a few cases, however, such as the one cited, is the change sufficiently great to be worthy of consideration in practice.

 

            It is helpful to note that all colorless sections, sections of cubic minerals, and basal sections of dimetric minerals are all non-pleochroic even when viewed more closely under polarized microscope. All other remaining sections may show pleochroism. Some may exhibit a striking change of color, other merely a slight change but still interesting when examined under geological polarizing microscope. In still others the absorptive powers of the mineral for the two vibration directions are so nearly equal that even the most imaginative observer fails to notice any change of shade or color.



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Wednesday, February 13th, 2008 at 3:49 am
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Characters of Minerals in Polarized Light
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