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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The light transmitted by the different minerals when evaluated in transmitted light of petrographic polarizing microscope varies both in amount and kind. Some minerals are almost opaque while others are perfectly transparent. In some case, the whole constituents of white light are allowed to pass through, these mineral are colorless. Other minerals, however, absorb some portion of the spectrum, with the result that the light reaching the eye has a certain color – the color complementary to that absorbed. This color supplies the viewer with the first means of distinction between the rock forming minerals.

 

            The mineral section will probably contain several fragments of any particular mineral that can be seen clearly under polarizing microscopes, and the inexpert viewer will naturally expect that all these fragments will exhibits the same color, or will agree in being colorless. But then it must be important to note that, while this may be the case, it is not necessarily so. Variation in the color of the different fragments as seen under polarized microscope is intimately related to the crystal symmetry. Should the mineral in question crystal in the cubic system, all fragments present in the section will be either colorless or will transmit the same color when viewed under the petrographic polarizing microscope. A good example of this is the mineral garnet, the common varieties of which transmit a pink color that is usually amazingly interesting when evaluated under geological polarizing light microscopes. On the other hand, mineral biotite, crystallizing in the monoclinic system, is a proper example of a mineral that varies in color according to its orientation in the section as exhibited under petrographic polarizing microscopes. The different fragments occupy all possible positions in the rock from which the section is cut. Consequently, a thin section supplies numerous cases. Crystallographically speaking, no two of these are identical, and each case has its own absorption. The result is that the different fragments of this mineral exhibits very different shades of brown that can be interesting when viewed under geological polarizing microscopes. It must be considered that other characters must be evaluated in order to demonstrate that such fragments are really the same in composition. There are also many cases, however, even in the same system, in which all sections of a colored mineral have much the same shade that is usually majestically displayed under polarizing microscopes. A good example of this is the common mineral augite. Mineral muscovite is also crystallizing in the monoclinic system, is typically colorless in all its sections. The relationship of color in thin section to crystal symmetry may be summarized as follows: In cubic system, all sections of one mineral are colorless or exhibits the same color when viewed under polarizing microscope. In other systems, the different sections are usually different in color, but all may be colorless or all may be similarly colored when viewed under polarized light microscopes.



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Wednesday, February 13th, 2008 at 3:32 am
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Minerals Characteristics in Ordinary Transmitted Light
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