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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

We are now in a position to consider the microscopic examination of minerals in thin sections when the lower nicol, the polarizer, is in position in the axis of the polarizing microscope underneath the stage.

 

            The light reaching the section has previously passed through the polarizer, and consequently performing its vibration parallel to the principal section of that nicol, which although the nicol is capable of rotation, we shall, for purposes of description, consider to be fixed in E.W. direction. If now a section containing the mineral biotite is placed on the stage of the petrographic polarizing microscope, and the stage is rotated, it will be seen that the mineral changes color from pale brown to dark brown, and that a rotation of 90 degrees is necessary in order to produce the maximum change.

 

            This property of certain minerals, exhibited very strikingly by biotite, of changing cooler when thus examined in polarized light of the geological polarizing microscope, is known as pleochroism. It supplies us with an extremely valuable means of distinguishing many minerals with the polarizing microscope.

 

            The case of biotite will be easily understood if it is remembered that such a section is capable of transmitting light vibration in two directions only. These two directions are at right angles, parallel and perpendicular to the cleavage direction. The first position is where the whole of the light transmitted vibrates in one of these directions, and the mineral absorbs its minimum amount of light. The other position is where the whole of the light transmitted vibrates in the other of these directions, and the mineral absorbs its maximum amount of light. In an intermediate position, some light is transmitted vibrating in each of the two vibration directions, each direction absorbs its own amount of light, and consequently the transmitted light is intermediate in the shade between two pleochroism extremes.

 

            Other common mineral affording good examples of pleochroism are, hornblende, which is commonly found pale green to dark green or pale brown to dark brown in color; tourmaline, which is usually pale brown to dark brown in color; andalusite, which is colorless to pink in color; aegerine, which is usually a rich yellow to blue green in color. In all these cases the minerals are doubly refracting, and, as a rule, their sections have two rectangular vibration directions with different powers of absorption. It must be understood, however, that the strength of the pleochroism depends on the direction in which the section is cut. For instance, basal section of biotite exhibits very feeble pleochroism when viewed under petrographic polarizing microscope, while basal sections of tourmaline are absolutely non-pleochoic. This crystallographic variation emphasizes the necessity for examining  as many fragment of the mineral as are available in the slide before an identification is made.     



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Wednesday, February 13th, 2008 at 3:48 am
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Characters of Minerals in Polarized Light
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