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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 section should be test by means of a one-fourth undulation mica plate after having obtained a uniaxial interference figure. The mica plate is usually introduced between the objective and the analyzer of the polarizing microscope in such a way that its vibration direction c, which is marked on the plate, makes an angle of 45-degree with the planes of vibration of the nicols.

 

            The interference figure may change or more or less disappear when this is done. Two dark spots or blotches will be found brought prominently into view. It can be also noticed that if the rings are still seen, they have expanded in the quadrants occupied by the dark spots and have contracted in the remaining quadrants. It would be now possible to determine the optical character of a section with the help of the observations found through the movement of the rings. The optical character of the section is usually so oblique to the optic axis that the dark spots are not found after the introduction of the mica plate.

 

            With the aid of the polarizing light microscope, it can be found that the line joining these dark spots is perpendicular to the direction of the mica plate. In optical mineralogy, if the finding appears like this, the optical character of the uniaxial mineral is said to be positive. On the other hand, if the line joining the dark spots coincides with the direction of the mica plate, then the optical character of the mineral is negative.

 

            By simply remembering the relationship of the line joining the dark spots with respect to the direction of the mica plate, the positive and negative character of the uniaxial mineral can be easily determined. During the test, the direction of the mica plate is actually not visible but it should be known as discussed in optical mineralogy books. The test can be made easily with the use of monochromatic or white light.

 

            In optical mineralogy, there are some tips on how to properly determine the positive or the negative character of the mineral. For instance, if the mica plate will not give satisfactory results like in the case when the double refraction of the crystal to be tested is very weak or the section is very thin, a selenite plate is recommended for use and cut the material into proper thickness in order for it to give the red color of the first order. This selenite plate must be introduced with its vibration direction making an angle of about 45-degrees with the planes of vibration of the nicols of petrographic polarizing light microscope. When closely evaluated, there will appear two blue and two red quadrants instead of the dark spots. The diagonally opposite quadrants are obviously of the same color.

 

            It is important to note that the test must be done using white light. In optical mineralogy, this test of determining the positive or negative character of uniaxial mineral should be done by also considering the blue quadrants as the equivalent of the dark spots in the preceding case.



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Monday, April 28th, 2008 at 3:27 am
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Optical Mineralogy
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