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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

It is not necessary that a mineral section, which has been prepared specially to give an interference figure, should have the same thickness as an ordinary rock section. In fact, it is only in the case of minerals with a very high double refraction that such a thin section gives the best results. As a rule, minerals in thin section, when examined in convergent polarized light of petrographic polarizing microscope, exhibit no color rings, but only a black cross or hyperbolae. Beginner mineralogists must be prepared, then, to find that the figures to be examined will be very variable as to the number of colored rings in the field. Even supposing the thickness to be constant, the number of rings in the field will, as has been seen, be dependent on the birefringence.

 

            Fortunately, however, there is one method, which can be used in order to determine the sign of any uniaxial mineral showing a complete figure. That method involves the use of a mica plate. This is a small rectangular flake of muscovite, cut in such a way that its slow vibration direction is parallel to the length of the rectangle. It should be mounted on a glass slip ½ in. in width. Having obtained the figure, the mica plate should be inserted into the slot immediately above the objective of the polarized light microscope. It has the effect of producing expansion in one pair of opposite quadrants, and contraction in the other pair. Where expansion takes place, two black spots appear near the center of the field. There are two cases. When the double refraction is positive, a straight line joining the two spots would lie perpendicular to the length of the plate, and when negative, parallel to the length.

 

            The same method may be used in order to determine the sign of a biaxial mineral, such as biotite for example, with small optic axial angle.



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Friday, February 15th, 2008 at 5:07 am
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Mineral characters Between Crossed Nicols in Convergent
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