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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

Interference figures of uniaxial minerals can be best viewed with the use of petrographic polarizing microscopes. In optical mineralogy, a dark cross that can be found with or without rings can be found as shown by sections positioned perpendicular to the optic or vertical axis c. When the section is closely evaluated, it can be found that the figure is symmetrical to the center, as the optical behavior of uniaxial mineral crystals is symmetrical to the optic axis. The planes of vibrations of the nicols are noticeably parallel to the arms of the cross that is found when the section is viewed between crossed nicols of the polarized light microscopes. It can be noticed also that the figure does not move when the stage of petrographic polarizing microscope carrying the sections is rotated.

 

            A portion of a dark cross, with or without rings is shown by the sections oblique to the optic axis. It should be noted that the center of the cross id not in the axis of rotation. It can be also noticed that the center would describe a circle and the arms would always maintain a parallel position when the stage of the polarizing microscope that bears the section is revolved. The center of the interference cross may be outside the field of view if the section is still more oblique to the optic axis, and the only portion that would be visible is the portions of the dark arms.

 

            A vague dark cross is shown by sections parallel to the optic axis. This, with the rotating stage, would dissolve into hyperbolic curves and would actually suggest a biaxial figure. This figure on the other hand would rapidly disappear in the direction of the optic axis. When the interference figures would show colors under the polarizing microscope, these colors are usually lower in order in the quadrants containing the optic axis. Thus, by the method of parallel light, the optical character of the mineral can be obtained when the position of the optic axis is known.

 

            The cross and the rings that are clearly and sharply defined are shown by the sections which are thick and have strong double refraction. There can be a number of rings that can be found crowded close together. A broad dark cross and no rings are usually shown by sections that are very thin and with weak double refraction. The interference figures will vary between these extremes and are dependent on the thickness of the section and the strength of the double refraction.

 

            In optical mineralogy, in order to obtain the most characteristic figure, a good observation must be made on sections about perpendicular to the optic axis. That is, sections which remain dark or nearly dark during the complete rotation of the stage of the petrographic polarizing microscope between crossed nicols in parallel light.



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