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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 biaxial minerals is one of the concerns of mineralogists in optical mineralogy. It can be best determined with the test made with the use of petrographic polarizing light microscope. The curves being nearly circular and a straight black bar bisecting these curves can be found as interference figure exhibited by sections perpendicular to an optic axis. This figure can be found whenever the trace of the plane of the optic axes coincides with the vibration direction of either nicol of the polarizing microscope. The bar changes into one arm of a hyperbola and back again into a bar when the stage of the petrographic polarizing microscope that carries the section is rotated. When closely examined, this arm or bar will rotate in the opposite direction to the motion of the stage.

 

            As previously noted, in optical mineralogy, sections of biaxial crystals that are found perpendicular to an optic axis usually do not remain dark during rotation of the stage of polarizing microscope between crossed nicols in parallel light. However, these sections are commonly found continuously or remain uniformly illuminated under several stage rotation of the petrographic polarizing microscope.

 

            Interference figures exhibited by sections perpendicular to the acute bisectrix appear like dark cross that seems to dissolve into two branches of a hyperbola, which again unite to form a cross. The interference figure exhibited by sections parallel to the bisectrix or somewhat oblique to the optic axis would show small ellipses and some black hyperbolic curve marks at different positions. The interference figures of some sections in other positions relative to the optic axes are less definite in appearance than the ones first described. The same conditions affect the appearance of all figures as in the case of uniaxial minerals. Indistinct dark crosses or hyperbolic curves, without any ellipse, are usually shown by very thin sections of weak double refractions.

 

            The most characteristic interference figure is usually shown by sections that are perpendicular to the acute bisectrix. But then, this section cannot generally be recognized except by an examination in convergent light of petrographic polarizing microscope. It is however important to note that this section never give the maximum interference color. The interference colors in different sections grade downward from optical normal, obtuse bisectrix, acute bisectrix, to optic axis. There are also some times where the acute bisectrix is normal to the cleavage.

 

            It is important to note that the uniaxial or biaxial character of mineral section shows a distinct bar as discussed in optical mineralogy. In uniaxial interference figure, a bar or the one arm of the cross moves in the same direction as the stage of the petrographic polarizing microscope rotates and always remains straight. The biaxial bar on the other hand rotates in the opposite direction to the stage and becomes curved.



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Monday, April 28th, 2008 at 3:23 am
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Optical Mineralogy
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