Leave a message

Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The methods used for testing uniaxial figures are employed when the axial angle is very small, so that the interference figure approaches that of a uniaxial crystal. However, in optical mineralogy, there are several methods used if the axial angle is large.

 

            The stage of the petrographic polarizing microscope is rotated until the plane of the optic axes makes an angle of about 45-degree with the planes of vibration of the crossed nicols or the cross-wires in the eyepiece after having obtained an interference figure from a section as nearly at right angles to the acute bisectrix as possible. The trace that can be found on the plane of the section is the line joining the points of emergence of the two optic axes.

 

            For the axis to be either at right angles or parallel to the plane of the optic axes of the mineral section, a quartz wedge is pushed in between the mineral section and the analyzer of the polarizing microscope.

 

            When the ellipse surrounding the points of emergence of the two optic axes on the convex sides of the hyperbola appear to expand or open out towards the center when the quartz wedge is pushed in with its axis parallel to the plane of the optic axes, the optical character of the mineral is positive. On the other hand, when the ellipses appear to expand or open out when the wedge is pushed in with its axis at right angles to the plane of the optic axes, the optical character is negative.

 

            When the ellipses expand, they move from the points of emergence of the optic axes towards the center of the interference figure, and then finally opening into the lemniscates which move outward from the plane of the optic axes.

 

            The test can be made but only the black hyperbolas without the ellipses can be found when evaluating the sections that are very thin and with weak double refraction. Then, colored ellipses will appear after pushing in the quartz wedge, which will act in the same way as the ellipses of the interference figure. These results are reversed for sections at right angles to the obtuse bisectrix.

 

            Rotate the section until the plane of its axes is 45-degree to the planes of vibration of the nicols when the section is perpendicular to one optic axis. It will be found that the interference figure will have the appearance of the hyperbola being convex towards the acute bisectrix. The one-fourth undulation mica plate should be inserted so that its direction is parallel to the plane of the optic axes. The hyperbola will move towards the acute bisectrix if the optical character of the section is positive. However, if it is negative the hyperbola will move away from it. If a gypsum plate is used, the blue color will appear on the convex side for positive character and on the concave side for the negative minerals.



Author:
Time:
Monday, April 28th, 2008 at 3:22 am
Category:
Optical Mineralogy
RSS:
You can follow any responses to this entry through the RSS 2.0 feed.
Navigation:
Click Here For Best Selection Of High Quality Polarizing Microscope