D. DETERMINATION OF OPTIC SIGN
By convention, if n > n the mineral is optically positive and if n < n , it is negative.
The accessory plate on your microscope is termed a mica plate. Clear the microscope stage of any slide, insert the mica plate, and cross the polars. You will see a first order red, a color sometimes referred to as the sensitive tint.

The mica plate is oriented so that the fast direction, the lower n, lies along the axis of the plate and the slow direction, higher n, lies at a right angle to the long axis of the plate.

Note that the plate is oriented at 45 degrees to the polars.
1. Sketch a uniaxial optic axis figure. Show the orientation of the e-rays and the o-rays in this figure. Explain qualitatively the color changes that occur in a positive optic axis figure when the mica plate is inserted.


2. Examine the slides with oriented quartz (1,2,3,4,5,6) BH-250-36, and calcite BH-250-35 and determine the optic signs of these minerals using the mica plate.


3. Next, go back to the thin section of quartz (1,2,3,4,5,6) and calcite-rich rock, BH 250-27, and 88-C5. Find grains that are oriented so you can determine optic sign. Are your results consistent with what you saw in question 2 above?


4. In the thin sections you will look at it is not always possible to find a perfectly or nearly perfectly oriented crystal. The best optic axis figure you may find might be highly off-center so you can see only one arm of an isogyre. In this case, how can you determine the optic sign? Explain using appropriate sketches.

 

Click here to print entire optics lab 7