A. BIREFRINGENCE
In uniaxial minerals light is transmitted as two discrete rays that are polarized in mutually perpendicular planes. One ray refracts according to Snell's Law and is called the ordinary ray, or the o-ray . The other does not refract according to Snell's Law and is called the extraordinary, or e-ray. The refractive indices in the directions these two rays travel are different and is manifested in the interference colors produced in the microscope under crossed polars. Birefringence of a crystal is the numerical difference between the maximum and the minimum indices of refraction. Interference colors are a function of birefringence and the thickness and orientation of a crystal. In thin sections of known thickness the birefringence of a mineral can be estimated by the maximum interference color observed in that mineral and can aid in the identification of minerals.


I. Look at one of the slides labeled BH-250-27 [6] (Baraboo) and BH56 [13] under crossed polars. What is the highest interference color that you can see for the dominant mineral (quartz (1,2,3,4,5,6) ) in these two slides? (You might look at a couple different slides.)

i. Assume that this slide is cut to standard thickness, 30 Microns. Use the Michel-Levy chart in your text (the one with the pretty rainbows of colors) to estimate the birefringence of quartz (1,2,3,4,5,6) . What is the value you get?


ii. How does this value compare with the one listed in your text or Bloss or Kerr?


iii. If you could identify quartz (1,2,3,4,5,6) by other means so you were sure you were looking at it, how could you tell if a quartz-bearing thin section was too thick?

II. quartz (1,2,3,4,5,6) has low birefringence. The next slide you will look at will contain a mineral of moderate birefringence. Since uniaxial minerals of moderate birefringence are relatively rare, we will use a common biaxial mineral, Pyroxene. Look at one of the slides labeled, BH-250-9 and locate a phenocryst that appears quite colorful under crossed polars. Some of these phenocryst are pyroxenes (Ask your TA to Show you). What are the highest interference colors you can find in Pyroxene? What value of birefringence does this correspond to (assuming standard slide thickness)?

III. Next, look at a slide labeled 88-A9 [11] (ask your TA to show you a Calcite grain.) Calcite is uniaxial mineral and in the thin sections under crossed polars it is the mineral that displays a whitish or pastel and almost iridescent interference color. What do you think are the highest order interference colors in calcite? What are the lowest? What can you say about the magnitude of birefringence of Calcite?


IV. Look at calcite 88-A9 in plane polarized light (analyzer out). Compare the relief of calcite to that of balsam as you rotate the stage. What do you see? What does this tell you about the birefringence of calcite?


Note: 88-A9 (1, 2, 3, ) also contains
i. Olivine
ii Ludwigite
Check these two minerals in Kerr and see if you can identify them.

 

 

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