Optical Mineralogy Pt.1- Plane & Cross Polarized Light, Birefringence, Pleochroism, etc. | GEO GIRL
Light slows down when traveling through thin sections, this is called retardation. The incident ray of light goes through one material into another and becomes refracted, which is why the ray traveling through the second material is called the refracted ray. The angle at which the light ray is refracted can help us figure out the refracted index (R.I.) of a mineral. But how can we figure out the R.I. of a mineral just by observing it in thin section? Relief. Relief, or relative contrast, which shows up on certain mineral grains in thin section as Becke lines (outlines of the crystal grains), helps us determine R.I. The more relief (the thicker and more apparent the Becke line), the higher the R.I. of the mineral relative to the R.I. of the epoxy that the thin section is mounted in.
In petrographic microscopes (or polarizing light microscopes), light is forced through one or more polarizers that cause the light to vibrate in specific directions. Observing a thin section in plane polarized light just means we are using one polarizer. Crossed polarized light just means we are observing the thin section with two polarizers inserted in the microscope.
Two perpendicularly vibrating light rays are produced when light travels through a thin section, and because these rays are out of phase, when they are recombined in the upper polarizer (or analyzer), they produce interference colors (or birefringence colors). Minerals with a high range of R.I. values have a high birefringence which causes them to produce beautiful, high order interference colors. Minerals with low birefringence produce duller, low order interference colors. Only anisotropic minerals produce interference colors. Isotropic minerals do not produce interference colors, they appear black under cross polarized light.
To measure extinction angles of anisotropic minerals you just need to rotate the microscope stage until your mineral goes into extinction (goes black) and record angle at which that occurs by using the ticks labeled on the stage.
Some minerals exhibit variations in color under plane polarized light as well. These is called pleochroism.
Reference: Earth Materials textbook: https://amzn.to/3DXOUnj
Color atlas of rocks & minerals in thin section: https://amzn.to/3JlbJDg
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Optical Mineralogy Part1:
0:00 light traveling through minerals
1:52 refractive index (R.I.)
4.35 relief & becke lines
6:09 plane & cross polarized light
10:33 isotropic vs. anisotropic minerals
10:47 birefringence & interference colors
13:01 orders of interference colors
14:02 accessory plates
18:00 extinction
19:10 extinction angles
21:05 length fast vs. length slow minerals
21:48 pleochroism
24:02 mineral identification chart
27:00 upcoming videos!
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