GEOL 212: Physical and Optical Mineralogy
OPTICAL CRYSTALLOGRAPHY--Extinction, Accessory Plates, and the Michel Levy Color Chart
NESSE: Read pages 127-130.
Light hitting a crystal can do one of three things. It can be transmitted (moved along the crystal lattice), absorbed in the crystal lattice, or reflected. Opaque minerals are those that absorb all light, even in thin section. These minerals may also reflect certain colors on their surface (like the gold color of pyrite). Non-opaque minerals transmit at least some or all of the light entering them.
ALL light coming thru the micrscope passes thru a polarizer on the bottom that orients it in only one plane. Thus, when the upper polarizer is out (not engaged) the light coming thru is called plane polarized light or plane light. Clear minerals do not absorb any color. Minerals that absorb certain wavelengths or combinations of wavelengths have a characteristic color. If the absorbtion differs along different axes, then the color varies as you spin the stage. This is called pleochrosim. The type of pleochroism is characteristic of certain minerals, just as birefringence is.
Minerals go extinct during rotation under crossed polars (both polarizers in, polarized light). Every time one of the 2 rays lines up with the polarizing directions, the mineral goes extinct (it loses 1 ray, so you can think of it becoming temporarily isotropic). See Fig. 7.19 on page 127. Whether the extinction is parallel to a crystallographic feature such as cleavage or grain elongation depends on whether the crystallographic axes line up with the optical axes.
Birefringence is dependent on the difference between the slow and fast indices of refraction, or in the case of minerals where a,b,c are not equal, the maximum and minimum indices of refraction. Another measure of the index of refraction is called relief. This is how well grain edges, cleavage and other fractures, stand out in plane light. In thin sections this is a function of how much the minerals index (or indices) of refraction differ from those of the epoxy and glass the mineral is mounted on (N=1.54). If the mineral has a similar n, it is called low relief, higher n is moderate relief, very high n produces high relief. These are all called positive relief as Nmineral >Nepoxy. If Nmineral < Nepoxy, the relief is negative and just the opposite occurs. Isometric minerals (which are isotropic) have relief just like other minerals, but it is constant in all crystallographic directions. Just as pleochroism and birefringence varie with orientation, so does relief in anisotropic minerals. Minerals with a large difference in indices of refraction may show low and high relief in different orientations.
What you need to know:
Understand what causes extinction and pleochroism. What separates pleochroism from a non-pleochroic color. Understand how to read the Michel-Levy color chart (the repetitive color patterns and how they differ for 1st, 2nd, and 3rd orde) and how to use the gypsum (1 wavelength) and mica (quarter wavelength plates). Know the difference between birefringence and retardation. Know what relief is and how it differs from birefringence.
READING FROM GREG FINN'S OPTICAL MINERALOGY PAGES
If you have comments or suggestions, email me at email@example.com