NESSE: Read pages 124-127. We are going to look at some different examples of minerals through the microscope. We will look at a variety of minerals with different birefringences. 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. What you need to know: The interference color or retardation is determined by the thickness of the mineral and by the difference in the indices of refraction (this is called the birefringence). Isotropic substances are black, that is no separation. Those with little difference in Nslow-Nfast have gray, white or yellow colors. Those with greater difference in N (no matter what the N is) go thru specific sequences of color with the highest order being pink-green in olivine or the creamy color of calcite/dolomite that is finely speckled with pink-green colors. Generally thin sections of rocks are cut to .03 mm thick (30 microns or 30,000 nm) so distance is constant. Check out figure 7.18 and the large fold out color chart in the back of the book. You should be able to read this chart and know how to read birefringence colors. The birerfringences of common minerals are given on the back of the chart. Extra Reading from Greg Finn's Optical Mineralogy Page Interference Phenomena Retardation Interference at the Upper Polar Monochromatic Light and the quartz wedge Polychromatic Light--The Michel-Levy Color Chart of Birefringence