PLM Fundamentals:
Angle of Extinction
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What is Extinction and Extinction Angle? |
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Under cross-polarized light an anisotropic mineral will change color as the stage is rotated (see the explanation of interference). At four positions, the grain will turn black, and is said to have reached EXTINCTION. |
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Extinction occurs whenever the vibration directions in the mineral grain are parallel to either of the vibration directions that either of the 2 perpendicular polarizing filters (polarizer and analyzer) allow to pass. |
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Each of the 4 positions of extinction during rotation is spaced 90 degrees apart. On minerals with a distinct long-axis one can imagine the mineral acting like a pointer. EXTINCTION ANGLE is a measure of angle between the up-down and/or side-to-side directions in the field of view of the microscope and the direction of length of the extinct minerals. |
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When a mineral goes extinct in a straight up-down or sideways orientation, the mineral is said to have... PARALLEL EXTINCTION like the long grain in the center of the picture below |
When a mineral goes extinct in an orientation that is neither up-down nor sideways, but rather some intermediate position, the mineral is said to have... INCLINED EXTINCTION like the striped (twinned) grain in the picture below |
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Example of Parallel Extinction |
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Example of Inclined Extinction |
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How to Determine Extinction Angle on a Polarized Light Microscope |
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Set up the microscope for cross-polarized light (Analyzer In and Gypsum Plate Out) with any immersion medium |
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Find a long grain to study and rotate the stage through 90 degrees while observing the mineral grain |
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Note when the grain goes extinct (turns black) |
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Does the mineral go extinct when it is oriented up-down and sideways? If so, then the mineral displays PARALLEL EXTINCTION. |
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Does the mineral go extinct when it is oriented at some intermediate angle? If so, then the mineral displays INCLINED EXTINCTION. |
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How would you describe the extinction angle of the prismatic mineral in the center of the example below?
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Example of Extinction |
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To see how your description compares to the instructor's...
