(Lab13)_Uniaxial_Interference_Figures_F10

(Lab13)_Uniaxial_Interference_Figures_F10 - Optical...

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1 Optical Mineralogy Lab 13 – Fall, 2010 Uniaxial Interference Figures
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2 Conoscopic Observation In order to observe an interference figure the microscope must be used in the conoscopic mode Conoscopic refers to the cone-shaped illumination obtained when the condenser lens is near the thin section This requires that the following conditions be met
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3 Conoscopic Technique A. Analyzer inserted and crossed with respect to polarizer (CN) B. Objective lens with a numerical aperture (N.A.) 0.65 must be used C. The condensing lens must be moved (or swing-out lens inserted) to focus the light on a small area D. The Bertrand lens must be inserted
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4 Choosing a Grain Choose a grain that stays in extinction or has very low colors You are trying to locate a grain with its optic axis perpendicular to the slide You want to be looking along the optic axis, or as close as you can possibly get – this produces a “centered” optic axis figure How close that is depends on the birefringence of the mineral
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5 Choosing a Grain, II For quartz, the grain must be almost black at all times, for olivine, first-order gray will do For calcite, any recognizable interference color will probably work Try to be at least in the lower 10% of the mineral's color range Sometimes you just can't do it with a given thin section, especially if the mineral you're dealing with has only tiny grains or very few of them
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6 Conoscopic vs. Orthoscopic Observation Diagram compares the two types of viewing
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7 Conoscopic Procedure Select a grain whose interference you wish to check Make sure the cover slip is facing up Move the grain to the center of the stage Be sure you are in CN (are the polars crossed?) Focus at low power Make sure you are not focused on a crack or impurity in the grain
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8 Conoscopic Procedure, II Increase to medium power, double check focus Move up to high power and double check focus Be sure to raise or flip in the auxiliary condenser lens
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9 Bertrand Lens Insert the Bertrand lens If your scope does not have a Bertrand lens, remove the eyepiece and look down the microscope tube An interference figure should appear – rotate the stage to see if there is any change
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10 No Interference Figure? Check that the microscope is in the correct configuration Check that the grain on high power is not focused on a crack or impurity Also check that the high power objective is properly centered
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11 Uniaxial Minerals The optical class uniaxial has minerals from two mineral systems: Tetragonal – A 4 Hexagonal o Rhombohedral division A 3 o Hexagonal division A 6 Each system has a unique high order axis, as shown – this is the optic axis
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12 Quadrant Labels The quadrants are labeled starting in the upper right and going counterclockwise Roman numerals are used to designate quadrants
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13 Optic Axis The optic axis is designated as the crystallographic “Z” axis
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(Lab13)_Uniaxial_Interference_Figures_F10 - Optical...

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