1.8 color and pleochroism

1.8 color and pleochroism - Mineral color and pleochroism...

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Unformatted text preview: Mineral color and pleochroism Color Many minerals are colorless True color Not interference colors Observed in plane polarized light Not crossed nicols Pleochroism Property of having two or more true colors Occurs only in anisotropic minerals Each principal vibration direction has a unique color Biotite a pleochroic mineral Specific color displayed when vibration direction parallel to polarized direction Slow ray has one color Fast ray another color Color intermediate if neither direction parallel to polarized direction Uniaxial Pleochroism Described by "pleochroic formula" Multiple types of formulas: 1. 2. 3. Color of and rays Greater absorbance e.g. > or > "strongly" or "weakly" pleochroic Determination of formula Find grain with = 0 Find grain with maximum This is value of color This has both and Already know , so other color must be Grains seen in plane polarized light (not crossed nicols) Fig. 730 Biaxial Pleochroism Biaxial minerals may have three colors: Procedure similar to uniaxial minerals, but more complex One for , , and Find extinct section color Find maximum this grain has and colors Determine fast and slow direction with accessory plate Extinction Four Categories: Parallel extinction feature parallel to cross hairs at extinction Inclined extinction extinction when at angle Symmetrical extinction occurs in minerals with two cleavages: bisect cleavage No extinction angle minerals with no elongation or cleavage Parallel Inclined Symmetrical No extinction angle Extinction may not be uniform Physically deformed minerals Minerals with variable chemical composition (chemically zoned) Extinction angle Inclined extinction angle between long axis of mineral grain prominent cleavage Twins Other crystallographic feature Extinction angle Long direction, also parallel to cleavage Rotate stage until crystallographic feature is parallel to cross hairs Record angle on goniometer Rotate stage until mineral is extinct Now mineral vibration direction is parallel to polarized light direction Amount of rotation is extinction angle Fig. 731 Possible to determine chemical composition from extinction angle MichelLevy technique MichelLevy Technique Section cut perpendicular to {010} Albite twin lamellae Cut of mineral must be with {010} plane vertical, b crystallographic axis horizontal Characteristics: High plagioclase = volcanic Sharp boundaries between twins Twin lamellae have same interference colorsLow plagioclase = plutonic Albite Nafeldspar b Z Z b Feldspars Triclinic minerals: Two cleavages Many types of twins Extinction angles show relationship between X YZ axes (indicatrix axes) and abc axes (crystallographic axes) Z b Z b Anorthite Cafeldspar p. 215 Sign of Elongation Length fast: elongate direction of mineral parallels fast vibration direction Length slow: elongate direction of mineral parallels slow vibration direction Also called negative elongation Length fast and length slow depends on cut of grain Also called positive elongation Determination Orient grain with vibration direction and length about 45 to polarized direction Use accessory plate to determine addition or subtraction of retardation Determines if fast or slow ray Vibration directions parallel to accessory plate If addition, length slow (positive elongation) If subtraction, length fast (negative elongation) Length fast Length slow Fig. 731 Biaxial Minerals n = elongate n = elongate n = elongate Always length slow Always length fast Either length slow or length fast ...
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This note was uploaded on 07/06/2011 for the course GLY 5245 taught by Professor Staff during the Spring '11 term at University of Florida.

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