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Lecture 15 (11/15/2006)
Analytical Mineralogy
Part 2:Optical Properties of Uniaxial
Minerals
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Anisotropy
Indicies of refraction can vary in all minerals(except those in the isometric system)depending on the orientation of light ray.
Such minerals are said to be anisotropic.Isometric minerals, glass, liquids and gasses
have a single refraction index valueregardless of the orientation of light rays.Such substances are said to be isotropic.
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Optical IndicatricesA 3-d map of the indices of refraction for various vibration
directions of light raysOrientation of the indicatrix within a mineral is symmetrical with the
crystallographic axis
Isotropic
Isometric
Anisotropic Uniaxial
Tetragonal
Hexagonal
Anisotropic-Biaxial
OrthorhombicMonoclinic
Triclinic
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Isotropic Indicatrix
A sphere whose radius corresponds tothe characteristic refraction index- n
Diagram shows change in nfor differentwavelengths of light in same mineral
4861Blue5893Yellow6563 Red
n=c/v=c/
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Optical Recognition of Isotropic Minerals
From Bloss (1961)
Total Extinctionunder X-polars
Slowing of ray= shortening ofwavelength, butno change inpolarity
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Anistropic
Minerals
All randomly orientedanisotropic mineralscause double refraction(splitting) of light
resulting in mutuallyperpendicular-polarizedlight rays.
One ray has a higher n
(slow ray, or theordinary ray) than theother ray (the fast ray,or extraordinary ray)
Fast raySlow ray
f ( ) ( )
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Birefringence (), Retardation(), and
Interference Colors
= nslow ray
nfast ray
= d*
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Uniaxial Indicatrix
Optic Axis= C axis in tetragonaland hexagonal crystals
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Sections of Uniaxial Indicatrices= -= 0 (circular section)
= - (random section)
= - (principal section)maximum birefringence
Totalextinction inx-polar light
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Double Refraction
through Random
Sections of UniaxialMinerals
Single Refraction
through Circular
and Principal
Sections of Uniaxial
Minerals
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Re-Polarization of Light through a Non-circular
Section of the Uniaxial Indicatrix
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Extinction of Uniaxial Minerals
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Conoscopic
Interference
Figures of UniaxialMinerals
Orthoscopic Conoscopic
Isochromes zones ofequal retardation
Isogyres represent theareas where the and
vibration directions areoriented N-S, E-W
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Uniaxial
Optic Axis(OA)
Figure
Circular sectionparallel to stage
= 0
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Off-centered OA FigureRandom section parallel to stage, < 0, max
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Very Off-centered OA Figure
Random section parallel to stage, 0, < max
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Flash FigurePrincipal section parallel to stage, =max
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Determining the Optic Sign of
Uniaxial Minerals
Connect the quadrantsthat go down in color(to yellow), comparewith slow direction of
gypsum plate for sign
+
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Next Lecture 11/20/06
Optical Properties of Biaxial Minerals
Read: Klein p. 303-309Perkins and Henke, p. 22-27