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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