Transcript Lecture 15 (11/15/2006) Analytical Mineralogy Part 2: Optical Properties of Uniaxial

Lecture 15 (11/15/2006)
Analytical Mineralogy
Part 2:
Optical Properties of Uniaxial
Minerals
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 value
regardless of the orientation of light rays.
Such substances are said to be isotropic.
Optical Indicatrices
• A 3-d map of the indices of refraction for various vibration
directions of light rays
• Orientation of the indicatrix within a mineral is symmetrical with the
crystallographic axis
Isotropic
Isometric
Anisotropic – Uniaxial AnisotropicBiaxial
Orthorhombic
Tetragonal
Monoclinic
Hexagonal
Triclinic
Isotropic Indicatrix
A sphere whose radius corresponds to
the characteristic refraction index- n
n=c/v
=c/
6563Å Red
5893ÅYellow
4861ÅBlue
Diagram shows change in n for different
wavelengths of light in same mineral
Optical Recognition of Isotropic Minerals
Total Extinction
under X-polars
Slowing of ray
= shortening of
wavelength, but
no change in
polarity
From Bloss (1961)
Slow ray
Fast ray
Anistropic
Minerals
All randomly oriented
anisotropic minerals
cause double refraction
(splitting) of light
resulting in mutually
perpendicular-polarized
light rays.
One ray has a higher n
(slow ray, or the
ordinary ray) than the
other ray (the fast ray,
or extraordinary ray)
Birefringence (), Retardation(Δ), and
Interference Colors
 = nslow ray – nfast ray
Δ = d* 
Uniaxial Indicatrix
Optic Axis
= C axis in tetragonal
and hexagonal crystals
Sections of Uniaxial Indicatrices
 = ω-ω = 0 (circular section)
 = ε’- ω (random section)
= ε - ω (principal section)
maximum birefringence
Total
extinction in
x-polar light
Double Refraction
through Random
Sections of Uniaxial
Minerals
Single Refraction
through Circular
and Principal
Sections of Uniaxial
Minerals
Re-Polarization of Light through a Non-circular
Section of the Uniaxial Indicatrix
Extinction of Uniaxial Minerals
Orthoscopic
Conoscopic
Conoscopic
Interference
Figures of Uniaxial
Minerals
Isochromes – zones of
equal retardation
Isogyres – represent the
areas where the ω and ε’
vibration directions are
oriented N-S, E-W
Uniaxial
Optic Axis
(OA)
Figure
Circular section
parallel to stage
=0
Off-centered OA Figure
Random section parallel to stage,  < 0, « max 
Very Off-centered OA Figure
Random section parallel to stage, 
« 0, < max 
Flash Figure
Principal section parallel to stage, 
= max 
Determining the Optic Sign of
Uniaxial Minerals
+
Connect the quadrants
that go down in color
(to yellow), compare
with slow direction of
gypsum plate for sign
Next Lecture 11/20/06
Optical Properties of Biaxial Minerals
Read:
Klein p. 303-309
Perkins and Henke, p. 22-27