Analytic Mineralogy - Optical Properties Of Uniaxial Minerals p227

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