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