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Lecture 15 (11/15/2006)Analytical MineralogyPart 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 Anisotropic- Biaxial Orthorhombic Monoclinic Triclinic Anisotropic – Uniaxial Tetragonal Hexagonal Isotropic Isometric
Isotropic Indicatrix A sphere whose radius corresponds to the characteristic refraction index- n n=c/v =c/ 5893ÅYellow 4861ÅBlue 6563Å Red 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)
Anistropic Minerals Fast ray Slow ray 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 UniaxialMinerals Single Refraction through Circular and Principal Sections of UniaxialMinerals
Re-Polarization of Light through a Non-circular Section of the Uniaxial Indicatrix
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