Lecture 16 (11/20/2006) Analytical Mineralogy Part 3: Optical Properties of Biaxial Minerals

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 IsotropicIsometric Anisotropic – Uniaxial TetragonalHexagonal Anisotropic-BiaxialOrthorhombicMonoclinicTriclinic

Biaxial Indicatrix Principal vibration axes n greatest n n lowest n n intermediate n < ’<<’<

Circular Sections and Optic Axes Circular Section OpticAxes OpticPlane

2V and the Optic Sign + - Trace of Circular Sections

Random Section through the Biaxial Indicatrix Vibration plane parallel to stage Double refraction rays

Variable Birefringence within a Biaxial Mineral =0 =max

Biaxial Optic Axis Figures Look for a mineral with the lowest interference colors, i.e. ~0

Acute Bisectrix Figures (Bxa) Melatope (emergence of optic axes)

Off-centered Bxa figure

Determining the Optic Sign of Biaxial Minerals D + D - U U D D D D ’ is fast ray  is intermediate ’ is slow ray - + X X U U U U

Estimating 2V by Curvature of Isogyre

Estimating 2V by Separation of Isogyres

Extinction Angle Symmetrical ParallelInclined

Sign of Elongation slowray Example – Length slow Interference colors increase Slowing down the slow ray

Next Lecture 11/22/06 Principles of X-ray Mineralogy Read: Klein p