390 likes | 396 Vues
Light in Mi ner als II. Light in Minerals. Polarization: Vibration directions The Petrographic Microscope Measuring Index of Refraction Pleochroism Birefringence: Optical anisotropy The Indicatrix: Uniaxial Minerals Biaxial Minerals. Petrographic Microscope. Scope. Becke Line Method.
E N D
Light in Minerals • Polarization: Vibration directions • The Petrographic Microscope • Measuring Index of Refraction • Pleochroism • Birefringence: Optical anisotropy • The Indicatrix: • Uniaxial Minerals • Biaxial Minerals
Becke Line Method • As you increase the working distance, the white line moves to the medium of higher index. • If dispersion curves cross, liquid has higher slope and yellow moves in and blue moves out.
Dispersion • The index of refraction (velocity) varies with wavelength. • Blue is bent more than red.
Optical Anisotropy • Pleochroism: • Different absorption for light vibrating in different directions. • Mineral grains change color on rotation in plane-polarized light. • Birefringence: • Different velocities (index or refraction) for light vibrating in different directions. • Mineral grains change color on rotation between crossed polarizers
Pleochroism • Pleochroism is different absorption spectra for light vibrating in different directions • Pleochroism is observed non-cubic Fe-bearing minerals
Birefringence • Birefringence is the color of a grain in crossed polars. • Birefringence is the difference between the operative indices of refraction of a grain times the thickness of the grain. • The color observed is controlled by the orientation of the grain and the thickness of the grain.
The Optical Indicatrix • The vibration direction of light determines the operative index of refraction in anisotropic media. • The index of refraction is represented by an ellipsoid called the indicatrix. • The elliptical section perpendicular to the ray (propagation direction) determines the operative indices of refraction.
The Optical Indicatrix • For Hexagonal, Tetragonal and Trigonal crystals the indicatrix is an ellipsoid of rotation. There is one circular section, and these crystals are said to be uniaxial. • For Orthorhombic, Monoclinic, and Triclinic crystals the indicatrix is a general ellipsoid. There are two circular sections. These crystals are said to be biaxial.
Orthoscope • Normal imaging setup • Parallel light from below • Condenser out • Bertrand out
Conoscope • Convergent light • Condenser in • High power objective • Analyzer in • Bertrand in or ocular out
Conoscope • Find a grain near its optic axis • Gives a figure on back of objective lens • Figure gives optic class and optic sign • Optic class = isotropic, uniaxial, or biaxial • Optic sign = positive or negative
What to do when you look at a rock in thin section • Low Power, Plane-Polarized Light • High-Power Plane-Polarized light • Crossed Polarizers, orthoscope • Conoscope
Low Power, Plane-Polarized Light • Count the number of minerals present. • For Each Mineral, Note: • Grain size • Opacity • Color/Pleochroism • Relief and Approximate Indices of Refraction (work in from the edge) • Grain Habit or Shape
Low Power,Crossed Polarizers • Birefringent or Isotropic ? • Maximum birefringence • Extinction (parallel, inclined, symmetric, or asymmetric) • Twinning
High Power - Orthoscope • Check for oriented inclusions. • Look at accessory or other fine-grained phases. • Check optic class/sign of unknown minerals High Power - Conoscope
Reflected Light • Use reflected light to examine opaques • Color and contrast • Bireflectance • Inclusion textures