Light in Minerals 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 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.

I grain > I oil

Igrain > Ioil

I grain < I oil

I grain = I oil

Dispersion The index of refraction (velocity) varies with wavelength. Blue is bent more than red.

Optical Anisotropy Pleochroism: Birefringence: 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

Pleochroism: Tourmaline in PPL

Birefringence

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.

The Uniaxial Indicatrix

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

Uniaxial Optic Axis Figure

Uniaxial Sign

The Biaxial Indicatrix

Biaxial Figures

Estimate 2V from Optic Axis Figure

Thin Section of Gabbro

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. High Power - Conoscope Check optic class/sign of unknown minerals

Reflected Light Use reflected light to examine opaques Color and contrast Bireflectance Inclusion textures

Thin Section of Gabbro