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Home , Pleochroism

Light in II Behavior of Light in Minerals

• Absorption (light is absorbed by materials) – Color (absorption is a function of wavelength) – Pleochroism (absorption is a function of direction)

• Refraction (light travels slowly in some materials) – Dispersion (velocity is a function of wavelength) – (velocity is a function of direction)

Refraction and Reflection Dispersion • When light strikes a polished surface of a material it is split into two rays. • One is reflected and the other refracted.

Pleochroism: Behavior of Light in Minerals in

• Absorption (light is absorbed by materials) Plane-Polarized Light – Color (absorption is a function of wavelength) – Pleochroism (absorption is a function of direction)

• Refraction (light travels slowly in some materials) – Dispersion (velocity is a function of wavelength) – Birefringence (velocity is a function of direction)

1 Birefringence: Birefringence The difference in the indices

Birefringence: The difference in the indices Light in Minerals • : Vibration directions • The Petrographic Microscope • Measuring Index of Refraction • Pleochroism • Birefringence: Optical • The Indicatrix: • Uniaxial Minerals • Biaxial Minerals

Scope Petrographic Oculars Microscope Bertrand Lens Analyzer

Accessory Plate

Objectives

Stage

Polarizer

Focus

2 I grain > I oil 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.

Igrain > Ioil I grain < I oil

Dispersion • The index of refraction (velocity) I grain = I oil varies with wavelength. • Blue is bent more than red. • Oil has higher dispersion than

3 Optical Anisotropy Pleochroism • Pleochroism: – Different absorption for light vibrating in • Pleochroism is different directions. different – Mineral grains change color on rotation absorption in plane-polarized light. spectra for light vibrating in • Birefringence: different – Different velocities (index or refraction) directions for light vibrating in different directions. • Pleochroism is – Mineral grains change color on rotation observed non- between crossed polarizers cubic Fe-bearing minerals

Pleochroism: Birefringence: Tourmaline in The difference in the indices Plane-Polarized Light

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.

4 The Optical Indicatrix The Optical Indicatrix • For Hexagonal, Tetragonal and • The vibration direction of light Trigonal the indicatrix is an determines the operative index of ellipsoid of rotation. There is one refraction in anisotropic media. circular section, and these crystals • The index of refraction is represented are said to be uniaxial. by an ellipsoid called the indicatrix. • For Orthorhombic, Monoclinic, and • The elliptical section perpendicular to Triclinic crystals the indicatrix is a the ray (propagation direction) general ellipsoid. There are two determines the operative indices of circular sections. These crystals are refraction. said to be biaxial.

Orthoscope The Uniaxial Indicatrix

• Normal imaging setup • Parallel light from below • Condenser out • Bertrand out

Conoscope Conoscope

• Convergent light • Find a grain near its optic axis • [Condenser in] • Gives a figure on back of objective • High power lens objective • Figure gives optic class and optic • Analyzer in sign • Bertrand in or ocular out • Optic class = isotropic, uniaxial, or biaxial • Optic sign = positive or negative

5 Uniaxial Uniaxial Sign Optic Axis Figure

Uniaxial Sign The Biaxial Indicatrix

Positive + Negative -

Biaxial Biaxial Sign Figures

Positive + Negative -

6 Estimate 2V of Gabbro from Optic Axis Figure

What to do when you look Low Power, at a rock in thin section Plane-Polarized Light • Count the number of minerals • Low Power, present. – Plane-Polarized Light • For Each Mineral, Note: – Crossed polars – Grain size • High-Power – Opacity – Plane-Polarized light – Color/Pleochroism • Crossed polars – Relief and Approximate Indices of Refraction (work in from the edge) – Grain Habit or Shape • Conoscope (check out a few grains of each mineral)

Low Power, High Power - Orthoscope Crossed Polarizers • Check for oriented inclusions. • Birefringent or Isotropic ? • Look at accessory or other fine- • Maximum birefringence grained phases. • Extinction (parallel, inclined, symmetric, or asymmetric) High Power - Conoscope • Twinning • Check optic class/sign of unknown minerals

7 Textures

Reflected Light • Phenocrysts: large crystals that grow in volcanic rock prior to eruption. • Use reflected light to examine • Cumulates: early-formed crystals that settle opaques in gabbros. – Color and contrast • Exsolution: crystals that grow inside other – Bireflectance crystals (sub solidus growth). – Inclusion textures • Twinning: orientation accidents in growth or sub-solidus ordering. • Zoning: concentric changes in composition as crystals grow. • Reaction rims: chemical reactions at crystal rims.

Sandstone Texture (cumulate)

Zoned Phenocryst Twinned K-Feldspar Phenocryst

8 Twinned Plagioclase Twinned Feldspar Feldspar (growth twins)

Exsolved Pyroxene Twinned Exsolved Pyroxene

Exsolved Pyroxene Zoned Pyroxene with cumulate olivine

9 Deformation Twinning in Deformation Twinning Calcite

Wadsleyite lamellae in olivine Reaction Rim on

Optics Terms

• Dispersion • Polarizer • Pleochroism • Analyzer • Index of refraction • Indicatrix • Anisotropy • Becke line • Birefringence • Refraction • Uniaxial • Diffraction • Biaxial • Conoscope • Orthoscope

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