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Light in Minerals 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)– Birefringence (velocity is a function of direction)

Refraction and Reflection• When light strikes a polished surface

of a material it is split into two rays.• One is reflected and the other

refracted.

Dispersion

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)– Birefringence (velocity is a function of direction)

Pleochroism:Tourmaline in

Plane-Polarized Light

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Birefringence:The difference in the indices

Birefringence

Birefringence:The difference in the indices 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

Oculars

Bertrand LensAnalyzer

Accessory Plate

Objectives

Stage

Polarizer

Focus

Scope

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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.• Oil has higher dispersion than mineral

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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

Pleochroism:Tourmaline in

Plane-Polarized Light

Birefringence:The difference in the indices

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.

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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 IndicatrixOrthoscope

• 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

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Uniaxial Optic Axis

Figure

Uniaxial Sign

Uniaxial Sign

Positive + Negative -

The Biaxial Indicatrix

Biaxial Figures Biaxial Sign

Positive + Negative -

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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 – Crossed polars

• High-Power – Plane-Polarized light • Crossed polars

• Conoscope (check out a few grains of each mineral)

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 optic class/sign of unknown minerals

High Power - Conoscope

• Check for oriented inclusions.• Look at accessory or other fine-

grained phases.

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Reflected Light

• Use reflected light to examine opaques– Color and contrast– Bireflectance – Inclusion textures

Textures• Phenocrysts: large crystals that grow in

volcanic rock prior to eruption.

• Cumulates: early-formed crystals that settle in gabbros.

• Exsolution: crystals that grow inside other crystals (sub solidus growth).

• Twinning: orientation accidents in crystal growth or sub-solidus ordering.

• Zoning: concentric changes in composition as crystals grow.

• Reaction rims: chemical reactions at crystal rims.

Sandstone Texture (cumulate)

Zoned Feldspar Phenocryst Twinned K-Feldspar Phenocryst

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Twinned Plagioclase Feldspar (growth twins)

Twinned Feldspar

Exsolved Pyroxene Twinned Exsolved Pyroxene

Exsolved Pyroxene Zoned Pyroxene with cumulate olivine

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Deformation Twinning in Calcite

Deformation Twinning Kyanite

Wadsleyite lamellae in olivine Reaction Rim on Coesite

Optics Terms• Dispersion• Pleochroism• Index of refraction• Anisotropy• Birefringence• Uniaxial• Biaxial• Conoscope• Orthoscope

• Polarizer• Analyzer• Indicatrix• Becke line• Refraction• Diffraction