Light in Minerals II

Light in Minerals II
Light in Minerals
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Petrographic
Microscope
Becke Line Method
Polarization: Vibration directions
The Petrographic Microscope
Measuring Index of Refraction
Pleochroism
Birefringence: Optical anisotropy
The Indicatrix:
Uniaxial Minerals
Biaxial Minerals
Scope
I
grain
>I
oil
• 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.
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Igrain > Ioil
I
grain
=I
I
grain
<I
oil
Dispersion
oil
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
• The index of refraction (velocity)
varies with wavelength.
• Blue is bent more than red.
Pleochroism
• Pleochroism is
different
absorption
spectra for light
vibrating in
different
directions
• Pleochroism is
observed noncubic Fe-bearing
minerals
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Pleochroism:
Tourmaline in PPL
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
• 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.
Birefringence
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 Uniaxial Indicatrix
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Orthoscope
• Normal imaging
setup
• Parallel light from
below
• Condenser out
• Bertrand 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 Sign
Conoscope
• Convergent light
• Condenser in
• High power
objective
• Analyzer in
• Bertrand in or
ocular out
Uniaxial
Optic
Axis
Figure
The Biaxial Indicatrix
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Biaxial
Figures
Thin Section of Gabbro
Estimate 2V
from Optic
Axis Figure
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:
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–
–
–
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
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High Power - Orthoscope
• Check for oriented inclusions.
• Look at accessory or other finegrained phases.
High Power - Conoscope
Reflected Light
• Use reflected light to examine
opaques
– Color and contrast
– Bireflectance
– Inclusion textures
• Check optic class/sign of unknown
minerals
Thin Section of Gabbro
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