Anisotropic minerals/interference

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Anisotropic minerals/interference
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• Isotropic minerals velocity of light same in all
  directions. 1 index of refraction
• Isometric crystal system
• Anisotropic minerals velocity of light varies.
• Light split into two rays vibrate perp. to each
  other
• 2 velocities, 2 indices of refraction

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1) Light passes through the lower polarizer
Plane polarized light
Unpolarized light
PPLplane polarized light
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2) Insert the upper polarizer
west (left)
east (right)
Now what happens? What reaches your eye?
Why would anyone design a microscope that
prevents light from reaching your eye???
XPLcrossed nicols (crossed polars)
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3) Now insert a thin section of a rock
west (left)
Unpolarized light
east (right)
Light vibrating E-W
Light vibrating in many planes and with many
wavelengths
How does this work??
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Conclusion has to be that minerals somehow
reorient the planes in which light is vibrating
some light passes through the upper polarizer
But, note that some minerals are better magicians
than others (i.e., some grains stay dark and thus
cant be reorienting light)
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4) Note the rotating stage
Most mineral grains change color as the stage is
rotated these grains go black 4 times in 360
rotation-exactly every 90o
These minerals are anisotropic
Glass and a few minerals stay black in all
orientations
These minerals are isotropic
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All anisotropic minerals can resolve light into
two plane polarized components that travel at
different velocities and vibrate in planes
that are perpendicular to one another
Some light is now able to pass through the upper
polarizer
fast ray
slow ray
mineral grain

• When light gets split
• velocity changes
• rays get bent (refracted)
• 2 new vibration directions
• usually see new colors

plane polarized light
W
E
lower polarizer
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Calcite double refraction

• Two rays
• Two velocities
• Each anisotropic mineral has one orientation so
  that it behaves as if isotropic
• Optic axis

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Birefringence

• Difference between two indices of refraction
• ? Nslow-nfas
• Fixed number
• Quartz ? 0.009

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Retardation

• Amount that slow ray lags behind fast ray
• Depends on thickness of mineral
• ? d(ns-nf)

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Interference colors

• 1 wavelength
• Slow and fast rays exit mineral, resolved into
  vibration direction at upper polar
• If in phase, vector perpendicular to polarizer,
  so cancel

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Interference colors

• 1 wavelength
• Rays constructively interfere, light passed
  through upper polar

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• Scopes polychromatic light
• Some colors destructively interfere, some color
  constructively interfere
• What we see if color that passes upper polar

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Mineral properties interference
colors/birefringence

• Colors one observes when polars are crossed
  (XPL)
• Color can be quantified numerically d
  nhigh - nlow

Now do question 4
More on this next week
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Estimating birefringence
1) Find the crystal of interest showing the
highest colors (D depends on orientation) 2) Go
to color chart thickness 30 microns use 30
micron line color, follow radial line through
intersection to margin read birefringence
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Example Quartz w 1.544 e 1.553
Data from Deer et al Rock Forming Minerals John
Wiley Sons
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Example Quartz w 1.544 e 1.553
Sign?? () because e gt w e - w
0.009 called the birefringence (d) maximum
interference color (when seen?) What color is
this?? Use your chart.
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Color chart
Colors one observes when polars are crossed
(XPL) Color can be quantified numerically
d nhigh - nlow
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Example Quartz w 1.544 e 1.553
Sign?? () because e gt w e - w
0.009 called the birefringence (d) maximum
interference color (when see this?) What color is
this?? Use your chart. For other orientations
get e' - w progressively lower color Rotation
of the stage changes the intensity, but not the
hue Extinct when either privileged direction N-S
(every 90o) and maximum interference color
brightness at 45o 360o rotation 4 extinction
positions exactly 90o apart
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If this were the maximum interference color seen,
what is the birefringence of the mineral?
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Extinction

• Anisotropic minerals go dark (extinct) every 90
  in cross polars

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Extinction angle
Extinction behavior is a function of the
relationship between indicatrix orientation and
crystallographic orientation
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Extinction angle parallel extinction

• All uniaxial minerals show parallel extinction
• Orthorhombic minerals show parallel extinction

(this is because xtl axes and indicatrix axes
coincide)
orthopyroxene
PPL
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Extinction angle - inclined extinction
Monoclinic and triclinic minerals indicatrix
axes do not coincide with crystallographic
axes These minerals have inclined extinction
(and extinction angle helps to identify them)
clinopyroxene
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• Parallel extinction
• Inclined extinction
• Symmetrical extinction (if two cleavage
  directions)
• Undulatory extinction