Biaxial interference figures, sign determination, and other properties

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Biaxial interference figures, sign determination,
and other properties
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Biaxial minerals

• 2 optic axes
• 3 indices of refraction
• X Y Z
• N?ltn?ltn?
• Axis in acute angle of 2V is Bxa
• Obtuse angle of 2V is Bxo
• If X is in Bxa, biaxial negative
• If Z is in Bxa, biaxial positive
• Optic plane X-Z plane
• Optic normal Y axis

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Biaxial interference figures

• Obtained the same way as uniaxial
• Only way to tell if mineral is biaxial or uniaxial

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Biaxial interference figures

• Acute bisectrix figure
• Grains with intermediate interference colors
• Isogyres centered around melatope

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Biaxial interference figures

• Acute bisectrix figure
• Grains with intermediate interference colors
• Isogyres centered around melatope

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Biaxial interference figures

• Acute bisectrix figure
• Grains with intermediate interference colors
• Isogyres centered around melatope

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Formation of isogyres

• Light extinct in cross-polars when parallel to
  both polarizers

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Formation of isogyres

• Light extinct in cross-polars when parallel to
  both polarizers

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Optic axis figure

• Looking down one of the optic axes if 2V lt30

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Optic axis figure

• Looking down one of the optic axes if 2V gt 50

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Optic axis figure

• Looking down one of the optic axes if 2V gt 50

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Obtuse bisectrix figure

• Looking down Bxo
• Grains with intermediate interference colors

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Obtuse bisectrix figure

• Looking down Bxo
• Grains with intermediate interference colors

• After 5-15 rotation, isogyres out of field of
  view

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Optic normal figure

• Optic normal (Y-axis) is vertical
• Grains show max interference colors
• Flash figure

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Optic normal figure

• Optic normal (Y-axis) is vertical
• Grains show max interference colors
• Flash figure

• Rotate stage only few degrees, isogyres gone

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Off centered figure

• With no optic axis, or axis is vertical, grain is
  randomly oriented
• Common orientation
• Good luck!

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Optic sign determination
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Biaxial optic signacute bisectrix figure
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Biaxial Optic Sign
B(-)
100 gray 550 650 blue
add
subtract
add
100 gray - 550 450 yellow
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Biaxial Optic Sign
B()
sub
add
sub
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Optic axis figure
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Determining 2V

• Look at curvature of isogyre
• Or separation of isogyres (Fig. 7.52 in Nesse)

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Best minerals for sign determination

• Lowest interference colors, puts optic axis
  closest to vertical

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Other optical properties

• Extinction angle angle between length of mineral
  or prominent cleavage and extinction

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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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Twinning
Presence and style of twinning can be diagnostic
Twins are usually most obvious in XN (upper
polarizer in)
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Twinning - some examples
Clinopyroxene (augite)
Plagioclase
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Cleavage
Most easily observed in PPL (upper polarizer
out), but visible in XN as well

• No cleavages quartz, olivine
• 1 good cleavage micas
• 2 good cleavages pyroxenes, amphiboles

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Cleavage
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Cleavage
random fractures, no cleavage olivine
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(No Transcript)
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Sign of Elongation
If g elongation will always add length slow
If a elongation will always subtract length
fast
g
a
U(-) will also length fast
U() will also length slow
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Sign of Elongation
If b elongation Sometimes will add length
slow Sometimes will subtract length fast
g
b
b
a
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Pleochroism in biaxial minerals

• Since 3 indices of refraction 3 colors (1 per
  axis)
• Light parallel to X axis is least strongly
  absorbed. Z is most strongly absorbed
• Hornblende x yellow y pale green, z dark
  green

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Pleochroism in biaxial minerals

• For Y, find grain nearly isotropic, uncross
  polars
• For X and Z, find grain with highest interference
  colors
• Rotate to extinction, then 45 clockwise, slide
  in accessory plate, determine if addition or
  subtraction
• If color increases, addition, slow ray, Z ray
• Rotate another 45 clockwise, uncross polars and
  note color
• For other axis, rotate 90 from here

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Review techniques for identifying unknown
minerals

• Start in PPL
• Color/pleochroism
• Relief
• Cleavages
• Habit

• Then go to XN
• Birefringence
• Twinning
• Extinction angle
• Uniaxial or biaxial?
• 2V if biaxial
• Positive or negative?

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Go to Nesse or similar book

• Chemical formula
• Symmetry
• Uni or biaxial, () or (-)
• RIs lengths of indicatrix axes
• Birefringence
• 2V if biaxial

• Diagrams
• Crystallographic axes
• Indicatrix axes
• Optic axes
• Cleavages
• Extinction angles

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Another example
Crystallographic axes a, b, c
Indicatrix axes X, Y, Z or e, w
Then read text re color, pleochroism, habit,
cleavage, twinning, distinguishing features,
occurrence make sure properties match your
observations. If not, check another mineral