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Biaxial interference figures, sign determination, and other properties

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Axis in acute angle of 2V is Bxa. Obtuse angle of 2V is Bxo. If X is in Bxa, biaxial negative ... Obtuse bisectrix figure. Looking down Bxo. Grains with ... – PowerPoint PPT presentation

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Title: Biaxial interference figures, sign determination, and other properties


1
Biaxial interference figures, sign determination,
and other properties
2
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

3
Biaxial interference figures
  • Obtained the same way as uniaxial
  • Only way to tell if mineral is biaxial or uniaxial

4
Biaxial interference figures
  • Acute bisectrix figure
  • Grains with intermediate interference colors
  • Isogyres centered around melatope

5
Biaxial interference figures
  • Acute bisectrix figure
  • Grains with intermediate interference colors
  • Isogyres centered around melatope

6
Biaxial interference figures
  • Acute bisectrix figure
  • Grains with intermediate interference colors
  • Isogyres centered around melatope

7
Formation of isogyres
  • Light extinct in cross-polars when parallel to
    both polarizers

8
Formation of isogyres
  • Light extinct in cross-polars when parallel to
    both polarizers

9
Optic axis figure
  • Looking down one of the optic axes if 2V lt30

10
Optic axis figure
  • Looking down one of the optic axes if 2V gt 50

11
Optic axis figure
  • Looking down one of the optic axes if 2V gt 50

12
Obtuse bisectrix figure
  • Looking down Bxo
  • Grains with intermediate interference colors

13
Obtuse bisectrix figure
  • Looking down Bxo
  • Grains with intermediate interference colors
  • After 5-15 rotation, isogyres out of field of
    view

14
Optic normal figure
  • Optic normal (Y-axis) is vertical
  • Grains show max interference colors
  • Flash figure

15
Optic normal figure
  • Optic normal (Y-axis) is vertical
  • Grains show max interference colors
  • Flash figure
  • Rotate stage only few degrees, isogyres gone

16
Off centered figure
  • With no optic axis, or axis is vertical, grain is
    randomly oriented
  • Common orientation
  • Good luck!

17
Optic sign determination
18
Biaxial optic signacute bisectrix figure
19
Biaxial Optic Sign
B(-)
100 gray 550 650 blue
add
subtract
add
100 gray - 550 450 yellow
20
Biaxial Optic Sign
B()
sub
add
sub
21
Optic axis figure
22
Determining 2V
  • Look at curvature of isogyre
  • Or separation of isogyres (Fig. 7.52 in Nesse)

23
Best minerals for sign determination
  • Lowest interference colors, puts optic axis
    closest to vertical

24
Other optical properties
  • Extinction angle angle between length of mineral
    or prominent cleavage and extinction

25
Extinction angle
Extinction behavior is a function of the
relationship between indicatrix orientation and
crystallographic orientation
26
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
27
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
28
Twinning
Presence and style of twinning can be diagnostic
Twins are usually most obvious in XN (upper
polarizer in)
29
Twinning - some examples
Clinopyroxene (augite)
Plagioclase
30
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

31
Cleavage
32
Cleavage
random fractures, no cleavage olivine
33
(No Transcript)
34
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
35
Sign of Elongation
If b elongation Sometimes will add length
slow Sometimes will subtract length fast
g
b
b
a
36
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

37
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

38
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?

39
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

40
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
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