Title: Biaxial interference figures, sign determination, and other properties
1Biaxial interference figures, sign determination,
and other properties
2Biaxial 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
3Biaxial interference figures
- Obtained the same way as uniaxial
- Only way to tell if mineral is biaxial or uniaxial
4Biaxial interference figures
- Acute bisectrix figure
- Grains with intermediate interference colors
- Isogyres centered around melatope
5Biaxial interference figures
- Acute bisectrix figure
- Grains with intermediate interference colors
- Isogyres centered around melatope
6Biaxial interference figures
- Acute bisectrix figure
- Grains with intermediate interference colors
- Isogyres centered around melatope
7Formation of isogyres
- Light extinct in cross-polars when parallel to
both polarizers
8Formation of isogyres
- Light extinct in cross-polars when parallel to
both polarizers
9Optic axis figure
- Looking down one of the optic axes if 2V lt30
10Optic axis figure
- Looking down one of the optic axes if 2V gt 50
11Optic axis figure
- Looking down one of the optic axes if 2V gt 50
12Obtuse bisectrix figure
- Looking down Bxo
- Grains with intermediate interference colors
13Obtuse bisectrix figure
- Looking down Bxo
- Grains with intermediate interference colors
- After 5-15 rotation, isogyres out of field of
view
14Optic normal figure
- Optic normal (Y-axis) is vertical
- Grains show max interference colors
- Flash figure
15Optic normal figure
- Optic normal (Y-axis) is vertical
- Grains show max interference colors
- Flash figure
- Rotate stage only few degrees, isogyres gone
16Off centered figure
- With no optic axis, or axis is vertical, grain is
randomly oriented - Common orientation
- Good luck!
17Optic sign determination
18Biaxial optic signacute bisectrix figure
19Biaxial Optic Sign
B(-)
100 gray 550 650 blue
add
subtract
add
100 gray - 550 450 yellow
20Biaxial Optic Sign
B()
sub
add
sub
21Optic axis figure
22Determining 2V
- Look at curvature of isogyre
- Or separation of isogyres (Fig. 7.52 in Nesse)
23Best minerals for sign determination
- Lowest interference colors, puts optic axis
closest to vertical
24Other optical properties
- Extinction angle angle between length of mineral
or prominent cleavage and extinction
25Extinction angle
Extinction behavior is a function of the
relationship between indicatrix orientation and
crystallographic orientation
26Extinction 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
27Extinction 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
28Twinning
Presence and style of twinning can be diagnostic
Twins are usually most obvious in XN (upper
polarizer in)
29Twinning - some examples
Clinopyroxene (augite)
Plagioclase
30Cleavage
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
31Cleavage
32Cleavage
random fractures, no cleavage olivine
33(No Transcript)
34Sign 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
35Sign of Elongation
If b elongation Sometimes will add length
slow Sometimes will subtract length fast
g
b
b
a
36Pleochroism 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
37Pleochroism 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
38Review 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?
39Go 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
40Another 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