1. Mineral color and pleochroism
True color
Not interference colors
Observed in plane polarized light
Not crossed nicols
Most minerals are colorless

2. Pleochroism Property of having two or more true colors
Occurs only in anisotropic minerals
Each principal vibration direction has a unique color
Preferentially absorbs selected wavelengths of light

3. Pleochroism – Glaucophane (amphibole – Na, Mg, Fe – Silicate) Pleochroism – orthopyroxene – (Ca, Fe, Mg – Silicate)

4. Color depends on which vibration direction parallels polarized direction
Slow ray has one color
Fast ray another color
Color intermediate if neither direction parallel to polarized direction
Pleochroic formula
Relationship of color to index of refraction (a, b, g, e, w) that shows the color

5. Pleochroic formula Multiple types of formulas:
Color of e, w, a, b, or g rays
Greater absorbance e.g. w > e or e > w
“strongly” or “weakly” pleochroic

6. Determination of formula – uniaxial minerals
Find grain with d = 0
This is value of w color
Find grain with maximum d
This has both e and w
Already know w, so other color must be e

7. Grains seen in plane polarized light (not crossed nicols)
Direction of polarized light
Grains seen in plane polarized light (not crossed nicols)
1st grain (not shown) – complete extinction
Viewed in plane polarized light gives w color
2nd grain (shown) – provides w and e colors
Since know w already, the other color is e
Determine fast and slow with accessory plate
Fig. 7-30

8. Biaxial Pleochroism Biaxial minerals may have three colors:
One for a, b, and g
Procedure similar to uniaxial minerals, but more complex
Find extinct section – b color
Find maximum d – this grain has a and g colors
Determine fast and slow direction with accessory plate

9. Vibration directions parallel to accessory plate
If addition, color associated with ng
If subtraction, color associated with na
Remember – check color without analyzer in na ng
Fig. 7-31

10. Extinction Four Categories:
Parallel extinction – feature (usually cleavage) parallel to cross hairs at extinction
Inclined extinction – extinction when feature is at an angle to cross hairs
Symmetrical extinction – occurs in minerals with two cleavages: bisect cleavage
No extinction angle – minerals with no elongation or cleavage

11. Parallel
Inclined
No extinction angle
Symmetrical
Fig. 7.32

12. Extinction may not be uniform
Physically deformed minerals
Minerals with variable chemical composition (chemically zoned)
Undulatory Extinction
Zoned Extinction

13. Extinction angle Inclined extinction - angle between
long axis of mineral grain
prominent cleavage
Twins
Other crystallographic feature

14. Long direction, also parallel to cleavage
Extinction angle
Long direction, also parallel to cleavage
Rotate stage until crystallographic feature is parallel to cross hairs
Record angle on goniometer
Rotate stage until mineral is extinct
Now mineral vibration direction is parallel to polarized light direction
Amount of rotation is extinction angle
Fig. 7-31

15. Possible to determine chemical composition from extinction angle
Michel-Levy technique

16. Michel-Levy Technique
Section cut perpendicular to {010}
Albite twin lamellae
Cut of mineral must be with {010} plane vertical, b crystallographic axis horizontal b a b b
NaAlSi3O8
CaAl2Si2O8
Characteristics:
Sharp boundaries between twins
Twin lamellae have same interference colors
High plagioclase = volcanic
Low plagioclase = plutonic
Fig & 12.17

17. Feldspars - Triclinic minerals: Two cleavages Many types of twins
Albite
Na-feldspar
NaAlSi3O8
Feldspars - Triclinic minerals:
Two cleavages
Many types of twins
Extinction angles show relationship between X-Y-Z axes (indicatrix axes) and a-b-c axes (crystallographic axes) Z b X b Z X
An0 to An10
An30 to An50 Z Z b b X X
Anorthite
Ca-feldspar
CaAl2Si2O8
An50 to An70
An90 to An100
p. 245

18. Sign of Elongation
Length fast: elongate direction of mineral parallels fast vibration direction
Also called negative elongation
Length slow: elongate direction of mineral parallels slow vibration direction
Also called positive elongation
Length fast and length slow depends on cut of grain

19. Determination
Orient grain with vibration direction and length about 45º to polarized direction
Use accessory plate to determine addition or subtraction of retardation
Determines if fast or slow ray

20. Vibration directions parallel to accessory plate
If addition, length slow (positive elongation)
If subtraction, length fast (negative elongation)
Length fast
Length slow
Fig. 7-31

21. Orthorhombic Minerals (biaxial)
ng = elongate
na = elongate
nb = elongate
Either length slow or length fast
Always length slow
Always length fast