1. Use of the petrographic microscope
Optical Mineralogy
Optical Mineralogy
Use of the petrographic microscope

2. Why use the microscope?? Identify minerals (no guessing!)
Determine rock type
Determine crystallization sequence
Document deformation history
Constrain P-T history
Note weathering/alteration

3. The petrographic microscope
Also called a polarizing microscope
In order to use the scope, we need to understand a little about the physics of light, and then learn some tools and tricks…

4. light travels as waves What happens as light moves through the scope?
light source
your eye
light ray
waves travel from source to eye
wavelength, l
I = f(A)
amplitude, A
light travels as waves
Frequency = # of waves/sec to pass a given point (hz)
f = v/l v = velocity
(in physics v = c, but no longer)

5. Refraction Incident ray and reflected ray: Refracted ray:
1)  of incidence i =  of reflection r'
2) coplanar “plane of incidence”
(^ plane of interface)
Refracted ray:
1) Slower in water (or glass)
2)  r   i
Depends on D velocity
Incident i
Reflected r’
air
water r
Refracted

6. What happens as light moves through the scope?
Each photon vibrates as a wave form in a single plane
propagation direction
Light beam = numerous photons, each vibrating in a different plane
Vibration in all directions ~ perpendicular to propagation direction
planes of
vibration
Light beam = numerous photons, all vibrating in the same plane
vibration directions
Polarized Light
Unpolarized Light

7. 1) Light passes through the lower polarizer
west (left)
east (right)
Plane polarized light “PPL”
Unpolarized light
Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases

8. XPL=crossed nicols (crossed polars)
2) Insert the upper polarizer
south (front)
north (back)
Black!! (“extinct”)
west (left)
east (right)
Now what happens?
What reaches your eye?
Why would anyone design a microscope that prevents light from reaching your eye???
XPL=crossed nicols (crossed polars)

9. Review: With any isotropic substance (spherical indicatrix), when the analyzer is inserted (= “crossed-nicols” or “XPL”) no light passes  extinct, even when the stage is rotated
Note: the gray field should also be extinct (glass and epoxy of the thin section are also isotropic), but is left lighter for illustration
Liquids, gases, amorphous solids such as glass, and isotropic minerals (isometric crystal system) stay black in all orientations

10. The refractive index (or index of refraction)
of a medium is a measure of how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.

11. Mineral properties in PPL: relief
Relief is a measure of the relative difference in n between a mineral grain and its surroundings
Relief is determined visually, in PPL
Relief is used to estimate n
garnet: n =
quartz: n =
epoxy: n = 1.54
Quartz has low relief
Garnet has high relief

12. Mineral properties in PPL: relief
Relief is a measure of the relative difference in n between a mineral grain and its surroundings
Relief is determined visually, in PPL
Relief is used to estimate n
olivine
plag
olivine: n =
plag: n =
epoxy: n = 1.54
- Olivine has high relief
- Plagioclase has low relief

13. What causes relief?
Difference in speed of light (n) in different materials causes refraction of light rays, which can lead to focusing or defocusing of grain edges relative to their surroundings
nxtl > nepoxy
nxtl < nepoxy
nxtl = nepoxy
Hi relief (+)
Lo relief (+)
Hi relief (-)

14. Now insert a thin section of a rock in XPL
Light vibrating in many planes and with many wavelengths
Light and colors reach eye!
west (left)
north
Unpolarized light
south
east (right)
Light vibrating E-W
How does this work??

15. & Interference colors and birefringence: Anisotropic minerals,
cause polarized light to be split into two rays as it travels through a grain.
The rays may not travel at the same velocity or follow the exact same path.
A value, termed birefringence, describes the difference in velocity of the two rays.
&
When the rays emerge from the grain, they combine to produce interference colors.

16. Interference colors are only seen in XP light!
The colors may be any hue in the specturm. As birefringence increases, the colors repeat (see birefringence chart).
To describe interference colors we must specify both a hue and an order (e.g., 2nd order red; see birefringence chart ).

17. Minerals act as magicians!!
Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer
plag
olivine
PPL
XPL
Minerals act as
magicians!!

18. Consider rotating the crystal as you watch: D
Polarized light has a component of each
Splits  two rays
one is O-ray with n = w
other is E-ray with n = e
When the rays exit the crystal they recombine w e
polarizer

19. Color chart Colors one observes when polars are crossed (XPL)
Color can be quantified numerically: d = nhigh - nlow

20. What interference color is this?
If this were the maximum interference color seen, what is the birefringence of the mineral?

21. Pleochroism
Changes in absorption color in PPL as rotate stage (common in biotite, amphibole…)
Pleochroic formula:
Example: Tourmaline: e = dark green to bluish w = colorless to tan
Can determine this as just described by isolating first w and then e E-W and observing the color

22. Hornblende as stage is rotated

23. Biotite as stage is rotated

24. Olivine