1. Optical Mineralogy in a Nutshell
Use of the petrographic microscope in three easy lessons
Part I
© Jane Selverstone, University of New Mexico, 2003
Used and modified with permission.

2. Why use the microscope?? Identify minerals (no guessing!)
Determine rock type
Determine crystallization sequence
Document deformation history
Observe frozen-in reactions
Constrain P-T history
Note weathering/alteration
Fun, powerful, and cheap!

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. What happens as light moves through the scope?
light source
your eye
light ray
waves travel from source to eye
light travels as waves
wavelength, l
amplitude, A

5. What happens as light moves through the scope?
Microscope light is white light,
i.e. it’s made up of lots of different wavelengths;
Each wavelength of light corresponds to a different color
Can prove this with a prism,
which separates white light into its
constituent wavelengths/colors

6. What happens as light moves through the scope?
plane of
vibration
vibration direction
propagation direction
light vibrates in
all planes that contain
the light ray
(i.e., all planes
perpendicular to
the propagation
direction

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

8. 2) Insert the upper polarizer
south (front)
north (back)
Black!!
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. 3) Now insert a thin section of a rock
Light and colors reach eye!
west (left)
Unpolarized light
east (right)
Light vibrating E-W
Light vibrating in many planes and with many wavelengths
How does this work??

10. Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer
But, note that some minerals are different. Some grains stay dark and thus can’t be reorienting light even when the stage is rotated.

11. 4) Note the rotating stage
Most mineral grains change color as the stage is rotated; these grains go black 4 times in 360° rotation-exactly every 90o
These minerals are anisotropic
Glass and a few minerals stay black in all orientations
These minerals are isotropic

12. Some generalizations and vocabulary
All isometric minerals (e.g., garnet) are isotropic – they cannot reorient light. These minerals are always black in crossed polars.
All other minerals are anisotropic – they are all capable of reorienting light.
All anisotropic minerals contain one or two special directions that do not reorient light.
Minerals with one special direction are called uniaxial
Minerals with two special directions are called biaxial

13. All anisotropic minerals can resolve light into two plane polarized components that travel at different velocities and vibrate in planes that are perpendicular to one another
Some light is now able to pass through the upper polarizer
fast ray
slow ray
mineral grain
When light gets split:
velocity changes
rays get bent (refracted)
2 new vibration directions
usually see new colors
plane polarized light W E
lower polarizer

14. Anisotropic minerals:
A brief review…
Isotropic minerals: light does not get rotated or split; propagates with same velocity in all directions
Anisotropic minerals:
Uniaxial - light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds
Biaxial - light entering in all but two special directions is resolved into 2 plane polarized components…
Along the special directions (“optic axes”), the mineral thinks that it is isotropic - i.e., no splitting occurs
Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to xtl axes

15. How light behaves depends on crystal structure
Isotropic
Uniaxial
Biaxial
Isometric
All crystallographic axes are equal length
crystallographic axes measure the sides of the unit cell
Same light speed everywhere
Hexagonal, trigonal, tetragonal
All axes  c are equal but c is unique
Light splits in two
except along optic axis
Orthorhombic, monoclinic, triclinic
All axes are unequal in length
Two optic axes
Let’s use all of this information to help us identify minerals

16. Mineral properties: color & pleochroism
Color is observed only in PPL
Not an inherent property - changes with light type/intensity
Results from selective absorption of certain l of light
Pleochroism results when different l are absorbed differently by different crystallographic directions -
rotate stage to observe
hbl
hbl
plag
plag
Plagioclase is colorless
Hornblende is pleochroic in olive greens

17. Mineral properties: Index of refraction (R.I. or n)
Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocity
the normal to the interface is show as a vertical black line n1 n2
n2>n1
n2<n1
n is a function of crystallographic orientation in anisotropic minerals
isotropic minerals: characterized by one RI
uniaxial minerals: characterized by two RI
biaxial minerals: characterized by three RI
n gives rise to 2 easily measured parameters: relief & birefringence
Discussion: the ray in the higher-index medium is closer to the normal.
A “normal” is a perpendicular to the boundary between the substances.

18. Mineral properties: 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
- Plag has low relief

19. 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 (-)

20. Mineral properties: interference colors/birefringence
Colors one observed when polars are crossed (XPL)
Color can be quantified numerically: d = nhigh - nlow

21. Use of interference figures
Find a grain that stays dark with crossed polars when the stage is rotated. Either the grain is isotropic, or anisotropic and the optic axis is aligned with the microscope optics
With crossed polars, the condenser in, the diaphragm open, and either the bertand lens in, OR the eyepiece removed, you will see a very small, circular field of view with one or more black isogyres
Rotate the stage and watch the isogyre(s)
Biaxial or
If Biaxial, isogyres define curve that rotates with stage, or cross that breaks up as stage is rotated
Uniaxial
If Uniaxial, isogyres define cross; arms remain N-S/E-W as stage is rotated

22. Use of interference figures, continued…
You can determine the optic sign of the mineral:
Rotate stage until isogyre is concave to NE (if biaxial)
Insert gypsum accessory plate
Note color in NE, immediately adjacent to isogyre --
Blue = (+)
Yellow = (-)
Uniaxial
(+)
(+)
Biaxial

23. A brief review…
Isotropic minerals: light does not get rotated or split; propagates with same velocity in all directions
Anisotropic minerals:
Uniaxial - light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds
Biaxial - light entering in all but two special directions is resolved into 2 plane polarized components…
Along the special directions (“optic axes”), the mineral thinks that it is isotropic - i.e., no splitting occurs
Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to xtl axes
You are now well on your way to being able to identify all of the common minerals (and many of the uncommon ones, too)!!