1. Optical Mineralogy in a Nutshell
Use of the petrographic microscope in three easy lessons
Jane Selverstone, University of New Mexico 2003
Tark Hamilton, Camosun College, 2013
Original powerpoint slides created by Jane Selverstone, 2003
Edited and annotated by Tark Hamilton, 2013
Part I

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
EW versus NS directions of polarizer and analyser are always set to 90° but vary from one make of scope to another.
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?
EW versus NS directions of polarizer and analyser are always set to 90° but vary from one make of scope to another.
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
EW versus NS directions of polarizer and analyser are always set to 90° but vary from one make of scope to another.
Light vibrating in many planes and with many wavelengths
How does this work??

10. 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
Minerals act as
magicians!!
Reorientation is really a vector addition/subtraction game between the light and the electrical vibration direction in the crystal field within minerals. It is called refraction rather than magic!
But, note that some minerals are better magicians than others (i.e., some grains stay dark and thus can’t be reorienting light)

11. 4) Note the rotating stage
Most mineral grains under XPL, 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
Now do question 1

12. Some generalizations and vocabulary
All isometric minerals (e.g., garnet, diamond) are isotropic – they have only 1 refractive index in all directions and cannot reorient light. These minerals are always black in crossed polars (XPL).
All other minerals are anisotropic – they are all capable of reorienting light (acting as magicians). They split the incident light into 2 components whose directions are controlled by the crystal lattice
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
Velocity always < c in vacuum
RI is optical density, slowness of light in crystal
The degree of bending depends on Snell’s law: sin(i)/n(i) = sin(r)/n(r), and on path length. The thicker the grain the more the bending.
plane polarized light W E
lower polarizer

14. A brief review…
Isotropic minerals: light does not get rotated or split; propagates with same velocity in all directions. (Cubic)
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. (Hexagonal & Tetragonal)
Biaxial - light entering in all but two special directions is resolved into 2 plane polarized components. (Orthorhombic, Monoclinic, Triclinic)
Along the special directions (“optic axes”), the mineral thinks that it is isotropic - i.e., no splitting occurs
The 2 vibration directions are 90° apart, accounting for 4 extinction directions upon 360° rotation.
Compared to the long direction of mineral crystals or cleavages, uniaxial XLs have parallel extinction while biaxial crystals have either symmetric or inclined extinction.
Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to crystallographic (XL) axes.

15. How light behaves depends on crystal structure (there is a reason you took mineralogy!)
Isotropic
Uniaxial
Biaxial
Isometric
All crystallographic axes are equal
Hexagonal - Trigonal, Tetragonal
All axes  c are equal but c is unique
Orthorhombic, Monoclinic, Triclinic
All axes are unequal
3, 2 or no right angles in unit cell shape
Let’s use all of this information to help us identify minerals

16. Mineral properties: color & pleochroism
Mineral true 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 wavelengths l are absorbed differently by different crystallographic directions -
rotate stage to observe mainly in XPL (can mask birefringence)
hbl
Hornblende pleochroism can vary from tan or yellow to greens and browns. Biotite is generally pleochroic in straw-brown-red brown shades. Hypershtene has pale pink and green pleochroism. Titaniferous augite can be pink or purplish brown.
hbl
plag
plag
Plagioclase is colorless
Hornblende is pleochroic in olive greens
Now do question 2

17. Mineral properties: Index of refraction (R.I. or n)
Refractive index is optical density or slowness of light interacting with electrons in crystal
Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocity 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

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
(appears bolder)
Plag has low relief
(less distinct grains)

19. What causes relief? Now do question 3
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 (-)
This focussing or defocussing appears as a thing bright or even coloured line around the grain margins called the Becke Line.
Now do question 3

20. Mineral properties: interference colors/birefringence
False Interference Colors one observed when polars are crossed (XPL)
Color can be quantified numerically as birefringence: d = nhigh - nlow
While birefringence (delta) is an absolute value, it varies for 2 reasons. When the mineral is cut or lies in different positions, this can be less than the full value due to only getting the projection of the RI vector. This is like the difference between your height and your shadow length near noon! The other reason is minerals have variable substitutions of Fe for MG, AL for Si etc that affects the RI to vary within a small range depending on exact composition. Finally the birefringence colour you get to see depends on grain size and thickness. For really tiny grains in rocks the birefringence of the underlying mineral can wash through and show false values. For really thick grains, the refraction increases as the “lens” of the mineral gets thicker.
Now do question 4
More on this next week…

21. Use of interference figures, continued…
You will see a very small, circular field of view with one or more black isogyres -- rotate stage and watch isogyre(s)
uniaxial
If uniaxial, isogyres define cross; arms remain N-S/E-W as stage is rotated
biaxial or
If biaxial, isogyres define curve that rotates with stage, or cross that breaks up as stage is rotated
Figures are taken in XPL with conoscopic illumination and the bertrand lens or pinhole condenser under the ocular switched into the optic train. The Black isogyres coincide with vibration directions of the ordinary and extraordinary rays. The isochromatic curves, due to conoscopic light show coloured rings of equal retardation. This figure is for a fairly birefringent mineral like Calcite having a big difference between the 2 RI’s.
For biaxial minerals, there are 2 optic axes, each having its own curved boomerang shaped black isogyre. When the intermediate refractive index is really close to either the big or small one, the optic axes are really close together and almost touch in the extinction position like the nearly centered acute bisectrix figure on the right.
The measure of optic axes separation in degrees is called 2V. For a 60° wide field of view the 2V on the right is about ~15°. For the biaxial figure to the left of the 2 biaxial ones, this is nearly a centered optic axis figure. You can judge the 2V 2 ways here: by the separation between OA’s using the full field of view or by looking at how straight or how kinked the isogyre is. When the single isogyre is straight across like a propeller blade the 2V is 90° and the intermediate refractive index (beta) is exactly halfway between the high and low RI’s (alpha and gamma). The 2V here is about 65-70° like the clinopyroxene augite or an iron rich olivine.

22. Use of interference figures, continued…
Now 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 = (-)
Now do question 5
uniaxial
(+)
When colours go up in the NE quadrant (addition, fast plate on fast RI direction in mineral) the colours go up to blue and the mineral has a positive optic sign (epsilon or gamma is much greater than omega or alpha,beta).
When colours subtract it is fast on slow and it is negative.
(+)
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)!!