1. Optical Mineralogy
Technique utilizing interaction of polarized light with minerals
Uses a polarizing microscope
Oils - Grain mounts
Thin sections – rocks
Primary way to observe minerals
Important:
cheap, quick, easy
Only way to determine textures

2. Why use microscopes? Visual properties for ID – e.g. texture
Color – may be variable
Cleavage (may not see, often controls shape)
Shape (depends on cut of mineral)
Only observable with microscope
Separate isotropic and anisotropic minerals and many other optical properties

3. Polarizing Microscope
Ocular
Bertrand lens
Analyzer, upper polarizer, nicols lens
Accessory Slot
Objective
Polarizer, typically oriented N-S

4. Slightly more modern version
Trinocular head
Reflected light source
Analyzer, upper polarizer, nicols lens
Accessory plate
Objectives
Vernier scale
conoscope
Internal light source, polarized

5. Four common settings for microscopic observations of thin sections:
Plane polarized light, analyzer (upper polarizer, nicols lens) out
Plane polarized light, analyzer in (cross nicols)
Conoscopic polarized light, bertrand lens in
Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slot

6. Quartz crystals in plane polarized light
Setting #1: No upper analyzer
Setting #2: Upper analyzer inserted
Quartz crystals in plane polarized light
Same quartz crystals with analyzer inserted (cross polarizers aka crossed nicols)

7. Setting # 3: Conoscopic polarized light, bertrand lens in, highest magnification
Setting #4: Conoscopic polarized light, bertrand lens in, gypsum plate in accessory slot, highest magnification

8. Characteristics of light
Electromagnetic energy
derived from excess energy of electrons
Energy released as electrons drop from excited state to lower energy shells – perceived as “light”
Particle, Wave or both
Particles = photons
For mineralogy, consider light a wave
Important wave interference phenomenon

9. Light has both electrical and magnetic energy
Light as wave
Energy vibrates perpendicular to direction of propagation
Light has both electrical and magnetic energy
Two components vibrate perpendicular to each other
Electrical component interacts with electrical properties of minerals, e.g. bond strength, electron densities

10. Electric vibration direction
Magnetic vibration direction
For mineralogy – we’ll only consider the electrical component
Fig. 7-2

11. Properties of light
Wavelength
Amplitude
Velocity

12. Relationship and units of properties
l = wavelength, unit = L, color of light
A = amplitude, unit = L, intensity of light
v = velocity, unit = L/t, property of material
f = frequency – e.g. how often a wave passes a particular point, unit = 1/t
f = v/l, frequency is constant, v and l variable

13. Visable light spectrum Full range of electromagnetic radiation
l (nm)
f (hertz)
1 Å
Visable light spectrum
100 Å
Full range of electromagnetic radiation
1 nm = 10-9 m
Fig. 6-6

14. If two light waves vibrate at an angle to each other:
Vibrations interfere with each other
Interference creates a new wave
Direction determined by vector addition
Vibration directions of single wave can be split into various components
Each component has different vibration direction

15. Two light waves A & B interfere to form resultant wave R
Note – two waves have the same v and l
Electrical components only
Two light waves A & B interfere to form resultant wave R
One light wave X has a component V at an angle 
Fig. 7-3

16. Light composed of many waves
Wave front = connects same point on adjacent waves
Wave normal = line perpendicular to wave front
Light ray (Ray path) = direction of propagation of light energy, e.g. direction of path of photon
Note: wave normal and light ray are not necessarily parallel

17. Wave normal and ray path not always parallel
Wave front connects common points of multiple waves
It is the direction the wave moves
Ray path is direction of movement of energy, e.g., path a photon would take
Fig. 7-2c

18. Wave normal and ray paths may be coincident
Propogation of light through Isotropic material
Wave normal and ray paths may not be coincident
Propogation of light through Anisotropic material
Fig. 7-2d and e

19. Anisotropic materials
Wave normals and ray paths are parallel
Velocity of light is constant regardless of direction in these minerals
Anisotropic materials
Wave normals and ray paths are not parallel
Velocity of light is variable depending on direction of wave normal and ray path
These difference have major consequences for interaction of light and materials

20. Birefringence demonstration?????????

21. Polarized and Non-polarized Light
Vibrates in all directions perpendicular to direction of propagation
Occurs only in isotropic materials
Air, water, glass, etc.
Fig. 7-4

22. Non-Polarized Light
Light vibrates in all directions perpendicular to ray path
Multiple rays, vibrate in all directions
Highly idealized – only 1 wavelength
Fig. 7-4

23. Polarized light Vibrates in only one plane
Generation of polarized light:
In anisotropic material, light usually resolves into two rays
Two rays vibrate perpendicular to each other
The energy of each ray absorbed by different amounts
If all of one ray absorbed, light emerges vibrating in only one direction
Called “Plane Polarized Light”

24. Polarized light vibrates in only one plane: “Plane-polarized light”
Anisotropic medium: light split into two rays. One fully absorbed
Fig. 7-4b

25. Polarization also caused by reflection:
“Glare”
Raybans cut the glare

26. Interaction of light and matter
Velocity of light depends on material it passes through
In vacuum, v = 3.0 x 1017 nm/sec = 3.0 x 108 m/sec
All other materials, v < 3.0 x 1017 nm/sec

27. f = v/l When light passes from one material to another f = constant
If v increases, l also must increase
If v decreases, l decreases
Vair > Vmineral
f = v/l

28. Isotropic vs. Anisotropic
Isotropic geologic materials
Isometric minerals; also glass, liquids and gases
Electron density identical in all directions
Think back to crystallographic axes
Direction doesn’t affect the electrical property of light
Light speed doesn’t vary with direction
Light NOT split into two rays

29. Anisotropic geologic materials:
Minerals in tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems
Interactions between light and electrons differ depending on direction
Light split into two rays – vibrate perpendicular to each other
Light speed depends on direction of ray and thus vibration direction

30. Reflection and Refraction
Light hitting boundary of transparent material
Some reflected
Some refracted
Reflected light
Angle of incidence = angle of reflection
Amount controls luster

31. Angle of incidence, i = angle of reflection, r
For reflection:
Angle of incidence, i = angle of reflection, r
Light ray
“reflective” boundary
Fig. 7-6a

32. Refracted light Angle of incidence ≠ angle of refraction
Angle of refraction depends on specific property, Index of refraction, n
n = Vv/Vm
Vv = velocity in a vacuum (maximum)
Vm = velocity in material
Note – n is always > 1
Big N means slow v
Little n means fast v

33. Angle of refraction given by Snell’s law
Wave normal
n=low, fast v
N=big, slow v

34. Snell’s law works for isotropic and anisotropic material if:
 are angles between normals to boundary
Direction is wave normal, not ray path

35. Measuring n important diagnostic tool
Not completely diagnostic, may vary within minerals
More than one mineral may have same n
n can’t be measured in thin section, but can be estimated

36. P. 306 – olivine information}
Optical properties
Indices of refraction {

37. Critical Angle - CA A special case of Snell’s law
Light going from low to high index material (fast to slow, e.g. air to mineral)
Can always be refracted
Angle of refraction is smaller than angle of incidence

38. Light going from high to low index material
May not always be refracted
Light is refracted toward the high n material
At some critical angle of incidence, the light will travel along the interface
If angle of incidence is > CA, then total internal reflection
CA can be derived from Snell’s law

39. Critical angle is when angle of refraction = 90º
All internal reflection
N = high
High index to low index material: light cannot pass through boundary if angle of incidence > CA
Critical angle is when angle of refraction = 90º
n = low
Fig. 7-7

40. Dispersion Material not always constant index of refraction
n = f(l)
Normal dispersion, within same material:
n higher for short wavelengths (blue)
n lower for long wavelengths (red)

41. Fig. 7-8

42. Because of dispersion, important to determine n for particular wavelength
Typically n given for l = 486, 589, and 656 nm
Common wavelengths for sunlight