Transcript Optical Mineralogy

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
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
Polarizing Microscope
Ocular
Bertrand lens
Analyzer, upper
polarizer, nicols lens
Accessory Slot
Objective
Polarizer,
typically oriented
N-S
Slightly more modern version
Trinocular
head
Analyzer, upper
polarizer, nicols
lens
Objectives
conoscope
Internal light
source, polarized
Reflected
light source
Accessory
plate
Vernier
scale

Four common settings for microscopic
observations of thin sections:
1. Plane polarized light, analyzer (upper
polarizer, nicols lens) out
2. Plane polarized light, analyzer in (cross
nicols)
3. Conoscopic polarized light, bertrand lens in
4. Conoscopic polarized light, bertrand lens in,
gypsum plate in accessory slot
Setting #1: No upper analyzer
Quartz crystals in plane
polarized light
Setting #2: Upper analyzer inserted
Same quartz crystals
with analyzer inserted
(cross polarizers aka
crossed nicols)
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
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

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
Electric vibration
direction
Magnetic vibration
direction
For mineralogy – we’ll only consider the
electrical component
Fig. 7-2
Properties of light
Wavelength
Amplitude
Velocity

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
Visable light
spectrum
1 nm = 10-9 m
f (hertz)
1Å
100 Å
Full range of electromagnetic radiation
l (nm)
Fig. 6-6

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
Electrical
components
only
Note – two
waves have the
same v and l
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

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
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
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

Isotropic materials



Anisotropic materials



Wave normals and ray paths are parallel
Velocity of light is constant regardless of
direction in these minerals
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

Birefringence demonstration?????????
Polarized and Non-polarized Light

Non-polarized light


Vibrates in all directions perpendicular to
direction of propagation
Occurs only in isotropic materials

Air, water, glass, etc.
Fig. 7-4
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
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”
Anisotropic medium:
light split into two
rays. One fully
absorbed
Polarized light
vibrates in only one
plane: “Planepolarized light”
Fig. 74b

Polarization also caused by reflection:


“Glare”
Raybans cut the glare
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

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
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

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
Reflection and Refraction

Light hitting boundary of transparent
material



Some reflected
Some refracted
Reflected light


Angle of incidence = angle of reflection
Amount controls luster
For reflection:
Angle of incidence, i = angle of reflection, r
Light ray
“reflective” boundary
Fig. 7-6a
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

Angle of refraction given by Snell’s law
Wave normal
n=low, fast v
sin 1 n2

sin 2 n1
N=big, slow v

Snell’s law works for isotropic and
anisotropic material if:


 are angles between normals to boundary
Direction is wave normal, not ray path

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
P. 306 – olivine information
Indices of
refraction
{
}
Optical
properties
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

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
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
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)
Fig. 7-8

Because of dispersion, important to
determine n for particular wavelength


Typically n given for l = 486, 589, and 656
nm
Common wavelengths for sunlight