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
Part I
Why use the microscope??
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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!
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…
What happens as light moves through the scope?
your eye
light travels
as waves
amplitude, A
wavelength,
l
light ray
waves travel from
source to eye
light source
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
What happens as light moves through the scope?
propagation
direction
plane of
vibration
vibration
direction
light vibrates in
all planes that contain
the light ray
(i.e., all planes
perpendicular to
the propagation
direction
1) Light passes through the lower polarizer
west
(left)
Unpolarized light
Plane polarized light
east
(right)
PPL=plane polarized light
Only the component of light vibrating in E-W
direction can pass through lower polarizer –
light intensity decreases
2) Insert the upper polarizer
west (left)
north
(back)
east (right)
south
(front)
Black!!
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)
3) Now insert a thin section of a rock
west (left)
Unpolarized light
east (right)
Light vibrating E-W
Light vibrating in
many planes and with
many wavelengths
Light and colors
reach eye!
How does this work??
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!!
But, note that some minerals are better magicians than others
(i.e., some grains stay dark and thus can’t be reorienting light)
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
Now do
question 1
These minerals
are isotropic
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
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
plane polarized
light
W
E
lower polarizer
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
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.
How light behaves depends on crystal structure
(there is a reason you took mineralogy!)
Isotropic
Isometric
– All crystallographic axes are equal
Uniaxial
Biaxial
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
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
hbl
-Plagioclase is colorless
-Hornblende is pleochroic in olive greens
plag
plag
Now do question 2
Mineral properties: Index of refraction (R.I. or n)
Refractive index is optical density or slowness of
light interacting with electrons in crystal
n=
velocity in air
velocity in mineral
n2>n1
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
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 has high relief
(appears bolder)
-Plag has low relief
(less distinct grains)
plag
olivine
olivine: n=1.64-1.88
plag:
n=1.53-1.57
epoxy: n=1.54
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
Hi relief (+)
Lo relief (+)
nxtl > nepoxy
nxtl = nepoxy
Now do question 3
Hi relief (-)
nxtl < nepoxy
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
Now do question 4
More on this next week…
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)
or
uniaxial
If uniaxial, isogyres define
cross; arms remain N-S/E-W
as stage is rotated
biaxial
If biaxial, isogyres define curve that
rotates with stage, or cross that
breaks up as stage is rotated
Use of interference figures, continued…
Now determine the optic sign of the mineral:
1. Rotate stage until isogyre is concave to NE (if biaxial)
2. Insert gypsum accessory plate
3. Note color in NE, immediately adjacent to isogyre -
Blue = (+)

Yellow = (-)
Now do question 5
uniaxial
biaxial
(+)
(+)
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)!!