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.
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
How does this work?
Light and colors
reach eye!
“Some minerals polarize light into two rays vibrating at right
angles to one another. …where these…are not parallel to … the
polariser and analyser, … light reach[es] the eye…. ”
Shelley 1975 p 23
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.
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
Some generalizations and vocabulary
• All isometric minerals (e.g., Garnet, Fluorite,
Halite) 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
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
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
In the picture, the slow ray has
traveled fewer wavelengths
compared to the fast ray. The
number of wavelengths difference
is called the RETARDATION.
fast ray
slow ray
mineral
grain
plane polarized
light
W
E
lower polarizer
(For white light of many colors)
“…certain colors will be out of
phase, while others will be in
phase. …instead of white light, we
see … interference colour that
results from the …colours’” that
do not cancel .Shelley 1975 p25
The colours are produced by the
difference in speed in the fast and
slow rays, also known as
birefringence.
Extinction
Recall: All anisotropic minerals can resolve light
into two plane polarized components that vibrate in
planes that are perpendicular to one another
• Since the plane polarized components are
perpendicular, they form a plus shape +
• Four times a rotation, they align with the
polarizing filters, and go dark.
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
How light behaves depends on crystal structure
Isotropic
Same light speed everywhere
Uniaxial
Light splits in two
except along optic axis
Biaxial
Two optic axes
Isometric
– All crystallographic axes are equal length
crystallographic axes measure the sides of the unit cell
Hexagonal, trigonal, tetragonal
– All axes  c are equal but c is unique
Orthorhombic, monoclinic, triclinic
– All axes are unequal in length
Let’s use all of this information to help us identify minerals
Mineral properties: color & pleochroism
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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
-Plagioclase is colorless
-Hornblende is pleochroic in olive greens
plag
plag
Mineral properties: Index of refraction (R.I. or n)
n=
velocity in air
velocity in mineral
Suppose vair = 1
Then n = 1/vmin
n2>n1
Light is refracted when it passes from one
substance to another; refraction is
accompanied by a change in velocity
n1
n1
n2
n2
n2<n1
the normal
to the
interface is
show as a
vertical
black line
• 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.
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
- Plag has low relief
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
Hi relief (-)
nxtl < nepoxy
Mineral properties: interference colors/birefringence
• interference colors observed when polars are crossed (XPL)
• Color can be quantified numerically:
birefringence d = nhigh - nlow
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 (Zeiss: flip it into optic
path, Olympus:move the substage up), 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)
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…
You can 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 = (-)
Uniaxial
Biaxial
(+)
(+)
A brief review…
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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)!!