Optical Mineralogy in a Nutshell
Use of the petrographic microscopein three easy lessons
Part I
© Jane Selverstone, University of New Mexico, 2003
Used and modified with permission.
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!
The petrographic microscope
Also called apolarizingmicroscope
In order to use the scope, we need to understand a little aboutthe physics of light, and then learn some tools and tricks…
What happens as light moves through the scope?
light source
your eye
light ray
waves travel fromsource to eye
wavelength,
amplitude, A
light travelsas waves
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?
light vibrates in
all planes that contain
the light ray
(i.e., all planes
perpendicular to
the propagation
direction
plane of
vibration
vibrationdirection
propagationdirection
What happens as light moves through the scope?
1) Light passes through the lower polarizer
west(left)
east(right)
Plane polarized light
PPL=plane polarized light
Unpolarized light
Only the component of light vibrating in E-Wdirection can pass through lower polarizer –light intensity decreases
2) Insert the upper polarizer
west (left)
east (right)
Now what happens?
What reaches your eye?
Why would anyone design a microscope thatprevents light from reaching your eye???
XPL=crossed nicols(crossed polars)
south(front)
north(back)
Black!!
3) Now insert a thin section of a rock
west (left)
east (right)
Light vibrating E-W
Light vibrating inmany planes and withmany wavelengths
How does this work?
“Some minerals polarize light into two rays vibrating at rightangles to one another. …where these…are not parallel to … thepolariser and analyser, … light reach[es] the eye…. ”
Shelley 1975 p 23
Unpolarized light
Light and colorsreach eye!
Minerals somehow reorient the planes in whichlight is vibrating; some light passes through theupper polarizer
But, note that some minerals are different. Some grainsstay dark and thus can’t be reorienting light even when thestage is rotated.
4) Note the rotating stage
Most mineral grains change color as the stage isrotated; these grains go black 4 times in 360°rotation-exactly every 90o
Glass and a few minerals stayblack in all orientations
These mineralsare anisotropic
These mineralsare 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 allcapable of reorienting light.
•All anisotropic minerals contain one or two specialdirections 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 polarizedcomponents that travel at different velocities and vibrate inplanes that are perpendicular to one another
mineralgrain
plane polarizedlight
fast ray
slow ray
lower polarizer
W
E
Some light is nowable to passthrough theupper polarizer
When light gets split:
-velocity changes
-rays get bent (refracted)
-2 new vibration directions
-usually see new colors
different velocities All anisotropic minerals can resolve light into two plane polarizedcomponents that travel at different velocities and vibrate inplanes that are perpendicular to one another
mineralgrain
plane polarizedlight
fast ray
slow ray
lower polarizer
W
E
RETARDATION.In the picture, the slow ray hastraveled fewer wavelengthscompared to the fast ray. Thenumber of wavelengths differenceis called the RETARDATION.
interference colour (For white light of many colors)“…certain colors will be out ofphase, while others will be inphase. …instead of white light, wesee … interference colour thatresults from the …colours’” thatdo not cancel .Shelley 1975 p25
birefringence.The colours are produced by thedifference in speed in the fast andslow rays, also known asbirefringence.
Extinction
•Since the plane polarized components areperpendicular, they form a plus shape +
•Four times a rotation, they align with thepolarizing filters, and go dark.
Recall: All anisotropic minerals can resolve lightinto two plane polarized components that vibrate inplanes that are perpendicular to one another
•Isotropic minerals: light does not get rotatedor split; propagates with same velocity in alldirections
•Anisotropic minerals:
•Uniaxial - light entering in all but one special directionis resolved into 2 plane polarized components thatvibrate perpendicular to one another and travel withdifferent speeds
•Biaxial - light entering in all but two special directionsis resolved into 2 plane polarized components…
–Along the special directions (“optic axes”), themineral thinks that it is isotropic - i.e., no splittingoccurs
–Uniaxial and biaxial minerals can be furthersubdivided into optically positive and opticallynegative, depending on orientation of fast and slowrays relative to xtl axes
A brief review…
Isotropic
Uniaxial
Biaxial
How light behaves depends on crystal structure
Isometric
– All crystallographic axes are equal length
crystallographic axes measure the sides of the unit cell
Orthorhombic, monoclinic, triclinic
–All axes are unequal in length
Hexagonal, trigonal, tetragonal
– All axes c are equal but c is unique
Let’s use all of this information to help us identify minerals
Same light speed everywhere
Light splits in two
except along optic axis
Two optic axes
Mineral properties: color & pleochroism
• Color is observed only in PPL
• Not an inherent property - changes with light type/intensity
• Results from selective absorption of certain of light
• Pleochroism results when different are absorbeddifferently by different crystallographic directions -
rotate stage to observe
plag
hbl
plag
hbl
-Plagioclase is colorless
-Hornblende is pleochroic in olive greens
Mineral properties: Index of refraction (R.I. or n)
Light is refracted when it passes from onesubstance to another; refraction isaccompanied by a change in velocity
n1
n2
n2
n1
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
•Discussion: the ray in the higher-index medium is closer to the normal.
•A “normal” is a perpendicular to the boundary between the substances.
the normalto theinterface isshow as averticalblack line
Suppose vair = 1
Then n = 1/vmin
Mineral properties: relief
• Relief is a measure of the relative difference in nbetween a mineral grain and its surroundings
• Relief is determined visually, in PPL
• Relief is used to estimate n
olivine
plag
olivine:n=1.64-1.88
plag:n=1.53-1.57
epoxy:n=1.54
- Olivine has high relief
- Plag has low relief
What causes relief?
nxtl > nepoxy
nxtl < nepoxy
nxtl = nepoxy
Hi relief (+)
Lo relief (+)
Hi relief (-)
Difference in speed of light (n) in different materials causesrefraction of light rays, which can lead to focusing ordefocusing of grain edges relative to their surroundings
Mineral properties: interference colors/birefringence
• interference colors observed when polars are crossed (XPL)
• Color can be quantified numerically: birefringence = nhigh - nlow
Use of interference figures
•Find a grain that stays dark with crossed polars when thestage is rotated. Either the grain is isotropic, or anisotropicand the optic axis is aligned with the microscope optics
•With crossed polars, the condenser in (Zeiss: flip it into opticpath, Olympus:move the substage up), the diaphragm open, andeither the bertand lens in, OR the eyepiece removed, you willsee a very small, circular field of view with one or more blackisogyres
•Rotate the stage and watch the isogyre(s)
Uniaxial
If Uniaxial, isogyres definecross; arms remain N-S/E-Was stage is rotated
Biaxial
or
If Biaxial, isogyres define curve thatrotates with stage, or cross thatbreaks 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
(+)
(+)
•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 2plane polarized components that vibrate perpendicular to one anotherand travel with different speeds
•Biaxial - light entering in all but two special directions is resolved into 2plane polarized components…
–Along the special directions (“optic axes”), the mineral thinks thatit is isotropic - i.e., no splitting occurs
–Uniaxial and biaxial minerals can be further subdivided intooptically positive and optically negative, depending on orientationof fast and slow rays relative to xtl axes
A brief review…
You are now well on your way to being able to identify all of thecommon minerals (and many of the uncommon ones, too)!!