Optical Mineralogy in a Nutshell

Use of the petrographic microscopein three easy lessons

Part I

© Jane Selverstone, University of New Mexico, 2003Used 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, λ

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 itsconstituent wavelengths/colors

What happens as light moves through the scope?

light vibrates inall planes that containthe light ray(i.e., all planesperpendicular tothe propagationdirection

plane ofvibration

vibrationdirection

propagationdirection

What happens as light moves through the scope?

1) Light passes through the lower polarizerwest(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-WLight vibrating inmany planes and withmany wavelengths

How does this work??

Unpolarized light

Light and colorsreach eye!

Conclusion has to be that minerals somehowreorient the planes in which light is vibrating;some light passes through the upper 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 stageMost 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) are isotropic –

they cannot reorient light. These minerals arealways 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 in

planes that are perpendicular to one another

mineralgrain

plane polarizedlight

fast ray

slow ray

lower polarizerW 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

• Isotropic minerals: light does not get rotatedor split; propagates with same velocity in alldirections

• Anisotropic minerals:• Uniaxial - light entering in all but one special direction

is 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 lengthcrystallographic 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 twoexcept 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 λ of light• Pleochroism results when different λ are absorbed

differently 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

n2n2

n1

n2>n1 n2<n1

n =velocity in air

velocity in mineral

• 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

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

plag

olivine: n=1.64-1.88plag: n=1.53-1.57epoxy: n=1.54

- Olivine has high relief- Plag has low relief

What causes relief?

nxtl > nepoxy nxtl < nepoxynxtl = 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 or

defocusing of grain edges relative to their surroundings

Mineral properties: interference colors/birefringence• Colors one observed when polars are crossed (XPL)• Color can be quantified numerically: δ = 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, the diaphragm open,and either the bertand lens in, OR the eyepiece removed, youwill see a very small, circular field of view with one or moreblack isogyres

•Rotate the stage and watch the isogyre(s)

UniaxialIf 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 plate3. 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 2

plane 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)!!