Optical Mineralogy in a Nutshell

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# Optical Mineralogy in a Nutshell - PowerPoint PPT Presentation

Optical Mineralogy in a Nutshell. Use of the petrographic microscope in three easy lessons. Part I. Why use the microscope??. Identify minerals (no guessing!) Determine rock type Determine crystallization sequence Document deformation history Observe frozen-in reactions

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Optical Mineralogy in a Nutshell

Use of the petrographic microscope in three easy lessons

Part I

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 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…

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?

Can prove this with a prism,

which separates white light into its

constituent wavelengths/colors

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

propagation direction

light vibrates in

all planes that contain

the light ray

(i.e., all planes

perpendicular to

the propagation

direction

plane of

vibration

vibration direction

What happens as light moves through the scope?

west (left)

east (right)

Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases

1) Light passes through the lower polarizer

Unpolarized light

Plane polarized light

PPL=plane polarized light

north (back)

south (front)

Black!!

2) Insert the upper polarizer

west (left)

east (right)

Now what happens?

Why would anyone design a microscope that prevents light from reaching your eye???

XPL=crossed nicols (crossed polars)

Light and colors reach eye!

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??

Minerals act as

magicians!!

Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer

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 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

Now do question 1

Some generalizations and vocabulary

• All isometric minerals (e.g., garnet) 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 (acting as magicians).
• 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

• When light gets split:
• velocity changes
• rays get bent (refracted)
• 2 new vibration directions
• usually see new colors

plane polarized light

W

E

lower polarizer

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

Isometric

• All crystallographic axes are equal
• Hexagonal, trigonal, tetragonal
• All axes c are equal but c is unique

Orthorhombic, monoclinic, triclinic

• All axes are unequal

How light behaves depends on crystal structure (there is a reason you took mineralogy!)

Isotropic

Uniaxial

Biaxial

Let’s use all of this information to help us identify minerals

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 l of light
• Pleochroism results when different l are absorbed differently by different crystallographic directions -
• rotate stage to observe

hbl

hbl

plag

plag

• Plagioclase is colorless
• Hornblende is pleochroic in olive greens

Now do question 2

n1

n2

n2

n1

n2>n1

n2<n1

Mineral properties: Index of refraction (R.I. or n)

Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocity

• 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

- 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

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

Hi relief (+)

Lo relief (+)

Hi relief (-)

nxtl > nepoxy

nxtl = nepoxy

nxtl < nepoxy

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

Now do question 3

Mineral properties: interference colors/birefringence

• Colors one observes when polars are crossed (XPL)
• Color can be quantified numerically: d = nhigh - nlow

Now do question 4

More on this next week…

or

uniaxial

biaxial

If uniaxial, isogyres define cross; arms remain N-S/E-W as stage is rotated

If biaxial, isogyres define curve that rotates with stage, or cross that breaks up as stage is rotated

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)

Use of interference figures, continued…

• Now determine the optic sign of the mineral:
• Rotate stage until isogyre is concave to NE (if biaxial)
• Insert gypsum accessory plate
• 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)!!