optical identification mineral

Some review…Optical mineral properties ONLY visible in PPL: Color – not an interference color!. Optical mineral properties visible in PPL or XPL: Cleavage – number and orientation of cle

Optical Mineralogy in a Nutshell

Use of the petrographic microscope in

three easy lessons

Part III

Some review…

Optical mineral properties ONLY visible in PPL:

Color – not an interference color! (for that, see below)

Pleochroism – is there a color change while rotating stage?

Relief – low, intermediate, high, very high?

Optical mineral properties visible in PPL or XPL:

Cleavage – number and orientation of cleavage planes

(may need higher magnification and at different grains )

Habit – characteristic form of mineral (sometimes better in XPL)

Optical mineral properties ONLY visible in XPL:

Birefringence – use highest order interference color to describe

Twinning – type of twinning, orientation

Extinction angle – parallel or inclined? Angle?

Isotropic vs anisotropic minerals – 100% extinct in XPL?

uniaxial or biaxial …

Some generalizations and vocabulary

• All isometric minerals (e.g., garnet) and glass are

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 (the “optic axes”) that do not reorient light

– Minerals with one special direction are called uniaxial

– Minerals with two special directions are called biaxial

• Uniaxial and biaxial minerals can be subdivided into

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

mineral grain

plane polarized light

When light gets split:

-velocity changes -rays get bent (refracted) -2 new vibration directions -usually see new colors

Calcite experiment and double refraction

We’ve talked about minerals as magicians -

now let’s prove it!

calcite

calcite

calcite

ca lcite

ordinary ray, ω

(stays stationary) extraordinaryray, ε

(rotates)

calcite

Uniaxial

Biaxial

How light behaves depends on crystal structure

(there is a reason you took mineralogy!)

Isometric

– All crystallographic axes are equal

Orthorhombic, monoclinic, triclinic

– All axes are unequal

Hexagonal, trigonal, tetragonal

– All axes ⊥ c are equal but c is unique

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

Simple guide to interference figures

• Get a good interference figure;

• Distinguish uniaxial and biaxial figures;

• Determine optic sign; and

• Estimate 2V

1) Choose a grain showing the lowest interference colors2) Move to the high-powered objective lens and refocus3) Open the sub-stage diaphragm as wide as possible4) Insert the condenser lens

5) Cross the polars

6) Insert the Bertrand lens

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)

uniaxial

If uniaxial , isogyres define

cross; arms remain N-S/E-W as

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

Remember determining optic sign last week with the gypsum plate?

530+100=630 nm = blue = (+) 530-100=430 nm = yellowish = (-) Addition = slow + slow

Time for some new tricks: the optical indicatrix

Thought experiment:

Consider an isotropic mineral (e.g., garnet)

Imagine point source of light at garnet center; turn light on for fixed amount of time, then map out distance traveled by light in that time

What geometric shape is defined by mapped light rays?

anisotropic minerals - uniaxial indicatrix

Uniaxial ellipsoid and conventions:

nω - nω = 0

therefore, δ =0: grain stays black

(same as the isotropic case)

Grain changes color upon rotation.

Grain will go black whenever indicatrix axis is E-W or N-S

n ω

anisotropic minerals - biaxial indicatrix

Alas, the potato (indicatrix) can have any orientation within a biaxial mineral…

… but there are a few generalizations that we can make

The potato has 3 perpendicular principal axes of

different length – thus, we need 3 different RIs

to describe a biaxial mineral

X direction = nα (lowest)

Y direction = nβ (intermed; radius of circ section)

Z direction = nγ (highest)

• Orthorhombic: axes of indicatrix coincide w/ xtl axes

• Monoclinic: Y axis coincides w/ one xtl axis

• Triclinic: none of the indicatrix axes coincide w/ xtl axes

2V: a diagnostic property of biaxial minerals

• When 2V is acute about Z: (+)

• When 2V is acute about X: (-)

• When 2V=90°, sign is indeterminate

• When 2V=0°, mineral is uniaxial

2V is measured using an interference figure…

How interference figures work (uniaxial example)

Biaxial interference figures

There are lots of types of biaxial figures… we’ll concentrate on only two

1 Optic axis figure - pick a grain that stays dark on rotation

Will see one curved isogyre

determine 2V from curvature of isogyre

determine sign w/ gyps

Estimating 2V

2 Bxa figure (acute bisectrix) - obtained when you are looking straight down between the two O.A.s Hard to find, but look for a grain with

intermediate δ

Biaxial interference figures

Use this figure to get sign and 2V:

Quick review:

Indicatrix gives us a way to relate optical phenomena to

crystallographic orientation , and to explain differences

between grains of the same mineral in thin section

2V z

Y

X Z

nβ

νγ

να

lo δ

Isotropic? Uniaxial? Biaxial? Sign? 2V?

All of these help us to uniquely identify unknown minerals.