Optical Mineralogy in a Nutshell

Use of the petrographic microscope in three easy lessons

Part III

Slides borrowed/adapted from Jane Selverstone (University of New Mexico) and John Winter (Whitman College)

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 describeTwinning – type of twinning, orientationExtinction angle – parallel or inclined? Angle?Isotropic vs. anisotropic minerals – 100% extinct in XPL?

Today we’ll break down anisotropic minerals intouniaxial or biaxial…

Some generalizations and vocabulary

• All isometric minerals (e.g., garnet) and glass 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 (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 optically positive and optically negative, depending on the orientation of fast and slow rays relative to the xtl axes

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

fast ray

slow ray

lower polarizerW E

Some light is now able to pass through the upper polarizer

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

O E

Fig 6-7 Bloss, Optical Crystallography, MSA

Fig 6-8 Bloss, Optical Crystallography, MSA

Calcite experimentCalcite experiment and double refractiondouble refraction

We’ve talked about minerals as magicians - now let’s prove it!

calcite

calcite

calcitecalciteordinaryray,

(stays stationary)extraordinary

ray, (rotates)

calcite

Isotropic

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 lens5) Cross the polars6) 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 stage is rotated

biaxial

or

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

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 plate3. Note color in NE, immediately adjacent to

isogyre -- Blue = (+) Yellow = (-)

uniaxial

biaxial

(+)

(+)

Without plateGypsum plate inserted

Remember determining optic sign last week with the gypsum plate?

slow

blue in NE = (+)

Gypsum plate has constant of 530 nm = 1st-order pink

Isogyres = black: =0Background = gray: =100

Add or subtract 530 nm:

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

Addition = slow + slowSubtraction = slow + fast

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?

Isotropic indicatrix

Soccer ball(or an orange)

Light travels the same distance in all directions;n is same everywhere, thus = nhi-nlo = 0 = black

anisotropic minerals - uniaxial indicatrix

quartz

calcite

c-axis

c-axis

Let’s perform the same thought experiment…

Uniaxial indicatrix

c-axisc-axis

Spaghetti squash = uniaxial (+)

tangerine = uniaxial (-)

quartz

calcite

Uniaxial ellipsoid and conventions:

(-) crystal: > oblate

(+) crystal: > prolate

Fig 6-11 Bloss, Optical Crystallography, MSA

n - n = 0therefore, =0: grain stays black (same as the isotropic case)

n

n

Propagate light along the c-axis, note what happens to it in plane of thin section

Grain changes color upon rotation. Grain will go black whenever indicatrix axis is E-W or N-S

n

n

This orientation will show the maximum of the mineral

n

n

n

n

n

n

n

n

n - n > 0therefore, > 0

N

S

W E

Now propagate light perpendicular to c-axis

anisotropic minerals - biaxial indicatrix

clinopyroxenefeldspar

Now things get a lot more complicated…

Biaxial indicatrix(triaxial ellipsoid)

The potato!

2Vz

There are 2 different ways to cut this and get a circle…

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

olivine augite

… 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 = n (lowest)Y direction = n (intermed; radius of circ. section)Z direction = n (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… More in a few minutes

How interference figures work (uniaxial example)

Bertrandlens

Sample(looking down OA)

substagecondensor

Converging lenses force light rays to follow different paths through the indicatrix

W E

N-S polarizerWhat do we see??

n

n

n

n

nn

nn

Effects of multiple cuts thru indicatrix

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

90° 60° 40°

See Nesse p. 103

determine sign w/ gyps

(+) (-)

Estimating 2V

OAPFig 11-5A Bloss, Optical Crystallography, MSA

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 .

Biaxial interference figures

Use this figure to get sign and 2V:

(+) 2V=20° 2V=40° 2V=60°

See Nesse p. 101

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

hi

lo

Isotropic? Uniaxial? Biaxial? Sign? 2V?All of these help us to uniquely identify unknown minerals.