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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:

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slide1

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)

slide2

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?

Today we’ll break downanisotropic minerals into

uniaxial or biaxial…

slide3

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

slide4

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

slide5

O E

Calcite experimentanddouble refraction

Fig 6-8 Bloss, Optical Crystallography, MSA

Fig 6-7 Bloss, Optical Crystallography, MSA

slide6

calcite

calcite

calcite

calcite

calcite

ordinary

ray, w

(stays stationary)

extraordinary

ray, e

(rotates)

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

slide7

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

simple guide to interference figures

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 colors

2) Move to the high-powered objective lens and refocus

3) Open the sub-stage diaphragm as wide as possible

4) Insert the condenser lens

5) Cross the polars

6) Insert the Bertrand lens

slide9

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)

slide10

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 = (-)

uniaxial

(+)

(+)

biaxial

Without plate

Gypsum plate inserted

slide11

blue in NE = (+)

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

Isogyres = black: D=0

Background = gray: D=100

Add or subtract 530 nm:

530+100=630 nm = blue = (+)

530-100=430 nm = yellowish = (-)

Addition = slow + slow

Subtraction = slow + fast

slow

Remember determining optic sign last week with the gypsum plate?

slide12

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

Time for some new tricks: the optical indicatrix

Thought experiment:

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

What geometric shape is defined by mapped light rays?

slide13

Light travels the same distance in all directions;

n is same everywhere, thus d = nhi-nlo = 0 = black

Isotropic indicatrix

Soccer ball

(or an orange)

slide14

Let’s perform the same thought experiment…

anisotropic minerals - uniaxial indicatrix

c-axis

c-axis

calcite

quartz

slide15

Uniaxial indicatrix

c-axis

c-axis

tangerine = uniaxial (-)

calcite

Spaghetti squash = uniaxial (+)

quartz

slide16

(+) crystal:

e > w

 prolate

(-) crystal:

w > e

 oblate

Fig 6-11 Bloss, Optical Crystallography, MSA

Uniaxial ellipsoid and conventions:

slide17

nw

nw

nw - nw = 0

therefore, d=0: grain stays black

(same as the isotropic case)

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

slide18

N

nw

nw

nw

nw

nw

W

E

ne

ne

ne

ne

ne

S

Now propagate light perpendicular to c-axis

ne - nw > 0

therefore, d > 0

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

This orientation will show the maximum d of the mineral

slide19

anisotropic minerals - biaxial indicatrix

feldspar

clinopyroxene

Now things get a lot more complicated…

biaxial indicatrix triaxial ellipsoid

2Vz

The potato!

Biaxial indicatrix(triaxial ellipsoid)

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

slide21

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

augite

olivine

slide22

… 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 = na(lowest)

Y direction = nb(intermed; radius of circ. section)

Z direction = ng(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
slide23

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

slide24

nw

nw

nw

nw

ne

ne

ne

ne

Effects of multiple cuts thru indicatrix

How interference figures work (uniaxial example)

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

Bertrand

lens

N-S polarizer

What do we see??

Sample

(looking down OA)

substage

condensor

W

E

slide25

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

determine sign w/ gyps

Will see one curved isogyre

(+)

(-)

determine 2V from curvature of isogyre

90°

60°

40°

See Nesse p. 103

Biaxial interference figures

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

estimating 2v
Estimating 2V

Fig 11-5A Bloss, Optical Crystallography, MSA

OAP

slide27

2V=20°

2V=40°

2V=60°

(+)

See Nesse p. 101

Biaxial interference figures

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 d.

Use this figure to get sign and 2V:

slide28

hi d

lo d

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

Isotropic? Uniaxial? Biaxial? Sign? 2V?

All of these help us to uniquely identify unknown minerals.