optical identification mineral

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 W inter (W hitman 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 describe
Twinning – type of twinning, orientation
Extinction angle – parallel or inclined? Angle?
Isotropic vs. anisotropic minerals – 100% extinct in XPL?
Today we’ll break down anisotropic minerals into
uniaxial 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
Some light is now
able to pass
through the
upper polarizer

fast ray
slow ray
mineral
grain

plane polarized
light

W

E

lower polarizer

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


Calcite experiment and double refraction
O

E

Fig 6-8 Bloss, Optical
Crystallography, MSA
Fig 6-7 Bloss, Optical
Crystallography, MSA


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

calcite
ca
lc
it
e
ca l

calcite

cit

e

calcite
ordinary
ray, ω
(stays stationary)

extraordinary
ray, ε
(rotates)


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

Isotropic

Isometric
– All crystallographic axes are equal

Uniaxial

Hexagonal, trigonal, tetragonal
– All axes ⊥ c are equal but c is unique

Biaxial

Orthorhombic, monoclinic, triclinic
– All axes are unequal

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 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


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)

or

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

biaxial
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 plate
3. Note color in NE, immediately adjacent to isogyre -
Blue = (+)

Yellow = (-)
uniaxial

(+)

(+)

biaxial

Without plate

Gypsum plate inserted


Remember determining optic sign last week with the gypsum plate?
blue in NE = (+)

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

w

Isogyres = black:
∆=0
Background = gray: ∆=100
Add or subtract 530 nm:
530+100=630 nm = blue = (+)
530-100=430 nm = yellowish = (-)
Addition = slow + slow
Subtraction = 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 δ = nhi-nlo = 0 = black


anisotropic minerals - uniaxial indicatrix
c-axis

c-axis

calcite
quartz

Let’s perform the same thought experiment…


Uniaxial indicatrix
c-axis

c-axis

tangerine = uniaxial (-)
Spaghetti squash = uniaxial (+)

quartz

calcite


Uniaxial ellipsoid and conventions:
Fig 6-11 Bloss, Optical
Crystallography, MSA

(-) crystal:
ω>ε
→ oblate

(+) crystal:
ε>ω
→ prolate


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

nω

χ=Ζ

nω

nε
νω

β=Ψ

α=Ξ

nω - nω = 0

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


Now propagate light perpendicular to c-axis

nε - nω > 0

N

therefore, δ > 0

ω

nnε ε

n ωnω

W

nnω
nω

n

nε
nε

ε

E

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

This orientation will show the maximum δ of the mineral


anisotropic minerals - biaxial indicatrix

clinopyroxene

feldspar

Now things get a lot more complicated…


Biaxial indicatrix
(triaxial ellipsoid)
OA

Z

2Vz

OA

2Vz
νγ

Y

nβ

νβ

να

nβ

The potato!

X
nγ

nγ
να

νβ

nα
νβ

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


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

a
Z

Z

olivine

c

augite

b
Y

b
X

a
X


… 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
OA

Z

OA

• When 2V is acute about Z: (+)

2Vz

• When 2V is acute about X: (-)
• When 2V=90°, sign is indeterminate

νγ

• When 2V=0°, mineral is uniaxial
Y

nβ

να

X

2V is measured using an interference figure…
More in a few minutes


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)

n

ε

substage

condensor

nω

n

nε

ω

nω
nε

n

ε

n

ω

Effects of multiple cuts thru indicatrix

W

E


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 sign w/ gyps

(+)

determine 2V from curvature of isogyre
90°

60°

40°

See Nesse p. 103

(-)