Review of Optical Mineralogy
GEOL 5310
Advanced Igneous and Metamorphic
Petrology
9/9/09
Nature of Light
Visible light is a form of electromagnetic
radiation, which can be characterized as
pulses or waves of electrical energy
 Travels in straight lines with a transverse
wave motion

Unpolarized light
Polarized light
Attributes of Light
Wavelength () - distance between wave peaks; measured in
angstroms (Å); defines color of visible light
Amplitude (A) - height of light waves; corresponds to the
intensity/brightness of light
Frequency () - number of light waves passing a fixed point per
second; measured in cycles/second
Velocity (v = ·); speed of light in a vacuum = 3·1018 Å/sec = c
e.g. for orange light in a vacuum,  = 6000Å,  = 5·1014 /sec
Light slows down as it passes through denser substances. Because
the frequency of light never changes as it passes through different
substances, a decrease in light velocity reflects a proportional
decrease in its wavelength.
Electromagnetic Spectrum
From Bloss, 1961
Reflection and Refraction of Light



When light passes from a low density medium (e.g. air)
into a higher density non-opaque medium (e.g. a mineral),
part will be reflected and part will be pass through, but be
bent and slowed – refracted.
Angle of reflection (r’) equals the incident angle (i)
Angle of refraction (r) will differ from the incident angle
depending on the change in velocity between the two
substances
Refractive Index and Snell’s Law
Index of Refraction – n
nsubstance = c / vsubstance
>1
light velocity in air ≈ c, so nair ~ 1
Snell’s Law- predicts the angle of refraction
at the interface of two substances with
different refractive indicies:
ni sin i = nr sin r
r = sin-1 (ni/nr x sin i)
Successive Refraction
Refraction, Relief, and the Becke
Line
Relief is the degree to which a phase stands
out from its surroundings and is an
expression of the contrast in index of
refraction
dark outline
Becke Line Test
From Bloss (1961)
Dispersion


Because n is related to light velocity, which is
related to wavelength ((v = ·), different
wavelengths of light will have different refraction
indicies within a particular substance
Illuminating a mineral with white light may thus lead
to color dispersion
Polarization of Light


Light emanating from a
point source vibrates in
all directions normal to
the propagation
direction
Light can be polarized
(made to vibrate in one
plane) by selective
absorption (OR) or by
reflectance (OL)
Anisotropy
Indicies of refraction can vary in all minerals
(except those in the isometric system)
depending on the orientation of light ray.
Such minerals are said to be anisotropic.
Isometric minerals, glass, liquids and gasses
have a single refraction index value
regardless of the orientation of light rays.
Such substances are said to be isotropic.
Optical Indicatrices
• A 3-d map of the indices of refraction for various vibration
directions of light rays
• Orientation of the indicatrix within a mineral is symmetrical with the
crystallographic axis
Isotropic
Isometric
Anisotropic – Uniaxial AnisotropicBiaxial
Orthorhombic
Tetragonal
Monoclinic
Hexagonal
Triclinic
Isotropic Indicatrix
A sphere whose radius corresponds to
the characteristic refraction index- n
n=c/v
=c/
6563Å Red
5893ÅYellow
4861ÅBlue
Diagram shows change in n for different
wavelengths of light in same mineral
Optical Recognition of Isotropic Minerals
Total Extinction
under X-polars
Slowing of ray
= shortening of
wavelength, but
no change in
polarity
From Bloss (1961)
Slow ray
Fast ray
Anistropic
Minerals
All randomly oriented
anisotropic minerals
cause double refraction
(splitting) of light
resulting in mutually
perpendicular-polarized
light rays.
One ray has a higher n
(slow ray, or the
ordinary ray) than the
other ray (the fast ray,
or extraordinary ray)
Birefringence (), Retardation(Δ), and
Interference Colors
 = nslow ray – nfast ray
Δ = d* 
Uniaxial Indicatrix
Optic Axis
= C axis in tetragonal
and hexagonal crystals
Sections of Uniaxial Indicatrices
 = ω-ω = 0 (circular section)
 = ε’- ω (random section)
= ε - ω (principal section)
maximum birefringence
Total
extinction in
x-polar light
Re-Polarization of Light through a Non-circular
Section of the Uniaxial Indicatrix
Extinction of Uniaxial Minerals
Orthoscopic
Conoscopic
Conoscopic
Interference
Figures of Uniaxial
Minerals
Isochromes – zones of
equal retardation
Isogyres – represent the
areas where the ω and ε’
vibration directions are
oriented N-S, E-W
Uniaxial
Optic Axis
(OA)
Figure
Circular section
parallel to stage
=0
Off-centered OA Figure
Random section parallel to stage,  < 0, « max 
Very Off-centered OA Figure
Random section parallel to stage, 
« 0, < max 
Flash Figure
Principal section parallel to stage, 
= max 
Determining the Optic Sign of
Uniaxial Minerals
+
Connect the quadrants
that go down in color
(to yellow), compare
with slow direction of
gypsum plate for sign
Biaxial Indicatrix
Principal
vibration
axes
greatest n
lowest n
intermediate
n
< ’<<’<
Circular Sections and Optic Axes
Circular
Section
Optic
Plane
Optic
Axes
Circular
Section
2V and the Optic Sign
Trace of
Circular
Sections
+
-
Random Section through the
Biaxial Indicatrix
Vibration
plane
parallel to
stage
Double
refraction
rays
=0
=max
Variable
Birefringence
within a
Biaxial
Mineral
Biaxial Optic Axis Figures
Look for a mineral with the
lowest interference colors,
i.e. ~0
Acute Bisectrix Figures (Bxa)
Melatope (emergence
of optic axes)
Determining the Optic Sign of Biaxial Minerals
D
U
+
U
D
U
U
X
D
+
D
D
-
U
X
’ is fast ray
 is intermediate
’ is slow ray
U
D
Estimating 2V by Curvature of
Isogyre
Estimating 2V by Separation of
Isogyres
Extinction Angle
Symmetrical
Parallel
Inclined
Sign of Elongation
Example – Length slow
slow
ray
Slowing down
the slow ray
Interference
colors increase