Optical Mineralogy
PG 201
By: Dr. Hossam Helba
Optical Mineralogy PG 201
Dr. Hossam Helba
1
What is Optical Mineralogy and
why do we study it?
Optical mineralogy is concerned with studying the optical properties of minerals
that depend on the manner by which visible light is transmitted through thin
sections of transparent or translucent minerals by using polarizing microscope.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Optical Mineralogy
Course Objectives
1. Applying the nature and properties of
visible light to examine thin sections of
transparent or translucent minerals
2. Use of the petrographic (polarizing)
microscope
3. Recognition of the diagnostic optical
characters of the rock-forming minerals
4. Ability to identify an unknown
mineral based on determining its optical
properties
Optical Mineralogy PG 201
Dr. Hossam Helba
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References
Nesse, W. D., 2004. Introduction to optical mineralogy. Oxford University
Press, 348 pp.
Kerr, P.F., 1977. Optical mineralogy (4th Edition). New York (McGraw Hill
Book Company), 492 PP .
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Lecture 1
Nature and Properties of Light
- Light is a form of energy that can be detected with the eye, and is
transmitted from one place to another at finite velocity.
- Visible light is a small portion of the electromagnetic spectrum , which
ranges from cosmic rays to radio waves. It has wavelengths of between
about 400 and 700 nm in a vacuum.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Optical Mineralogy PG 201
Dr. Hossam Helba
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Nature of light
Theories developed to determine the nature of light:
1- Wave theory by Higgins
2- Corpuscular theory by Newton
3- Electromagnetic wave theory by Maxwell
4- Quantum theory by Planck
Both the particle and wave
theories have been shown to
be correct and can be
considered as complementary
theories. But, because wave
theory describes effectively the
phenomena of polarization,
reflection, refraction, and
interference-the meet of an
optical mineralogy course- so
we treat light as
electromagnetic radiation.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Wave motion of light
It is a simple harmonic movement combined with regular forward motion.
Simple harmonic motion is regular movement in a circular path looks as if
it were projected on the diameter of the circle
Optical Mineralogy PG 201
Dr. Hossam Helba
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Wave Nomenclature
Direction of propagation
Wave crest
Wave bottom
Wave amplitude (A)
Velocity (v)
Wave length (λ)
Frequency (f)
v=f x λ
Velocity of light
in vacuum
3 x 1010 cm/sec
3 x 10 17 nm/sec
Optical Mineralogy PG 201
Dr. Hossam Helba
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Refractive index, n, of a material or substance is defined as the ratio of the
speed of light in a vacuum, VC , to the speed of light in a material through
which it passes, Vm
n = VC / Vm
- Note that the value of refractive index will always be greater than 1.0, since
Vm can never be greater than VC. In general, Vm depends on the density of the
material, with Vm decreasing with increasing density. Thus, higher density
materials will have higher refractive indices.
- The refractive index of any material depends on the wavelength of light
because different wavelengths are interfered with to different extents by the
atoms that make up the material. In general refractive index varies linearly with
wavelength.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Isotropic and anisotropic materials
- Isotropic material is a material in which the velocity of light is the same in all
directions. They include gases, liquids, amorphous materials (as glasses) and
minerals in the cubic system.
- Anisotropic material is a material in which light travels at a different speed in
different directions. They include minerals in tetragonal, hexagonal, trigonal,
orthorhombic, monoclinic and triclinic systems.
- Any isotropic material has a single constant refractive index for each
wavelength.
- Any anisotropic material is characterized by the phenomenon of double
refraction, where any ray passes through it splits into two rays vibrating
perpendicular to each other. Anisotropic materials will have a range of refractive
indices between two extreme values for each wavelength.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Reflection and Refraction
When light passes from one material to another at any angle other
than perpendicular to the boundary: Some reflected - Some refracted
For Reflection
Angle of incidence = angle of reflection
Optical Mineralogy PG 201
Dr. Hossam Helba
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For Refraction
- Angle of incidence ≠ angle of refraction
- The angle of refraction is dependent on the angle of incidence and the
refractive index ,n of the materials on either side of the interface according to
Snell’s law : ni sin (i) = nr sin (r)
- A high refractive index indicates low velocity and vice versa
Snell’s law
Optical Mineralogy PG 201
Dr. Hossam Helba
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Critical Angle and Total Internal Reflection
When light passes from a high
index material to a low index
material, the angle of refraction
will be larger than the angle of
incidence.
Light with an angle of incidence
greater than the critical angle
(CA) cannot be refracted into the
low-index material.
The critical angle is the angle of
incidence that yields an angle of
refraction of 90º
Optical Mineralogy PG 201
Dr. Hossam Helba
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Dispersion
- The index of refraction of a material is not the same for all wavelengths of
light, consequently, dispersion occurs .
- Normal dispersion, within same material:
n higher for short wavelengths (violet)
n lower for long wavelengths (red)
Optical Mineralogy PG 201
Dr. Hossam Helba
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Wave front and wave normal
The light passing through a mineral or through space commonly passes as a beam
of light rather than a single wave. In this case a wave front is a surface that
connects similar points on adjacent waves or connects points of the same phase. A
line constructed at right angles to the wave front is called the wave normal and
represents the direction that the wave is moving.
In isotropic materials (light velocity the same in all directions), the light ray and
wave normal coincide.
In anisotropic materials (light velocity different in different directions) the wave
normal and light ray directions usually are not parallel .
Optical Mineralogy PG 201
Dr. Hossam Helba
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Interference of light waves
If two waves travel along the same path, they can interfere with each other. The
distance that one wave lags behind the other is called the retardation (Δ). It can be
described either in terms of the distance in nanometers, or in terms of the number
of wavelengths that one wave lags the other.
If the two waves vibrate in the same plane three cases of interference occur:
1- When the retardation equals an integral number of wavelengths Δ = i λ
the two waves are in phase. They constructively interfere with each other to
produce a resultant wave that is the arithmetic sum of the two.
2- When the retardation equals ½ , 1 ½, 2 ½, etc. wavelengths, the two waves are
out of phase. They destructively interfere and cancel each other .
3- When the retardation is some intermediate value, the light is partially in phase
(or partially out of phase, if you prefer) and the interference is partially
constructive (or partially destructive).
Optical Mineralogy PG 201
Dr. Hossam Helba
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(1) Constructive interference
(2) Destructive interference
(3) Partially Constructive interference
Optical Mineralogy PG 201
Dr. Hossam Helba
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Polarization of light
Normal light vibrates equally in all direction perpendicular to its
path of propagation. If the light is constrained to vibrate in only one
plane, however, we say that it is plane polarized light.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Unpolarized and Polarized Light

Unpolarized light

Vibrates in all directions perpendicular to
direction of propagation
Occurs only in isotropic materials like isometric
crystals, Air, water, glass, etc.


Polarized light
Vibrates in only one direction

Plane, circular, and elliptical polarization
were recognized- we focus on plane
polarized light

Plane polarized light can be achieved by:
(1) Refraction
(2) Reflection

Optical Mineralogy PG 201
Dr. Hossam Helba
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Polarizer
Light vibrating in many
Planes and of many
wave lengthes
(unpolarized light)
Light vibrating in one plane
Optical Mineralogy PG 201
Dr. Hossam Helba
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Polarization by Refraction
- When light enters an anisotropic material, it experiences double refraction
(it is split into two rays that vibrate at right angles to each other, have
different velocities, and may have different absorption).
- This property can be used to produce polarized light by removing one of the
two plane polarized rays. This is done either by selective absorption or by
utilizing the critical angle effect.
- Selective absorption provides the
basis for polarization with
polarizing films. A variety of
anisotropic materials, among them
tourmaline, have the property of
strongly absorbing light vibrating in
one direction and transmitting light
vibrating at right angles.
Optical Mineralogy PG 201
Dr. Hossam Helba
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A second means of eliminating one of the two plane
polarized rays produced by double refraction utilizes
the critical angle effect. The most commonly known
device using this principle is the Nicol prism, which is
constructed of clear calcite (iceland spar). A crystal of
calcite is cut on the diagonal, and glued back together
with balsam cement, which has an index of refraction
of 1.537. When light enters the calcite, it is split into
two plane polarized rays that have different velocities
and that vibrate at right angles to each other. Each ray
has its own index of refraction. The cut through the
crystal is oriented so that the ordinary ray (O) strikes
the boundary between the balsam cement and the
calcite at greater than the critical angle, hence it is
internally reflected and absorbed by black paint on the
side of the prism. The extraordinary ray (E), with its
lower index of refraction, is able to enter the balsam
cement, so it is transmitted through the prism to
emerge at the top as plane polarized light.
Optical Mineralogy PG 201
Dr. Hossam Helba
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Polarization by Reflection
When unpolarized light strikes a
smooth surface, such as a piece of
glass, a smooth table top, or the
surface of a lake, the reflected light
is polarized so that its vibration
direction is parallel to the reflecting
surface. The reflected light is not
completely plane polarized unless
the angle between the reflected and
refracted rays is 90° .
Optical Mineralogy PG 201
Dr. Hossam Helba
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Polarizing microscope and thin sections
Although there are numerous designs of polarizing
microscopes, all of them have the following
fundamental parts :
- Illumination part
- Substage assembly (including lower polarizer)
- Stage
- Objective lenses
- Upper polarizer (analyzer)
- Bertrand lens
- Ocular lens
Optical Mineralogy PG 201
Dr. Hossam Helba
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Typical design of Zeiss Polarizing microscope
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Slightly more modern version
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Dr. Hossam Helba
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Preparing the microscope for work

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

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Adjusting illumination
Justifying cross polars
centering the microscope
Delineating the polarizer vibration by using of biotite
or tourmaline slides
Adjusting the cross hairs of the ocular with the
analyzer and polarizer vibrations
Making calibration of vision view
Optical Mineralogy PG 201
Dr. Hossam Helba
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Optical Mineralogy PG 201
Dr. Hossam Helba
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Optical properties
Described by polarizing microscope



Using normal and plane polarized light (PPL)
 Colour & pleochroism – crystal shape – relief & twinkling –
refractive index – cleavage - alteration
Using crossed polarized light (C.N)
 Isotropism – interference colour – birefringence-extinction –
twinning – elongation sign
Using convergent crossed polarized light
 Interference figure – optic sign – birefringence – optic angle
Optical Mineralogy PG 201
Dr. Hossam Helba
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