Optical Mineralogy Basics

Optical Mineralogy Course (801202)

• Course offered during the first semester of 2024/2025
• Taught by Dr. Sanaa Al-Zyoud
• Department of Applied Earth and Environmental Sciences
• Faculty of Earth and Environmental Sciences, Al-al-Bayt University

Course Description

• Focuses on the optical properties of rock-forming minerals
• Covers light properties and theories
• Explains plane polarized light (PPL) and cross-polarized light (XPL)
• Includes the study of igneous, metamorphic, and sedimentary rocks
• Aims to teach data analysis and interpretation skills
• Focuses on a scientific approach to problem-solving

Course Outcomes (COs)

• Students will distinguish different types of light interaction.
• Students will link light properties to the physical properties of minerals.
• Students will evaluate mineral properties optically.
• Students will distinguish minerals optically.

Textbook

• Title: Introduction to Optical Mineralogy
• Author: William D. Nesse
• Publisher: Oxford University Press, Inc.
• Year: 1991
• Edition: Second Edition
• Website: https://www.amazon.com/Introduction-Optical-Mineralogy-William-Nesse/dp/0195060245

Course Content

• Week 1: Introduction, nature of light, electromagnetic radiation, perception of color, interaction of light and matter, plane polarized light
• Week 2: Petrographic microscope, illuminator, substage assembly, microscope stage, objective lenses, upper polar, Bertrand lens
• Week 3: Refractometry, relief, Becke line method
• Week 4-8: Optics of Isotropic and Anisotropic Materials, Identification of Isotropic & Anisotropic minerals, optics of anisotropic minerals, Interference of Phenomena, Determining Thickness of a Sample, Determining birefringence from the color charts
• Week 8: Midterm Exam
• Week 9-12: Extinction, Accessory Plates, Sign of Elongation, Relief, Pleochroism, Uniaxial Optics, Optic Sign, Crystallographic Considerations, Uniaxial Indicatrix, Birefringence and Interference Colors, Extinction, Pleochroism, Interference Figure, Selecting Grains to Give Interference Figures, Determining Indices of Refraction, Biaxial Optics, Biaxial Indicatrix, Crystallographic Orientation of Indicatrix Axes, Biaxial Interference figures
• Week 13: Identification of Minerals, Descriptive Features, Cleavage, Twinning, Alteration, Association, Tactics, Opaque Minerals, Non-minerals
• Week 14: Optical Properties of Non-silicate (Selected)
• Week 15: Optical Properties of Silicate (Selected)
• Week 16: Final Exam

Light as a Tool of Examination

• Describes the role of light in examining the structure of minerals

What is Light?

• A form of energy
• Transmitted with finite velocity
• Part of the electromagnetic spectrum
• Ranges from cosmic rays to radio waves
• Visible light is a small portion of this spectrum (390nm - 770nm)

How the Light Transfer in Microscope

• Diagram illustrating light path in a microscope, including components like eyepieces, analyzer, ordinary ray, birefringent specimen, polarizer

Electromagnetic Radiation and Light Waves

• Light's relationship to electromagnetic radiation and wavelength
• Formula linking velocity(V) and wavelength(λ) with frequency(F).

Wave Front, Wave Normal

• Definition of wave front and wave normal in light waves.

Components of Light and Propagation

• Light has two main components: electric and magnetic vector.
• As light propagates, it interacts with the crystal lattice with potential changes in speed & direction.

Light Components

• Light is described having electric and magnetic components.
• These vibrate perpendicular to the direction of propagation

Isotropic and Anisotropic Minerals

• Isotropic minerals: Same light velocity in all directions. Examples: volcanic glass, isometric minerals like fluorite, garnet, halite
• Anisotropic minerals: Different light velocities depending on the direction of light travel. Examples: tetragonal, hexagonal, orthorhombic, monoclinic, triclinic systems.

Refractive Index, Dispersion, Polarization, and Double Refraction

• The Snell-Descartes Law
• Light velocity in vacuum
• Refractive index (n) definition
• Refractive index varies with temperature and wavelength

Refractive Index

• Refractive index of water - 1.333
• Light travels slower in denser media.

Refractive Index

• Snell's law
• Relationship between angles of incidence and refraction for light passing between two media like air and a mineral.

Refractive Index

• Diagram illustrating refraction, critical angle, and total internal reflection.

Polarization of Light

• Explains the restriction of light vibration to a single plane
• Polarized light microscopy: Reveals details about crystal structures, birefringence, pleochroism, and refractive index

Generating Polarized Light

• Methods for generating polarized light, like selective absorption, reflection, refraction, and scattering

Polarization by Selective Absorption

• Absorption of light based on orientation relative to the polarizing filter

Polarization in Crystals and Color Effects

• Double refraction in crystals, which split polarized light into two rays
• Pleochroism; mineral color changes with rotation of sample

Polarization by Reflection

• Polarization happens partially or fully, depending on the angle of incidence
• Brewster's angle for full polarization

Brewster's Angle and Its Calculation

• Calculation of Brewster's angle using Snell's Law.
• Practical applications in photography and optical instruments

Crossed Polarizers in Microscopy

• Polarized Light Microscopy
• Identifying minerals through birefringence, pleochroism, and extinction angles

Polarization by Scattering

• How polarization occurs due to scattering of light off particles in the atmosphere
• Applications in astronomy

Summary of Polarization in Mineralogy

• Polarized light's role in optical mineralogy, mineral structure, properties
• Various polarization methods
• Applications across diverse scientific fields.

Lecture (4)

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Double Refraction in Anisotropic Minerals: Example Calcite

• Anisotropic minerals' light propagation at varying velocities.
• Light split into separate rays within the crystal
• Double Refraction phenomenon

Ordinary and Extraordinary ray

• Description of ordinary and extraordinary rays in crystals
• How wave fronts relate to ray paths.

Fig. 2.8 Double refraction of calcite

• Visualization of double refraction in calcite crystals
• Demonstrates how the behavior is viewed using microscopes.

Optic axis

• Light direction unaffected by refraction or polarization along the c-axis
• Represents the crystallographic c-axis of the mineral's three-fold rotation

Retardation of Rays

• The difference in time the slow ray lags behind the fast ray
• Calculations related to the retardation, which impacts the mineral’s observed properties

Retardation

• Relation to thickness(d) of the mineral and the refractive indices for the two vibration directions

Retardation

• Velocity of the slow and fast rays, distance traveled, retardation, and calculation

Retardation

• Detailed calculation of retardation(Γ) and relationship between thickness(d), refractive indices(n), and velocity of light(V)

Retardation

• Definition and relationship of retardation to mineral thickness and refractive indices

Lecture (5)

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Minerals properties in PPL

• Using polarized light to observe the properties of rocks and minerals
• Microscopes used: Petrographic microscope, Polarizing microscope
• Properties to identify minerals

Minerals properties in PPL

• Fundamental principle: Most minerals, even dark-colored ones, transmit light when thin.
• How light passes through samples in a petrographic microscope

Minerals properties in PPL

• Optical mineralogy today largely uses thin sections.
• These are thin (0.03mm-30mm) slices of minerals or rocks mounted on glass-slides

How to make thin sections

• Steps involved in producing thin sections.
• Images illustrating the process

Minerals thin sections

• Practical applications for identification, composition, mineral relations, and origins.

Minerals Color

• Identifying opaque minerals, use of transmitted and reflected light microscopy to observe visible characteristics of minerals

Minerals Color

• Color of mineral in hand specimen vs. thin section. Limitations of hand specimen observations for identification
• Importance of identifying minerals.

Minerals Color

• Table of examples of opaque (non-transparent) and non-opaque (transparent) minerals, with their optical characteristics (e.g., isotropy, uniaxial, biaxial).

Pleochroism

• Variation in color from rotations of sample
• Color differences are caused by differential absorption of light. This is not related to interference colors

Pleochroism

• General term for phenomena in light absorption by different crystalline vibrational directions.
• Definitions for Dichroism and Trichroism within the context

Pleochroism

• Detailed explanation of the dichroism and trichroism phenomena in relation to polarized light

Minerals Color

• Images of minerals in thin section, displaying some observed characteristics

Relief

• Refractometry measurements
• Use of immersion oils for refractive index determination
• The difference in refractive indices in the mineral and surrounding medium.

Relief

• Relief is the degree to which a mineral grain stands out relative to mounting material like oil or Canada balsam.
• High Relief: Index difference is larger than certain amount.
• Low Relief: Index difference is minimal.

Relief

• Strong Relief: large index difference
• Moderate Relief: moderate index difference
• Low Relief: small index difference

Relief

• Positive and negative relief definitions, based on mineral's refractive index vs. immersion medium

Lecture (6)

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Becke Line

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