Optical Mineralogy Course 2024/2025

Optical Mineralogy Course (801202)

• Course offered by the Applied Earth and Environmental Sciences Department, Earth and Environmental Sciences Faculty, Al al-Bayt University
• First semester, 2024/2025
• Instructor: Dr. Sanaa Al-Zyoud
• Course aims to study optical properties of rock-forming minerals, including light properties and theories, plane polarized light, and its properties
• Students will study igneous, metamorphic, and sedimentary rocks
• Students will gain basic knowledge, data analysis, and interpretation techniques
• Course includes lectures, reports, assignments, and training using instrumentation and techniques

Course Outcomes

• Upon successful completion, students will be able to distinguish between different types of light interaction
• Students will understand the link between light and minerals' physical properties
• Students will be able to evaluate minerals' optical properties
• Students will be able to distinguish minerals optically

Textbook

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

Course Content

• Week 1: Introduction (nature of light, electromagnetic radiation, phases, perception of color, interaction of light and matter, plane polarized light) - Chapter 1
• Week 2: Petrographic Microscope (illuminator, substage assembly, microscope stage, objective lenses, upper polar, Bertrand lens) - Chapter 2
• Week 3: Refractometry (relief, Becke line method) - Chapter 3
• Week 5: Optics of Isotropic Materials (isotropic indicatrix, distinguishing between isotropic and anisotropic minerals, identification of isotropic minerals in thin sections) - Chapter 4
• Week 6: Optics of Anisotropic Minerals (introduction, interference phenomena, determining thickness of a sample, determining birefringence from the color chart, recognizing different orders of interference colors) - Chapter 5
• Week 8: Optics of Uniaxial and Biaxial Optics (optics sign, crystallography considerations, uniaxial indicatrix, birefringence and interference colors, extinction, pleochroism, interference figure, selecting grains to give interference figures, determining indices of refraction) - Chapters 6 and 7
• Week 9-10: Identification of Minerals (descriptive features, cleavage, twinning, alteration, association, tactics, opaque minerals, non-minerals) – Chapters 8, 9-10
• Week 11-15: Optical Properties of Minerals (silicates and non-silicates) - Chapters 11-15
• Week 16: Final Exam

Light as a Tool

• Light interacts with minerals' atomic structure
• Used to identify minerals in rocks using a petrographic microscope

What is Light?

• A form of energy, detectable by the eye
• Transmitted from one place to another at a finite speed
• Part of the electromagnetic spectrum (390-770 nm visible)

How Light Transfers in a Microscope

• Detailed diagram of the light path (see image)

Electromagnetic Radiation and Light Waves

• Light consists of vibrating electric and magnetic components at right angles to each other
• Vibration direction is perpendicular to propagation direction
• Described by velocity, frequency, and wavelength
• Velocity and wavelength are related mathematically (V = Fλ )

Wave Front, Wave Normal

• Wave front: parallel surfaces connecting equivalent points on adjacent waves
• Wave normal: perpendicular to wave front, represents propagation direction
• Light ray: direction of light energy propagation

Components of Light and Propagation

• Light has electric and magnetic components
• Components vibrate perpendicularly to propagation direction
• Interaction with crystal lattice affects direction and speed

Isotropic and Anisotropic Minerals

• Isotropic: same light velocity in all directions
• Examples: volcanic glass, cubic minerals (fluorite, garnet, halite)
• Anisotropic: different light velocity in different directions
• Examples: tetragonal, hexagonal, orthorhombic, monoclinic, triclinic minerals

Refractive Index

• Ratio of light velocity in vacuum to velocity in a medium
• Varies with temperature and wavelength
• Snell-Descartes law describes light refraction at boundaries between media

Polarization of Light

• Restriction of light vibration to a single plane
• Important in mineralogy for studying thin sections
• Polarized light microscopy reveals optical properties like birefringence, pleochroism, and refractive index

Polarization Techniques

• Selective absorption
• Reflection
• Refraction
• Scattering

Polarization in Crystals and Color Effects

• Double refraction: light splits into two rays in crystals
• Pleochroism: minerals change color with different viewing angles

Polarization by Reflection

• Reflection can polarize light, depending on the angle of incidence (Brewster's angle)

Brewster's Angle

• Specific angle where reflected light is fully polarized
• Calculated using Snell's Law and refractive indices

Crossed Polarizers

• Two polarizers at 90 degrees
• Block light unless a mineral sample introduces birefringence
• Used to identify minerals (birefringence, pleochroism, extinction angles)

Polarization by Scattering

• Light scattering in atmosphere polarizes light
• Used to study planetary atmospheres and detect exoplanets

Summary of Polarization in Mineralogy

• Polarized light studies minerals' internal structure and optical properties
• Techniques like selective absorption, scattering, reflection, double refraction generate polarized light

Minerals' Properties in Plane Polarized Light (PPL)

• Study rocks and minerals using PPL
• Petrographic microscopes for transmitted light
• Darkly colored minerals transmit light when thin enough

How to Make Thin Sections

• Detailed steps for preparing mineral samples to make thin sections
• Cutting, polishing, mounting, grinding, final polishing

Minerals Thin Sections

• Studying grain mounts or thin sections with transmitted light
• Reveal properties not discernible with other methods
• Mineral identification, composition, and relationships

Minerals Color

• Color is characteristic for specific minerals
• Be cautious in relying on color as the only identifying tool
• Color in thin sections, opaque minerals (studied under reflected light)

Pleochroism Defined

• Color variation in anisotropic minerals under polarized light, upon rotation
• Related phenomena: dichroism, trichroism
• Resulting from interference and selective absorption of light

Relief

• Mineral grain standing out from mount material, like oil, Canada balsam, or another mineral
• Strong, moderate, low relief relate to difference in refractive indices
• Refractometry technique using immersion oil of known refractive index to determine unknown mineral's refractive index

Becke Line

• Determination of relative refractive indices, to observe whether the mineral is higher or lower in refractive index than the oil medium

Additional Information

• Course materials include diagrams and images