Optical Mineralogy

Polarization

Orientation of light waves in a specific direction.

Refractive Index (R.I)

Ratio of light velocity in air to mineral.

Pleochroism

Color change of minerals with crystal rotation.

Thin Sections

30µm thickness for consistent mineral observations.

Plane Polarized Light (PPL)

Light vibrating in one plane, used in microscopy.

Electromagnetic Waves

Light considered as waves with electrical and magnetic components.

what are the 2 components of light

electrical and magnetic component that vibrate perpindicular to each other

Wavelength (λ)

Distance between peaks of a light wave, in nm.

Frequency

Number of waves passing a point per second.

Amplitude (A)

Intensity of light, related to wave height.

Snell's Law

Describes refraction of light between different media.

High Relief

Minerals with distinct edges, easily seen against background.

Low Relief

Minerals blending with mounting medium, indistinct edges.

Becke Line Test

Method to determine mineral's refractive index relative to medium.

How do you conduct a Becke Line Test?

Place a mineral under a microscope and adjust the stage to focus. Observe the mineral's edge: if a bright line appears as you move closer, the mineral has a higher refractive index; if it appears when moving away, it has a lower refractive index. This test distinguishes minerals based on refractive indices.

Optical Crystallography

Study of mineral structures through light interaction.

Visible Light Spectrum

Range of light wavelengths from 400 nm to 700 nm.

Electrical Component

Part of light interacting with mineral's electrical properties.

Polarizer

Filter allowing only specific light vibrations through.

Cleavage

Mineral's tendency to break along specific planes.

Opaque Minerals

Minerals that do not transmit light, appear black.

Mineral Identification

Determining mineral types through optical properties.

Crystal Structure

Arrangement of atoms in a mineral, affecting optical properties.

Light Refraction

Bending of light as it passes between materials.

why do we get relief?

light paths refract when travelling between different materials

Cross-polarization

Technique using two polarizers for mineral analysis.

Birefringence

Difference in refractive indices in anisotropic materials.

Single refraction

Light passes through material without splitting.

Double refraction

Light splits into two rays in anisotropic crystals.

Isotropic crystals

simple cubic structure; uniform; transparent in ppl, but don’t let light pass through (opaque) in xpl e.g., halite.

Anisotropic crystals

Variable properties depending on direction; 4 extinction position in 360 degrees of rotation and display interference colours in xpl; e.g., calcite.

Ordinary ray

Ray vibrating parallel to the crystal's symmetry axis.

Extraordinary ray

Ray vibrating perpendicular to the ordinary ray.

Refractive index (R.I.)

Measure of how much light bends in a material.

Relief

Visual contrast of a mineral grain in thin section.

Extinction

Minerals appear black at certain orientations in XPL.

Vibration direction

Orientation of light waves within the crystal structure.

Retardation (Δ)

Distance offset between two rays in birefringent materials.

Twinkling

Change in relief due to crystal rotation in PPL.

Anomalous interference colours

Unexpected colours seen in birefringent minerals.

Polysynthetic twinning

Repeated symmetrical patterns in crystal structure.

First-order colours

Interference colours produced by low birefringence; grey, white, yellow, red

Third-order birefringence

High birefringence resulting in distinct colour patterns;

Halite

Isotropic mineral with simple cubic crystal structure.

Calcite

Anisotropic mineral with complex trigonal crystal structure.

Muscovite

Mica with perfect cleavage and moderate relief.

Quartz

Common mineral, low relief, birefringence shows greys.

Chlorite

Green, pleochroic mineral with moderate relief.

Plagioclase

Colourless mineral with polysynthetic twinning.

Microcline

Non-pleochroic mineral with plaid twinning.