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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.
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