Optical Mineralogy Part II Quiz

The study of minerals using optical methods, particularly polarized light microscopy.

Light waves vibrating in a single plane.

Two polarizers with their axes oriented perpendicular to each other, used in microscopy.

The rotation of a birefringent crystal section between crossed polarizers resulting in a periodic change between dark and bright images.

The property of a material to refract light differently in different directions.

Measures how much light slows down when passing through a material.

Patterns of light and dark created by light interference when a mineral is viewed between crossed polarizers

Four specific orientations in which a crystal exhibits maximum brightness when viewed between crossed polarizers

The orientation of a crystal where light waves passing through it aren't split and appear black.

The difference in the optical path lengths traveled by the two refracted light rays.

The cancellation of light waves resulting from opposite directions of vibration.

Mineral with different refractive indices in different directions.

Light passing through has only two refractive index values.

Difference in the arrival time of the two rays, leading to the interference and cancellation of light

The distance separating the two polarized light rays after passing through a birefringent material.

The property of a material to refract light differently in different directions, causing the two polarized light rays to travel at different speeds.

Retardation is directly proportional to birefringence and the thickness of the birefringent material.

The two polarized rays are in phase, leading to maximum constructive interference. All light is blocked by the analyzer, resulting in darkness or extinction.

The two polarized rays are out of phase, leading to destructive interference. All light passes through the analyzer.

White light, containing a spectrum of wavelengths, interacts with a birefringent material, causing different wavelengths to interfere constructively or destructively. This results in specific colors being transmitted or blocked, creating interference colors.

Birefringence and retardation are maximum when the optic axis is parallel to the stage, and minimum when it's perpendicular to the stage. Intermediate orientations have intermediate values.

Ideally, thin sections used in optical mineralogy are 0.3 mm thick. However, errors can occur, leading to variation in thickness.

The analyzer, a second polarizer, is used to analyze the polarized light after it passes through the birefringent material. It blocks or transmits light depending on the retardation and the orientation of the analyzer relative to the polarized light.

Each specific retardation value corresponds to a unique set of interference colors observed in white light. This relationship allows us to identify minerals based on their retardation and the resulting colours.

The colour produced when light waves interfere with each other after passing through a birefringent crystal.

The difference in the optical path length travelled by two light waves passing through a birefringent crystal.

The distinctive purple colour at 551 nm retardation, where intermediate wavelengths are filtered out, leaving red and blue.

A chart that shows the relationship between retardation, crystal thickness, and interference colours.

The sequence of interference colours, subdivided based on the distinctive purplish reds (551 nm intervals).

The pale white colour produced at high retardation, where interference colours fade.

The difference in refractive indices between two directions in a crystal.

The defined thickness of thin sections (usually 25 or 30 µm) used for optical mineral identification.

Different grains of the same mineral show different interference colors because they have different orientations within the thin section, leading to variations in the amount of light retardation.

Light waves vibrating in a single plane, used in optical mineralogy to analyze mineral properties.

Light waves vibrating in all directions.

The route light takes through a mineral, including the direction it enters and exits.

The angle between a mineral's vibration direction and a reference feature (crystal edge, cleavage, twin plane) in a thin section.

When a mineral is anisotropic, its refractive indices differ for different light directions. This causes a phase shift between the two polarized rays, leading to constructive interference and bright colors.

Rotating the stage changes the alignment of the mineral's vibration directions relative to the polarizers. This causes periodic changes in light intensity, resulting in alternating bright and dark images.

Extinction positions indicate the alignment of the mineral's vibration directions with the polarizer's axes. By observing these positions, we can determine the optical properties and identify the mineral.

Extinction angles are particularly useful for characterizing monoclinic and triclinic minerals. These minerals have a unique alignment of their vibration directions, which affects their extinction angles.