Optical Mineralogy Quiz

Questions and Answers

What happens to the color observed at the thin edge of a quartz wedge?

Retardation causes all colors to blend equally.

All wavelengths are amplified.

All wavelengths cancel and the color appears black. (correct)

The color appears as a vivid blue.

If the retardation is 500 nm, what color is primarily produced at the upper polarizer?

Black

Green

Blue

Red (correct)

Which range of retardation produces first-order colors?

250 - 550 nm

Greater than 1100 nm

Less than 550 nm (correct)

0 - 250 nm

What is the perceived color when retardation corresponds to 2500 nm?

Creamy white Signup and view all the answers

How does birefringence affect the observed interference colors?

It can produce anomalous interference colors based on its variation. Signup and view all the answers

What equation is used to determine the thickness of a sample?

∆ = 𝑑 (𝑛 − 𝑛 ) Signup and view all the answers

What is the characteristic of second-order colors?

They result from retardation between 550 and 1100 nm. Signup and view all the answers

Why does mineral color influence the observed interference color?

It selectively absorbs specific wavelengths, altering perception. Signup and view all the answers

What are the two fundamental theories that describe the nature of light?

Particle Theory and Wave Theory Signup and view all the answers

What is the wavelength range for visible light in a vacuum?

400 - 700 nm Signup and view all the answers

What does the frequency of a light wave represent?

The number of wave crests passing a point per second Signup and view all the answers

How does the intensity of light relate to amplitude?

Intensity is directly proportional to the square of the amplitude Signup and view all the answers

What characterizes the orientation of electromagnetic radiation?

Electric and magnetic vectors vibrate at right angles to the direction of travel Signup and view all the answers

Which statement is true regarding isotropic materials?

Light ray and wave normal coincide or are parallel Signup and view all the answers

What is the relationship between wavelength (λ), frequency (f), and velocity (v) of light?

f = v / λ Signup and view all the answers

What distinguishes anisotropic materials in terms of light propagation?

Wave normal and light ray directions are not parallel Signup and view all the answers

What color does the shorter wavelength Becke line exhibit?

Bluish Signup and view all the answers

What occurs if the dispersion curves for mineral and immersion oil do not intersect?

One white Becke line is produced Signup and view all the answers

Under what condition will no Becke line be produced?

When the indices of the mineral and oil are matching Signup and view all the answers

How is the oblique illumination method used in examining grains?

By blocking some of the light from the microscope Signup and view all the answers

In the bracketing method, what is the purpose of preparing the first grain mount?

To determine the relief of the grains Signup and view all the answers

If the mineral index of refraction ($n_{min}$) is greater than the oil index ($n_{oil}$), what will be observed?

Grains will be bright on the side facing the darkened area Signup and view all the answers

How should the second grain mount be prepared if the relief in the first mount was moderate?

Using oil that is closer to the index of the first oil Signup and view all the answers

What should be considered when preparing the third grain mount?

Selecting oil based on the relief observed in previous mounts Signup and view all the answers

What is observed when a mineral with rhombohedral cleavage is cut perpendicular to the optic axis?

Uniform dark color appears when rotated between crossed polars. Signup and view all the answers

If a mineral is cut parallel to the optic axis in rhombohedral cleavage, what will be true of the extinction?

Extinction is symmetrical to two cleavages and parallel to the third. Signup and view all the answers

What occurs when a mineral with rhombohedral cleavage is cut in a random direction?

Extinction is not parallel or symmetrical to any cleavages. Signup and view all the answers

In the case of prismatic and pinacoidal cleavage, what happens when the mineral is cut perpendicular to the optical axis?

Prismatic cleavages are visible while pinacoidal cleavages are not. Signup and view all the answers

Which characteristic is true when a mineral with prismatic and pinacoidal cleavage is cut parallel to the optic axis?

Extinction is parallel to prismatic and pinacoidal cleavages. Signup and view all the answers

What can be observed about the birefringence when a mineral with prismatic and pinacoidal cleavage is cut randomly?

All cleavages may be visible with intermediate birefringence. Signup and view all the answers

What color is displayed in plane light when a mineral with rhombohedral cleavage is cut randomly?

An intermediate color between ω and ε is displayed. Signup and view all the answers

Which statement is true regarding the angles of cleavage for minerals with rhombohedral cleavage?

Cleavage angles intersect at either 60 degrees or 120 degrees. Signup and view all the answers

What indicates that a mineral is uniaxial when using the biaxial technique?

Two out of three indices of refraction are identical. Signup and view all the answers

Which of the following statements about biaxial minerals is true?

Biaxial minerals can be categorized into orthorhombic, monoclinic, and triclinic systems. Signup and view all the answers

In the construction of the biaxial indicatrix, where is $n_{eta}$ plotted?

Along the Y-axis Signup and view all the answers

How is the maximum birefringence of a biaxial mineral calculated?

As $n_{eta} - n_{eta}$. Signup and view all the answers

What defines the optic normal in biaxial minerals?

It is the Y-axis, which is perpendicular to the optic plane. Signup and view all the answers

What determines if a biaxial mineral is optically positive?

If the acute bisectrix Bxa lies on the Z-axis. Signup and view all the answers

Which axis bisects the obtuse angle between the optic axes in biaxial minerals?

The obtuse bisectrix. Signup and view all the answers

What describes the behavior of light rays in biaxial minerals?

Two rays are produced: the fast ray and the slow ray. Signup and view all the answers

Study Notes

Becke Lines and Interference Colors

Formation of two Becke lines occurs: a yellowish-orange line (longer wavelength) moving into the mineral and a bluish line (shorter wavelength) moving into oil.
Brightness of the color band corresponds to higher infrared (IR); a single white Becke line arises if dispersion curves do not intersect.
Only one Becke line is produced with monochromatic light sources, and no line is observed when mineral and oil indices are matched.

Oblique Illumination Method

Utilizes shadows cast by grains; bright grains indicate 𝑛𝑚𝑖𝑛 > 𝑛𝑜𝑖𝑙, while dark grains indicate the opposite.
Bracketing technique helps match unknown mineral indices by preparing multiple grain mounts with different immersion oils based on relief observations.

Orders of Interference Colors

Thin section thickness affects color perception; color increases with thickness.
Retardation values influence color appearance:
- 250 nm results in a light washout.
- 500 nm cancels blues and greens, displaying red.
- 2500 nm allows a creamy white appearance.
First-order colors (retardation < 550 nm) are most vivid; later orders progressively wash out into creamy white.

Anomalous Interference Colors

Result from differences in birefringence and retardation for different light wavelengths, leading to unique perceived colors.
Mineral color impacts interference colors by selectively absorbing certain wavelengths.

Determining Thickness of Samples

The formula ∆ = 𝑑(𝑛𝑠 − 𝑛𝑓) aids in measuring thickness.
Quartz is commonly used for determining thin section thickness due to its birefringence variability (0 - 0.009).

Nature of Light

Light is energy traveling at a finite speed, exhibiting particle (photons) and wave characteristics (electromagnetic radiation).
Electromagnetic radiation consists of electric and magnetic vectors perpendicular to the direction of travel; visible light ranges from 400 nm to 700 nm.

Wave Properties

Wave intensity and brightness correlate with the square of amplitude; wavefront connects similar wave points.
Light rays in isotropic materials coincide with wave normals, whereas in anisotropic materials, they do not.

Birefringence in Minerals

Different cutting angles of minerals yield various optical figures and extinction patterns:
- Cut perpendicular to the optic axis: uniform dark colors, circular indicatrix.
- Cut parallel to the optic axis: maximum birefringence with symmetrical extinction.
- Random cuts: visible cleavages with intermediate birefringence.

Biaxial Optics in Minerals

Biaxial minerals pertain to orthorhombic, monoclinic, and triclinic systems characterized by three refractive indices (𝑛α < 𝑛β < 𝑛γ).
Biaxial indicatrix is a triaxial ellipsoid; principal sections help determine vibration direction and refractive indices.

Optical Sign Determination

The optic sign depends on which axis bisects the 2V angle:
- Optically positive if Bxa is along the Z-axis.
- Optically negative if Bxa is along the X-axis.
- Optically neutral if 2V = 90 degrees.

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Description

This quiz focuses on the formation and characteristics of Becke lines in optical mineralogy. Participants will explore the concepts of wavelength dispersion and the colors associated with different wavelengths. Prepare to test your knowledge on this fascinating aspect of mineral examination.