Optical Mineralogy Part II Quiz
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What happens to a birefringent crystal section during a full rotation between crossed polarizers?

  • It only shows dark images.
  • It remains consistently bright.
  • It shows colors in every position.
  • It alternates between bright and dark images. (correct)

    • How many extinction positions are observed during a complete rotation of a birefringent crystal?

  • Three extinction positions.
  • Six extinction positions.
  • Two extinction positions.
  • Four extinction positions. (correct)

    • What are the orientations of maximum brightness in a birefringent crystal referred to as?

  • Extinction positions.
  • Polarized orientations.
  • Diagonal positions. (correct)
  • Birefringent positions.

    • What is indicated by the periodic change between bright and dark images in a birefringent crystal?

    <p>Birefringence effect.</p> Signup and view all the answers

    What is the function of the analyzer in the crossed-polarizers mode?

    <p>To produce polarizing effects.</p> Signup and view all the answers

    During the observation of a crystal, how many bright images are seen in between the extinction positions during rotation?

    <p>Four bright images.</p> Signup and view all the answers

    What can be identified as the principal observable behavior of birefringent crystals under polarized light?

    <p>Periodic extinction and emergence of brightness.</p> Signup and view all the answers

    What is the significance of crossed polarizers in optical mineralogy?

    <p>To observe birefringence and interference effects.</p> Signup and view all the answers

    What phenomenon generates interference colors in a colorless anisotropic crystal?

    <p>Polychromatic light</p> Signup and view all the answers

    At which retardation range do black and grey color tones dominate in interference colors?

    <p> = 0 - 200 nm</p> Signup and view all the answers

    Which interference color is noted at a retardation of 551 nm?

    <p>First-order red</p> Signup and view all the answers

    With increasing retardation, what happens to the intensity of interference colors?

    <p>They fade towards white</p> Signup and view all the answers

    What is the result of using shorter wavelengths in the interference spectrum at a retardation of 600 to 650 nm?

    <p>Appearance of blue interference color</p> Signup and view all the answers

    Which factor does NOT influence the accumulated retardation of waves in a crystal?

    <p>Temperature of the environment</p> Signup and view all the answers

    What did Michel-Lévy publish in 1888 regarding interference colors?

    <p>A graphical representation of retardation effects</p> Signup and view all the answers

    Which hue is common as one progresses through the Michel-Lévy interference color sequence?

    <p>Alternating greenish and reddish hues</p> Signup and view all the answers

    What occurs when light waves are in the extinction position while observing through crossed polarizers?

    <p>Waves pass through without changing vibration direction</p> Signup and view all the answers

    What happens to E-W vibrating waves when the crystal is rotated out of the extinction position?

    <p>They get split into two orthogonal waves</p> Signup and view all the answers

    In the context of birefringence, what is the retardation (Δ) defined as?

    <p>The difference in the refractive indices of the waves</p> Signup and view all the answers

    What occurs in case A, when the retardation corresponds to a phase shift of zero or whole multiples of λ?

    <p>Destructive interference occurs</p> Signup and view all the answers

    What factors determine the retardation (Δ) in the thin section of a mineral grain?

    <p>Thickness, birefringence, and angles of incidence</p> Signup and view all the answers

    In the formula for retardation, which variables represent the two different wave velocities?

    <p>Vslow and Vfast</p> Signup and view all the answers

    What is the effect of birefringence on the propagation of light in an anisotropic mineral?

    <p>It leads to the generation of distinct wave paths</p> Signup and view all the answers

    If the retarded waves described are only affected by the air, which equation best describes the relationship?

    <p>$, Δ = d(V_{air}/V_{slow} - V_{air}/V_{fast})$</p> Signup and view all the answers

    What causes different grains of the same mineral to show different interference colors?

    <p>Different light wavelengths interacting with the minerals</p> Signup and view all the answers

    How is light described when passing through a thin section of rock?

    <p>Light vibrates in many phases and wavelengths</p> Signup and view all the answers

    In what orientation does light vibrate as it interacts with the minerals in a thin section?

    <p>Primarily in an East-West direction</p> Signup and view all the answers

    Which type of light is used in examining thin sections of rocks?

    <p>Unpolarized light</p> Signup and view all the answers

    What is the primary factor that allows us to distinguish different interference colors in rock sections?

    <p>The interaction of light with mineral thickness variations</p> Signup and view all the answers

    Why might the interference colors appear differently when viewed from various angles?

    <p>Light refraction varies with different viewing angles</p> Signup and view all the answers

    What effect do multiple phases of light have when interacting with minerals in a thin section?

    <p>They enhance the colors of the minerals</p> Signup and view all the answers

    Which characteristic of a mineral does NOT contribute to its interference color?

    <p>The ambient temperature of the environment</p> Signup and view all the answers

    What is the primary optical behavior of isotropic minerals?

    <p>Their optical properties are identical in all directions.</p> Signup and view all the answers

    What causes minerals to become extinct every 90º of rotation in crossed polarizers?

    <p>The principal vibration directions align with polarizer orientations.</p> Signup and view all the answers

    What is an extinction angle (EA) used for?

    <p>To characterize monoclinic and triclinic minerals.</p> Signup and view all the answers

    During the rotation of a birefringent crystal section, how many extinction positions occur in a full rotation?

    <p>Four</p> Signup and view all the answers

    What happens to E-W vibrating plane-polarized light waves when passing through isotropic materials?

    <p>They maintain their E-W orientation.</p> Signup and view all the answers

    How does the optical behavior of anisotropic minerals differ from isotropic minerals?

    <p>They exhibit varied optical properties based on light direction.</p> Signup and view all the answers

    What is the significance of the four bright images observed during the rotation of birefringent crystals?

    <p>They are points of maximum brightness in the birefringent material.</p> Signup and view all the answers

    Which of the following statements about crossed polarizers is correct?

    <p>They allow for the observation of extinction in mineral grains.</p> Signup and view all the answers

    What occurs when the retardation corresponds to an integer number of wavelengths?

    <p>Light is blocked by the analyzer.</p> Signup and view all the answers

    Which condition corresponds to maximum constructive interference?

    <p>The retardation is an odd-number multiple of l/2.</p> Signup and view all the answers

    What happens when the retardation is a half-integer of wavelengths?

    <p>Light passes through the analyzer at full intensity.</p> Signup and view all the answers

    What does apparent birefringence refer to?

    <p>Difference in refractive index between two rays.</p> Signup and view all the answers

    What is the result of maximum birefringence and retardation when the c-axis is parallel to the stage?

    <p>Light exhibits maximum intensity.</p> Signup and view all the answers

    What effect does white light have on the observation of interference colors?

    <p>A spectrum of wavelengths is visible.</p> Signup and view all the answers

    What occurs when retardation is perpendicular to the c-axis of the crystal?

    <p>Light is transmitted without any birefringence.</p> Signup and view all the answers

    Which statement is true regarding the components of polarized light when retardation is an integer number of wavelengths?

    <p>Components resolve parallel to the original direction.</p> Signup and view all the answers

    What is the function of retardation in the context of birefringence?

    <p>It reflects the speed difference between the two rays.</p> Signup and view all the answers

    What is the impact of thickness on birefringence and retardation?

    <p>Thickness affects the retardation in thin sections.</p> Signup and view all the answers

    Study Notes

    Optical Mineralogy Part II

    • Optical mineralogy studies the identification of minerals using light
    • The process involves different types of light, including plane-polarized light and crossed-polarized light
    • Different observations are made with varying light types, enabling mineral identification
    • The different aspects of light passing through the mineral are observed
    • The nature of light, shape, cleavage, relief, refractive index, colour, absorption scheme, interference of light, extinction, twinning, birefringence, and indicatrix are observed
    • Petrological microscope is used for observations
    • Mineral identification is the main goal

    Observation with Analyzer Inserted (Crossed-polarizers Mode)

    • Extinction behaviour is a periodic change between bright and dark images when rotating a birefringent section between crossed polarizers
    • Four extinction positions separated by 90° each are observed
    • The maximum brightness position is called a diagonal position
    • Different extinction positions occur during a 360° stage rotation

    Interference Colours

    • White light produces a full spectrum of colours when used with crossed polarizers
    • The observed colours are dependent upon retardation, crystal orientation, and wavelength
    • The colours vary with retardation and show a distinctive colour pattern
    • Different colours imply different retardation values and wavelengths
    • Interference colours are unique and characteristic of a specific mineral

    Michel-Lévy Colour Chart

    • The chart shows a colour sequence using distinctive purplish reds
    • Colours change dependent on retardation
    • With increasing retardation, colours fade into a high-order white
    • The chart shows the relationship between retardation, crystal thickness, and birefringence
    • This chart was developed by Michel-Lévy

    Isotropism and Anisotropism

    • Isotropic minerals display identical optical properties in all directions
    • Light waves pass unimpeded through isotropic minerals
    • They remain black in all rotations
    • Anisotropic minerals show different optical properties in various directions
    • Their optical properties change with varying light directions and rotations of the section

    Extinction

    • Many mineral grains change color as the stage is rotated
    • Extinction of a mineral happens when the vibration directions are parallel to the polarizers
    • Extinction angle is the angle between vibration direction and morphological reference elements (crystal edge, cleavage, twin plane)
    • Extinction angles help characterize monoclinic and triclinic minerals

    Types of Extinction and Determination of the Angle

    • Three main types of extinction exist: parallel, symmetrical, and inclined
    • Parallel extinction occurs when the vibration directions are parallel to the morphological reference direction
    • Symmetrical extinction occurs when vibration directions bisect angles between equivalent morphological reference directions
    • Inclined extinction occurs when vibration directions are at any angle to the morphological reference direction apart from 0° and 90°
    • Extinction angle is determined by stage rotations for max extinction, and the difference between two readings, provides the extinction angle measure.

    Parallel Extinction

    • All uniaxial minerals exhibit parallel extinction
    • Orthorhombic minerals also exhibit parallel extinction (crystal and indicatrix axes coincide)

    Extinction Angle

    • Extinction angle is a function of the relationship between the orientation of the indicatrix and the crystallographic orientation
    • The angle between a vibration direction and the morphological reference element (crystal edge, cleavage) in a crystal section is referred to as the extinction angle.

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    Optical Mineralogy Part II Lecture PDF

    Description

    Test your knowledge on the key concepts of optical mineralogy, including the use of light for mineral identification through various observations. This quiz covers topics such as crossed-polarized light, birefringence, and the use of a petrological microscope.

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