Optical Properties Using Crossed Polarizers
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Questions and Answers

What type of twinning is best recognized in cross polarized light?

  • Simple twins
  • Polysynthetic twins (correct)
  • Penetration twins
  • All types of twins

    • Microcline exhibits characteristic cross-hatched twinning.

    True

    Name one mineral that exhibits penetration twinning.

    Staurolite

    __________ twins in plagioclase are characterized by their lamellar structure.

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

    Match the following minerals with their type of twinning:

    <p>Kyanite = Simple twin on (100) Cummingtonite = Thin twin lamellae on (100) Sanidine = Carlsbad twins Titanite = Simple twin on (100)</p> Signup and view all the answers

    Which mineral shows complex lamellar twinning?

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

    What does the equation $\delta = n_{high} - n_{low}$ represent in the context of optical properties?

    <p>The quantification of color in crossed polarizers mode</p> Signup and view all the answers

    Birefringence can change due to variations in the chemical composition of minerals.

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

    Chemical zoning in minerals often leads to the variation in their __________ colors.

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

    High birefringent minerals generally do not display their primary mineral color when viewed under crossed polarizers.

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

    What visual effect do wedge-shaped grain boundaries provide in mineral analysis?

    <p>They allow observation of the color spectrum related to crystal thickness.</p> Signup and view all the answers

    The interference colors can be estimated by counting the number of _______ bands at the wedge-shaped grain edges.

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

    Match the following minerals with their interference colors observed in CPL:

    <p>Forsterite = Varied interference colors based on orientation Calcite = High-order white Riebeckite = Masked color under crossed polarizers Biotite = Dominant primary color</p> Signup and view all the answers

    How are interference colors described according to the interference color chart?

    <p>By their hue and order</p> Signup and view all the answers

    The purplish red color in the interference color chart separates orders of interference colors.

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

    The first-order black color is observed in relation to decreasing _______ thickness.

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

    What is the highest interference color order recognized in the calcite grain?

    <p>3rd order red</p> Signup and view all the answers

    The interference colors of certain minerals only show grey to white colors in the first order of the interference color spectrum.

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

    What does birefringence refer to in the context of anisotropic minerals?

    <p>The variation of the refractive index for different wavelengths of light.</p> Signup and view all the answers

    In optically uniaxial minerals, the birefringence is maximum in sections parallel to the optic _____

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

    Match the mineral with its description of anomalous interference colors:

    <p>Melilite = Birefringence becomes zero for orange-yellow-green wavelengths Chlorite = Different anomalous colors depending on mineral structure and composition</p> Signup and view all the answers

    What minimizes the effect of birefringence in certain wavelengths for melilite?

    <p>Wavelengths in the orange-yellow-green range</p> Signup and view all the answers

    The maximum birefringence values are always equal for all minerals.

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

    What is the term for colors that deviate from normal interference colors in certain minerals?

    <p>Anomalous interference colors</p> Signup and view all the answers

    What is the correct equation to calculate birefringence?

    <p>$\delta = \Delta / d$</p> Signup and view all the answers

    Olivine has low relief in plane polarized light (PPL).

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

    What is the maximum interference color used to determine birefringence?

    <p>The color chart after Michel-Lévy.</p> Signup and view all the answers

    Sphene has __________ birefringence in crossed polarized light (CPL).

    <p>extremely high</p> Signup and view all the answers

    Match the minerals with their birefringence characteristics:

    <p>Quartz = Low birefringence Sphene = Extremely high birefringence Plagioclase = Low birefringence Calcite = Extremely high birefringence</p> Signup and view all the answers

    Which mineral is characterized by moderate birefringence?

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

    Twinning in minerals can occur due to crystal-structure-controlled intergrowths.

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

    What is the thickness of the standard thin section used in the procedure for determining birefringence?

    <p>25 µm</p> Signup and view all the answers

    Study Notes

    Optical Properties Using Crossed Polarizers (CPL)

    • Interference colors appear when polarizers are crossed
    • Color is quantified numerically: 8 = nhigh - nlow.
    • Hue and order are described using an interference color chart.
    • Orders are separated by a purplish red color.
    • Interference color estimation involves counting red bands at wedge-shaped grain edges.
    • Identifying interference colors can be challenging in highly birefringent minerals like carbonates (high-order white).
    • Wedge-shaped grain boundaries aid observation of color spectra related to decreasing crystal thickness, down to first-order black.
    • Interference colors from minerals may be masked by the mineral's own color (e.g., riebeckite, biotite), especially in strongly colored, highly birefringent minerals.
    • In such minerals, the primary color is dominant under crossed polarizers.

    Orders of Interference Colors

    • Interference colors are observed when polarizers are crossed.
    • Color description numerically: 8 = nhigh - nlow.
    • Both a hue and order need to be specified for interference colors.
    • Interference color charts help to describe these colors.

    Determination of Birefringence

    • Birefringence is a significant mineral property.
    • A mineral's birefringence value depends on its section's orientation.
    • In uni-axial minerals, birefringence varies from zero (perpendicular to the optic axis) to a maximum (parallel to the optic axis).
    • In bi-axial minerals, it varies from zero (perpendicular to an optic axis) to a maximum (parallel to an optic axis).
    • Maximum birefringence (Δn) is critical for mineral identification.

    Procedure for Determining Birefringence

    • Maximum birefringence is a diagnostic value for minerals.
    • Easily determined in thin sections with known thickness.
    • Identify grains with the highest interference colors.
    • Determine retardation from the color chart.
    • Calculate birefringence using the formula: δ = A/d.
    • Maximum birefringence can be found from charts (e.g., 30 micron lines combined with color follow radial lines).

    Twinning in Minerals

    • Twinning in minerals occurs through crystallographic relationships.
    • Each part of a twin can be a mirror image of the other, rotated relative to each other, or both.
    • Twinning is an important property for mineral identification.
    • Different twinning types include: simple, contact, penetration, multiple, and polysynthetic (lamellar).
    • Twinning can be observed under crossed polarizers in anisotropic minerals.
    • Different minerals exhibit different twinning patterns (e.g., cross-hatched twins in microcline).

    Zoning in Minerals

    • Optical properties of minerals with extensive solid solutions vary with chemical composition.
    • Chemical zoning in crystals can be identified by examining interference colors.
    • Plagioclase and titanaugites commonly show oscillatory zoning in volcanic rocks.

    Mineral Examples and Properties

    • Quartz and microcline have low birefringence.
    • Olivine has high relief in plane-polarized light and moderate-to-high birefringence in crossed polarizers.
    • Pyroxene has moderate relief and birefringence.
    • Calcite has extremely high birefringence.
    • Plagioclase has low (white to gray) birefringence.

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    Optical Properties Using Crossed Polarizers Mode (CPL) PDF

    Description

    Explore the fascinating world of optical properties as observed through crossed polarizers. This quiz delves into interference colors, their numerical quantification, and the complexities of identifying colors in birefringent minerals. Test your knowledge on how crystal structure influences color perception and the techniques used in mineralogy.

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