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 (A)

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 (B)</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 (D)</p> Signup and view all the answers

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

<p>True (A)</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 (B)</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 (C)</p> Signup and view all the answers

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

<p>True (A)</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 (B)</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 (B)</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 (C)</p> Signup and view all the answers

The maximum birefringence values are always equal for all minerals.

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

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

<p>False (B)</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 (B)</p> Signup and view all the answers

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

<p>True (A)</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

Flashcards

Interference Colors

The colors observed when polarized light passes through a mineral, indicating the mineral's birefringence (difference in refractive indices).

Delta (δ)

A numerical value representing birefringence, derived from the difference in refractive indices of a mineral.

Interference Color Chart

A chart that visually displays interference colors across various mineral thicknesses, allowing identification of mineral birefringence.

Order of Interference Colors

The number of red bands observed at a wedge-shaped grain edge, used to estimate the order of interference color.

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High-Order White

Minerals with high birefringence, exhibiting a white interference color due to overlapping interference colors of different orders.

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Wedge-Shaped Grain Boundary

A wedge-shaped grain boundary in a mineral, providing a visual spectrum of interference colors as the thickness decreases.

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Masked Interference Colors

The color of a mineral can sometimes mask the true interference color, especially in highly colored minerals.

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Dominant Mineral Color (under CPL)

In strongly colored minerals with high birefringence, the mineral's own color dominates, obscuring the interference color.

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Contact Twins

A twinning type where two or more crystal parts are intergrown along a common plane - like two puzzle pieces fitting together.

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Penetration Twins

A twinning type where two or more crystal parts intergrow with one penetrating the other - forming a more complex structure.

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Lamellar Twins

A twinning type where a crystal is divided into multiple parallel thin layers, resulting in a striated appearance.

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Polysynthetic Twins

A specific type of lamellar twinning found in plagioclase minerals, where the twin lamellae are extremely fine and parallel.

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Twinning in Polarized Light

The phenomenon where individual parts of a twinned crystal exhibit different colors when viewed under a microscope with polarized light.

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Chemical Zoning in Minerals

The variation in optical properties (like birefringence) within a mineral due to changes in its chemical composition.

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Oscillatory Zoning

A type of chemical zoning in minerals where there are alternating bands of different compositions, creating a banded appearance.

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Zoning

Variations in chemical composition within a mineral, often visible as color differences or bands under a microscope.

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Birefringence

The difference between the maximum and minimum refractive indices of a mineral (nz-nx) under polarized light.

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Maximum Interference Color

The highest interference color observed under cross-polarized light (CPL) in a thin section.

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Retardation

The degree to which light is delayed when it passes through a mineral.

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Thin Section Thickness

The thickness of a thin section, typically measured in micrometers. It's important for calculating birefringence.

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Birefringence Calculation

The difference in velocity of light passing through a mineral, as the velocity is dependent on the refractive index.

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Twinning

A symmetrical intergrowth of two or more crystal segments with a defined relationship.

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Low Birefringence

Minerals like quartz and microcline have low birefringence, meaning they produce weak interference colors under CPL.

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High Birefringence

Minerals like olivine, pyroxene, sphene, and calcite have high birefringence, meaning they produce bright and colorful interference colors under CPL.

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Calcite Interference Color

The highest interference color observed at the wedge-shaped margin of a calcite grain, where the thickness decreases. It corresponds to the 3rd order interference color in the spectrum.

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Anomalous Interference Colors

Deviation from the normal interference color scheme, resulting in leather brown, ink blue, or grey-blue colors instead of the expected grey to white colors in the 1st order. This occurs due to strong birefringence variation with different wavelengths.

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Maximum Birefringence

The maximum birefringence (nz-nx) exhibited by a biaxial mineral, often listed in mineral-optical tables for identification. It represents the greatest degree of birefringence possible within the mineral.

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Sections Perpendicular to the Optic Axis

Sections of a crystal that are perpendicular to the optic axis. They exhibit zero birefringence and therefore no interference colors.

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Sections Parallel to the Optic Axis/Plane

Sections of a crystal that are parallel to the optic axis in uniaxial minerals, or parallel to the optic plane in biaxial minerals. They show the maximum birefringence and most intense interference colors.

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Birefringence in Mineral Identification

A key property of anisotropic minerals (those that exhibit birefringence), used for identification. It is a measure of the difference in refractive index between two light rays passing through the mineral and is related to the intensity of interference colors observed.

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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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