Optical Properties of Common Rock-Forming Minerals | Geology Optional Notes for UPSC PDF Download

• Study of Optical Properties of Minerals
  • Introduction to Petrological Microscope
    • Explanation of the petrological microscope and its functions.
    • Common practice involves using lower power objectives like 2.5x or 5x for a wider view and better illumination.
    • Higher magnification objectives such as 10x or 40x can be utilized subsequently.
  • Optical Properties Examination Methods
    • Discussion on the polarising microscope's capabilities
    • Options for studying mineral optical properties in different positions:
      • Properties under plane-polarized light (analyser in and polariser out)
      • Properties between crossed nicols (both analyser and polariser in)
      • Additional optical studies if provision is available for polariser and analyser:
        • Properties under ordinary light (polariser and analyser out)
        • Properties under plane-polarized light (analyser in and polariser out)
        • Properties between Crossed Nicol (both polariser and analyser in)
    • Note on studying properties under ordinary light if the polariser is non-removable from the microscope's optical system.
  • Table of Optical Properties
    • List of optical properties of minerals studied under ordinary light, plane-polarised light, and between crossed nicols (Table 9.1).
• Optical Properties Under Ordinary Light
  • Explanation of optical properties observed under ordinary light.
  • Details on the characteristics and behavior of minerals under this lighting condition.
  • Examples to illustrate how different minerals exhibit unique optical properties when viewed under ordinary light.

Unit 9: Optical Properties of Minerals

Introduction

• When studying minerals under a microscope, it's essential to exclude light polariser and analyser from the optical system.
• If the polariser cannot be excluded, properties are studied under Plane Polarised Light.

Table 9.1: Optical Properties of Minerals

• Under Ordinary Light (both Plane Polarised polariser and Light analyser out):
• 
  
  • Form/Habit
  • Colour
  • Transparency
  • Refractive Index (R.I.)
  • Relief
  • Cleavage
  • Inclusion and Alteration
• Under Plane Polarised Light (analyser out):
• 
  
  • Pleochroism
  • Pleochroic haloes
  • Twinkling
• Between Cross Nicol (analyser in):
• 
  
  • Isotropism/Anisotropism
  • Polarisation/Interference colours
  • Extinction and extinction angle
  • Twinning, Enbu, Zoning, Alteration

Form (Section 9.3.1)

• Form relates to the crystalline nature of minerals and can be regular or irregular.
• Shapes observed under the microscope:
• 
  
  • Euhedral: Complete outline like hexagonal, rectangular, etc. (Example: Fig. 9.1a)
  • Subhedral: Partial outline of the mineral grain (Example: Fig. 9.1b)
  • Anhedral: Irregular shape with invisible grain boundaries (Example: Fig. 9.1c)

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Block 3: Optical Properties of Minerals

• Habit:
  • Reflects the natural state of growth of minerals.
  • Useful in recognizing well-developed minerals like garnet, zircon, and sphene.
  • Examples: Hornblende typically shows a prismatic habit, feldspars have a tabular habit, and mica is flaky.
  • Refer to Fig. 9.2 for microscopic views of mineral habits.
• Commonly Recognized Habits:
  • Equant: Mineral dimensions are nearly equal.
  • Prismatic or Columnar: Length exceeds width.
  • Acicular: Needle-shaped crystals that may be radiating.
  • Lath-shaped: Small, prismatic crystals (Fig. 9.3).
• Colour:
  • Color in minerals is tied to the visible light wavelength spectrum.
  • White light spans wavelengths from violet (around 390 nm) to red (760 nm).
  • Minerals display a diverse range of colors due to light absorption and reflection.

```In Block 3, the optical properties of minerals are discussed, focusing on the concept of habit, which reflects how minerals naturally grow. This characteristic is crucial for identifying specific minerals like garnet, zircon, and sphene. Different minerals exhibit distinct habits; for instance, hornblende typically shows a prismatic habit, while feldspars have a tabular habit, and mica appears flaky.Commonly recognized mineral habits include equant, where the mineral's dimensions are nearly equal; prismatic or columnar, where the length exceeds the width; acicular, featuring needle-shaped crystals; and lath-shaped, characterized by small, prismatic crystals.Additionally, the section covers the relationship between color and minerals. Color in minerals is associated with the wavelength of visible light. As white light contains a spectrum from violet to red, minerals display a wide array of colors due to light absorption and reflection.

Unit 9: Optical Properties of Minerals

• Natural Color of Minerals

  Minerals exhibit a range of colors, from colorless to various hues like brown and green. For example, quartz and calcite may show different colors in hand specimen but appear colorless in thin sections. On the other hand, minerals like biotite and hornblende maintain their distinct colors even at a thickness of 0.30mm.

• Identification Based on Color

  While color can provide some clues, it is not sufficient for mineral identification. Other optical properties are necessary to confirm the identity of minerals.

• Colored Minerals

  Colored minerals can display different shades depending on their orientation. Some commonly colored minerals are listed in Table 9.2.

  • Colored minerals may appear opaque or non-opaque in thin sections.

• Optical Properties of Minerals

  Minerals can be categorized as non-opaque or opaque based on their transparency. Non-opaque minerals become transparent or translucent at around 0.03mm. While some minerals may appear colored in hand specimen, they can be nearly colorless in thin sections.

  • Non-opaque minerals include quartz and feldspar, while examples of more strongly colored minerals are hornblende and biotite.
  • Opaque minerals, such as hematite, magnetite, and pyrite, appear black or brownish black under plane-polarized light (PPL).

• Commonly Colored Minerals

  Table 9.2 lists some commonly colored minerals, showcasing their colors in thin sections.

  • Biotite: Strong yellow, pale brown, blue/brown
  • Other minerals: Hypersthene, garnet, andalusite, hornblende, actinolite, chlorite, tourmaline, aegerine, staurolite, augite, tourmaline

• Relief

  Relief refers to the distinctness with which a mineral stands out from the embedding medium when viewed under a microscope in plane-polarized light. The surface relief of a mineral depends on the difference in refractive index between the mineral and the surrounding resin or cementing material.

  • The surface of a mineral appears rougher if there is a significant difference in refractive index, whereas a smoother appearance is observed when the refractive indices are similar.
  • Carbonate minerals may exhibit variations in surface relief compared to other minerals.

Block 3: Optical Properties of Minerals

• Specimen Mounting:
  • Commonly, Canada balsam with a refractive index of 1.54 is used as the mounting medium.
  • If a mineral has the same refractive index as Canada balsam, like halite (RI-1.54), it may not be clearly visible as it blends in with the medium.
• Relief in Minerals:
  • Relief is determined by the difference in refractive index between the mineral and the mounting medium.
  • Negative Relief:
    • Occurs when the mineral's refractive index is lower than that of the mounting medium.
    • Examples include leucite and augite.
  • Positive Relief:
    • Occurs when the mineral's refractive index is higher than that of the mounting medium.
    • Minerals with positive relief, like garnet and zircon, have well-defined borders and cleavage cracks.
• Categorization of Relief:
  • Poor Relief:
    • Minerals with a similar refractive index to Canada balsam have fuzzy borders that blend with the mounting medium, for example, quartz and feldspars.
  • Negative Relief:
    • Minerals with a lower refractive index than Canada balsam show unclear outlines, such as leucite and augite.
  • Positive Relief:
    • Minerals with a higher refractive index than Canada balsam exhibit distinct borders and cleavage cracks, like garnet and zircon.

Unit 9: Refractive Index

9.3.3 Refractive Index

• When light rays move from a higher refractive index (RI) medium to a lower RI medium, some of the light is reflected back.
• Relief in petrology is influenced by the mineral's RI and the surrounding medium.
• Determining RI using a petrological microscope involves observing if a mineral appears raised or depressed compared to Canada balsam.
• If a mineral seems raised, it has positive relief and a higher RI; if it appears depressed, it has negative relief and a lower RI than the embedding medium.
• Relative RIs of minerals can be studied using Becke's effect and the Becke's Line method, which involves central illumination.

Table 9.3: Descriptive Scheme for Relief of Minerals

• RI Range: 1.40 - 1.50
• RI Range: 1.50 - 1.58
• RI Range: 1.58 - 1.67
• RI Range: 1.67 - 1.76
• RI above 1.76

Becke's Test

• Assesses the RI of a mineral compared to substances like Canada balsam or epoxy.
• Objective movements must be minimal to avoid defocusing.
• Based on total internal reflection when light moves from a higher RI mineral to a lower RI one in a thin section.

Performing Becke's Test

• Focus on the mineral edge touching the mounting medium.
• Observe the bright Becke's line moving towards the higher RI material.
• Raise the focusing tube and monitor the movement of the Becke's line.

Remember to adjust the iris diaphragm for optimal illumination during these observations.

Block 3: Optical Properties of Minerals

• Optical Properties of Minerals:
  • If the Becke line moves towards the mineral body, it indicates that the mineral has a higher refractive index, resulting in positive relief.
  • Lowering the focusing tube allows for observing the movement of the bright Becke line. If it moves away from the margin towards the outside, the mineral has a lower refractive index than the mounting medium, suggesting negative relief.
  • Generally, when the objective is raised, the Becke line moves towards higher refractive indices.

Cleavage

• Cleavage:
  • Cleavage refers to a mineral's ability to break along well-defined crystallographic planes within its lattice structure.
  • It is more evident in thin sections, appearing as parallel straight lines, indicating specific crystallographic orientation.
  • For example, olivine shows cracks in thin sections, while mica or hornblende exhibit one or two distinct cleavages.
  • Minerals like pyroxenes and amphiboles display one-directional cleavage in certain sections and two-directional cleavage in others, with characteristic angles between cleavage sets.

Unit 9: Cleavage Angle and Optical Properties of Minerals

Method to Find Cleavage Angle:

• Keep one set of cleavage parallel to the cross wire and note down reading 'a'.
• Rotate the stage until the second set of cleavage is parallel to the same cross wire and note down reading 'b'.
• The cleavage angle is the difference between these two readings.

Minerals and Cleavage Sets:

• One Set: Minerals like Muscovite, biotite exhibit one set of cleavage.
• Two Sets: Minerals like hornblende, orthoclase, and augite have two sets of cleavage at specific angles.
• Three Sets: Calcite shows three perfect rhombohedral cleavages.
• Absent Cleavage: Minerals like quartz and olivine lack cleavage.

Cleavage Patterns:

• Minerals can have four directional cleavages in an octahedral pattern (e.g., fluorite) or six sets of cleavages (e.g., sphalerite).
• The number of visible cleavage sets depends on the section's orientation.

Inclusion and Alteration:

• Inclusions: Olivine contains numerous inclusions of opaque iron oxides, quartz, mica, and zircon.
• Alteration: Minerals may undergo alteration, leading to cloudy areas within the mineral due to reactions with water or carbon dioxide.
• For example, feldspars react with water to produce clay minerals, altering the appearance of the mineral grain in polarized light.
• Advanced alteration can completely replace the original mineral with a new mineral phase.

Block 3: Optical Properties of Minerals

Alteration in Olivine

• Olivine is a mineral that is prone to alteration, unlike some other minerals that do not readily undergo changes.
• When olivine crystals undergo alteration, a secondary mineral known as serpentine is formed along the cracks.

Types of Inclusions in Minerals

• Olivine Inclusions:
  • Regular Inclusions: These are well-defined crystals found within the mineral.
  • Irregular Inclusions: Typically fluid inclusions that are irregularly distributed within the mineral.
• Serpentine Inclusions:
  • Acicular Inclusions: These inclusions are needle-shaped and can be simple or radiating in structure.
• Zircon Inclusions:
  • Zircon can show regular fluid inclusions distributed within quartz crystals.

Unit 9: Optical Properties Under Plane Polarised Light

Introduction to Optical Properties of Minerals

• These optical properties can be observed in ordinary light as well as plane polarised light.
• Properties described below can only be observed in polarised light.

Properties Studied Under Plane Polarised Light:

• Form/habit
• Colour and Transparency: Some minerals exhibit different colors when viewed from different angles.
• Relief: The difference in brightness between the mineral and its surroundings.
• Refractive Index (R.I.): The extent to which light is bent when passing through the mineral.
• Cleavage: The way a mineral breaks along certain planes of weakness.
• Inclusion and Alteration: Presence of foreign materials within the mineral and any changes it undergoes.

Pleochroism

• Definition: Pleochroism is a significant property of anisotropic minerals where colors change on rotation.
• Explanation: Some minerals display different colors when viewed from different angles due to light absorption.
• Example: Tourmaline exhibits pleochroism, showing different colors along its length when viewed from different angles.

Pleochroic Haloes

• Definition: Haloes around certain minerals due to radiation damage.
• Explanation: These haloes are caused by the diffusion of radioactive elements in the mineral's surrounding environment.

Twinkling

• Definition: Effect observed in minerals where they appear to twinkle due to internal reflections.
• Explanation: Light bouncing off internal surfaces creates a twinkling effect in certain minerals.

Block 3: Optical Properties of Minerals

• Variation in Color

  Color variation in minerals results from the absorption of different wavelengths in various directions.

  Colored minerals exhibit changes in intensity and shade when rotated in plane-polarized light.

  Dichroism occurs when there is a change in only two shades. Uniaxial minerals of tetragonal, trigonal, and hexagonal systems demonstrate dichroism.

  Orthorhombic, monoclinic, and triclinic minerals (biaxial category) show more than two changes in color absorption.

  • Pleochroism Examples

    Biotite shifts from light yellowish-brown to greenish and dark brown.

    Hornblende changes from light green to dark green.

  • Pleochroic Minerals

    Biotite is a prime example of a pleochroic mineral, showing color changes perpendicular to its cleavage.

    Minerals in the biaxial category exhibit three main possible vibration directions (X, Y, Z).

  • Classification of Minerals

    Pleochroic Minerals:

    • Strongly Pleochroic: Biotite, hornblende, tourmaline, aegerine augite, staurolite
    • Faintly Pleochroic: Augite, andalusite

  • Non-Pleochroic Minerals: Quartz, feldspars, feldspathoids, olivine, apatite

  • Pleochroic Haloes

    Pleochroic haloes are circular areas found in some minerals like biotite, tourmaline, hornblende, etc., caused by radioactive inclusions such as zircon or apatite.

    These inclusions result from radioactive disintegration, producing alpha particles.

Unit 9: Optical Properties of Minerals

(a) Pleochroic Halos

• Pleochroism is the property of certain minerals to exhibit different colors when viewed from different angles.
• For example, Biotite displays pleochroism by changing from dark brown to green. It also showcases pleochroic haloes.
• Hornblende mineral demonstrates pleochroism by transitioning from light green to dark green.

(b) Twinkling in Anisotropic Minerals

• Anisotropic minerals, such as calcite and dolomite, exhibit twinkling under plane-polarized light when rapidly rotated.
• The twinkling effect arises from the rapid change in relief of these minerals.
• Calcite, for instance, has a refractive index of 1.66 for the ordinary ray and 1.49 for the extraordinary ray, while Canada balsam has a refractive index of 1.54.
• Due to its doubly refracting nature, calcite shows two vibration directions for transmitted light.

Block 3: Optical Properties of Minerals

• Contrasting Relief in Minerals

  In minerals, contrasting relief is observed in different situations such as borders and conspicuous cleavages. When the two vibration directions of calcite in a section align with the vibration direction of light from the polariser, distinct relief patterns are exhibited. Each vibration direction corresponds to its own Refractive Index (R.I). Rapid stage rotation causes a twinkling effect similar to stars.

• Optical Properties under Plane Polarised Light

  Previously, we explored optical properties under ordinary and plane polarised light. It's essential to spend a few minutes reviewing your progress before proceeding to the next section.

• SAQ 1

  • List of Optical Properties:
  • List of Inclusions:
  • Purpose of Becke's Test:
  • Definition of Pleochroism:

• Optical Properties Between Cross Nicols

  Transitioning from plane polarised light, we now focus on the optical properties observed between cross nicols. To achieve this configuration, a polariser must be inserted into the microscopic tube. When the mineral slide is not positioned between the two nicols, the field appears completely dark. Let's delve into the optical properties between cross nicols:

• Isotropism vs. Anisotropism

  Minerals can be categorized as isotropic or anisotropic. Isotropic minerals darken under cross nicols and remain dark upon stage rotation. Examples include minerals crystallizing in the cubic/isometric system, as well as amorphous substances like glass and Canada balsam.

Isotropic and Anisotropic Minerals

• Isotropic minerals appear completely dark between cross nicols.
• Example: Octahedral garnet showing isotropism.

Anisotropic Minerals

• Anisotropic minerals become completely dark four times in one complete rotation.
• Minerals in orthorhombic, monoclinic, triclinic, tetragonal, trigonal, and hexagonal systems are anisotropic.

Examples of Anisotropism

• Plagioclase mineral shows different colors and becomes dark four times in one complete rotation.
• Quartz and zircon exhibit anisotropism between cross nicols.

Interference Colors in Optical Mineralogy

• When white light passes through an anisotropic mineral, it splits into two polarized rays, the O-ray and E-ray, vibrating at 90 degrees to each other.
• The phase difference between these rays corresponds to the wavelength of a specific color in the spectrum.
• As different colors have different wavelengths, upon exiting the crystal, some colors may be delayed while others are not.
• Anisotropic minerals between extinction positions exhibit various colors due to the interference of the two rays.
• Interference colors are most intense at a 45-degree position between two extinct positions.

Order of Interference Colors with Examples

• First Order: Grey, white, light yellow, or sometimes light orange
  • Examples: Quartz, plagioclase, orthoclase, hypersthene, chlorite
• Second Order: VIBGYOR - sharp, distinct rainbow colors (violet, indigo, blue, green, yellow, orange, and red)
  • Examples: Augite, hornblende
• Third Order: VIBGYOR - repeated but faint rainbow colors
  • Examples: Muscovite, biotite
• Fourth and Higher Orders: Pale green and pink - very faint
  • Examples: Calcite, zircon

Birefringence and Optical Path Difference

• Birefringence measures the variance between the maximum and minimum refractive indices of a mineral.
• It's the discrepancy between the refractive indices of the extraordinary and ordinary rays.
• Calcite demonstrates high birefringence, while quartz and olivine present a range of interference colors.
• The colors observed depend on thickness, birefringence, and crystallographic orientation of the section.

Relationship between Birefringence, Optical Path Difference, and Thickness

• Birefringence is the phase difference between two rays and can be quantified as n1-n2.
• Optical path difference is the distinction in distance each ray travels and is visible as interference colors.
• For isotropic materials, the path difference is the thickness multiplied by the refractive index.
• In anisotropic materials, birefringence substitutes the refractive index in the formula.

Relationship on the Michel-Lévy Chart

• Interference colors can be categorized as first order, second order, and so on.
• Color bands progress from shades of grey and white to various colors like yellow, orange, pink, red, purple, blue, green, and back to yellow.
• The progression of colors is similar to a rainbow.
• By knowing the interference color and specimen thickness, the birefringence value can be determined using the Michel-Lévy chart.

Thickness and Birefringence

• Thickness of thin sections ranges from 0.06 mm to 0.01 mm.
• Birefringence values vary from 0.001 to 0.055 for different orders of interference.

Extinction

• Extinction occurs when the vibration directions of rays align with the nicols in a petrological microscope.
• Isotropic minerals reach extinction position four times during a 360° rotation between crossed nicols.
• Isotropic minerals are always in extinction position between crossed nicols.
• Anisotropic minerals have extinction positions separated by 90°.

Extinction and Interference Colors

• The brightest interference colors are visible midway between two extinction positions (45° from extinction).
• Rotating the stage reveals polarisation colors at their brightest at 45° from extinction.

Extinction Angle

• Extinction angle is the angle between crystallographic direction and maximum extinction position.
• This angle is measured concerning cleavage, prominent crack, or twin plane.

Types of Extinction

• Extinction position is typically referenced with respect to the crystal's outer edges or prominent cleavage direction.

Block 3: Optical Properties of Minerals

• Straight or Parallel Extinction:
  • Minerals exhibit straight or parallel extinction when they become dark parallel to the cross wire without any further rotation of the stage.
  • For example, hypersthene mineral demonstrates parallel extinction where the extinction is parallel to one set of cleavage.
  • Hypersthene typically shows low first-order interference colors and has two sets of cleavage at 90 degrees.
• Oblique or Inclined Extinction:
  • Oblique extinction occurs when a mineral does not appear dark parallel to the cross wire but instead at a certain angle upon rotating the stage.
  • Minerals like hornblende and augite often display oblique extinction.
  • To determine the extinction angle, start with the initial reading with cleavage parallel to the cross wire and rotate the stage until the mineral grain becomes dark or extinct.
  • The difference in angle between the initial and final readings gives the extinction angle.

Unit 9: Optical Properties of Minerals

Extinction Types

• Symmetrical Extinction:
• 
  
  • Minerals exhibiting symmetrical extinction have a square outline or a rhombic cross-section.
  • During observation, the mineral section becomes extinct parallel to the diagonal of the rhombic pattern.
  • For example, calcite demonstrates symmetrical extinction.
• Oblique Extinction:
• 
  
  • Oblique extinction is characterized by minerals that are extinct at an angular position.
  • An illustrative example is plagioclase, as depicted in Fig. 9.16.
• Wavy or Undulose Extinction:
• 
  
  • This type of extinction is identifiable by bands of darkness crossing a crystal unit during stage rotation.
  • It may result from strain and deformation within the crystal.
  • Quartz is an example of a mineral that exhibits wavy or undulose extinction, as shown in Fig. 9.17b.

Quartz Twinning and Optical Properties of Minerals

Twinning in Minerals

• During crystallization, minerals can form twinned crystals due to variations in pressure, temperature, etc.
• Twinning is commonly observed in feldspars.

Types of Twinning

• Carlsbad Twinning: This type of twinning displays dark and bright bands that alternate positions on rotation, seen in minerals like orthoclase.
• Polysynthetic Twinning: Characterized by multiple thin dark and bright bands that alternate positions on rotation, commonly found in plagioclase feldspars.

Optical Properties of Minerals

• Carlsbad twinning in orthoclase shows simple banding as seen in Figure 9.18.

Unit 9: Optical Properties of Minerals

Polysynthetic Twinning

• Polysynthetic twinning involves the presence of repeated thin layers of twinned crystal material.
• It is exemplified by plagioclase, a common mineral, where twinning can be observed.
• Cross hatch twinning is a specific form of polysynthetic twinning found in microcline, a type of potassium feldspar.

Polysynthetic Twinning

• Polysynthetic twinning is a phenomenon where crystals exhibit repeated thin layers of twinned material.
• For example, plagioclase demonstrates polysynthetic twinning, which can be observed under a microscope.

Cross Hatch Twinning

• Cross hatch twinning is a variation of polysynthetic twinning where twinning occurs in two directions at right angles.
• Microcline, a potassium feldspar mineral, is a notable example exhibiting cross hatch twinning.

Illustrative Examples

• Polysynthetic Twinning: Imagine a layered cake where each layer represents a twinned crystal structure within a mineral.
• Cross Hatch Twinning: Picture a checkerboard pattern where the squares represent the twinned areas in the mineral.

Block 3

Summarizing and Synthesizing Complex Information

• Expertise in condensing lengthy content into digestible formats.
• Specialization in summarizing complex information from PDF books.
• Providing clear definitions and illustrative examples.

Creating Comprehensive Test Materials

• Crafting informative and engaging test questions.
• Accurately assessing understanding of theoretical concepts.

Optimizing the Learning Experience

• Ensuring educational rigor while presenting information.
• Organizing notes in a visually appealing and well-structured manner.

Optical Properties of Quartz

Recap of Quartz Physical Properties

• Pleochroism: Not pleochroic
• Form: Typically anhedral but may be euhedral prismatic
• Quartz is the dominant mineral on Earth's crust. It is a tectosilicate and appears in various colors such as white, grey, purple, yellow, brown, black, pink, green, and red in hand specimens.
• Diagnostic characteristics of quartz include conchoidal fracture, vitreous or glassy luster, hardness of 7, and the absence of cleavage.

Study of Optical Properties with Figure 10.2

• Under Plane Polarised Light
• 
  
  • Color: Typically colorless and clear in thin sections

Optical Mineralogy Concepts

Block 3

• Cleavage: Absent
• Relief: Low
• Between Cross Polars
• Isotropism/Anisotropism: Anisotropic
• Interference colours: Maximum interference colours are first-order white and grey
• Extinction: Undulose or wavy extinction common, often with a fan-like pattern
• Twinning: Not seen in thin section

Diagnostic Features

• Colourless and clear
• Display first-order white or weak yellow interference colours
• Lack of cleavage
• Lack of alteration
• Shows undulatory extinction
• Wavy extinction
• Very low relief

Observation Techniques

• Synthetic Polarized Light (S-PPL)
• Theoretical Mineralogy (THE)
• Polarized Light (PPL)
• Experimental Optics (XP)

Examples

• When observing a mineral under a microscope with cross polars, if it lacks cleavage and displays first-order white or weak yellow interference colors, it likely belongs to this category.
• Undulatory extinction patterns, along with wavy extinction and very low relief, are key indicators of minerals falling under Block 3 in optical mineralogy.

Unit 10: Properties of Rock-Forming Minerals

• Quartz (Qtz)
  • Sketch of quartz with low relief in Plane-Polarized Light (PPL) and Cross-Polarized Light (XP)
  • Photomicrograph of quartz under PPL and between XP
  • Wavy extinction in quartz; elongated quartz grain with flaky muscovite (Musc)
• Feldspar Group Minerals
  • Orthoclase, microcline, and plagioclase
  • Make up about 51% of the Earth's continental crust by weight

10.4 Optical Properties of Feldspar Group Minerals

• Orthoclase
  • Physical properties: tectosilicate, potassium feldspar (KAISi³O)
  • Colors: flesh red, colorless, light grey
  • Diagnostic features: tabular habit, flesh red color, hardness of 6, two set cleavage at 90°
  • Optical properties under Plane-Polarized Light:
    • Color: Usually colorless in thin sections, may appear cloudy or pale brown due to alteration

Block 3: Orthoclase Mineral Characteristics

• Pleochroism: Not showing pleochroism
• Form: Subhedral or anhedral crystals
• Cleavage: Two sets of cleavage at 90 degrees; one set as perfect cleavage and the other set as imperfect
• Relief: Low relief
• Between Cross Polars:
• 
  
  • Isotropism/Anisotropism: Anisotropic nature
  • Interference colours: Maximum interference colors seen are first-order grey and white
  • Extinction: Oblique extinction, angle ranging from 0 to 12 degrees
  • Twinning: Carlsbad twinning is observed
• Diagnostic features:
• 
  
  • Orthoclase appears colorless, cloudy, or turbid
  • Shows first-order interference colors
  • Usually displays one or two distinct cleavage sets at 90 degrees
  • Exhibits Carlsbad twinning
• PPL (Plane-Polarized Light):
• 
  
  • Colorless and cloudy appearance
  • Two sets of cleavage at 90 degrees
• PPL XP (Crossed Polars under Plane-Polarized Light):
• 
  
  • (a) Optical Mineralogy
  • Carlsbad twinning
  • First-order colors
  • (b) See Figure 10.3: Orthoclase in thin section; a) Sketch of orthoclase with two set cleavage in PPL and XP; and b) Photomicrograph of orthoclase in PPL and Carlsbad twinning in XP

Please note the importance of understanding the diagnostic optical properties of orthoclase to enable its identification under the microscope.

Unit 10: Microcline

Physical Properties of Microcline:

• Microcline is a type of tectosilicate and potassium feldspar.
• It can exhibit various colors such as white, grey, yellowish, tan, salmon-pink, bluish green, and green.
• Key characteristics include a tabular habit, green color, hardness of 6-6.5, and two sets of cleavage at 90 degrees.

Optical Properties of Microcline:

• Under Plane Polarised Light:
  • Color: Typically colorless but may appear cloudy in thin sections.
  • Pleochroism: Non-pleochroic.
  • Form: Usually subhedral or anhedral plates or laths.
  • Cleavage: Two sets of cleavage at 90 degrees; one set perfect and one set poor.
  • Relief: Low.
• Between Cross Polars:
  • Isotropism/Anisotropism: Anisotropic.
  • Interference Colors: First-order light grey interference colors.
  • Extinction: Oblique extinction with an angle that varies up to 15 degrees.
  • Twinning: Exhibits distinctive cross hatch twinning pattern.

Diagnostic Features:

• Microcline is colorless, displays first-order interference colors, and typically shows one or two cleavages.
• It exhibits typical cross hatch twinning.

Cleavage:

• 2 sets at 90 degrees.

Properties of Rock-Forming Minerals:

• Microcline shows cross hatch twinning.

Block 3

• LG
• PPL
• XP

Optical Mineralogy

• Microcline in Thin Section
• 
  
  • Sketch of microcline with cleavage at 90° in PPL and XP
  • Photomicrograph of microcline in PPL and XP showing cross-hatched twinning
• Importance of Learning Optical Properties

Microcline in Thin Section

Microcline is a mineral that exhibits cleavage at 90 degrees when viewed in Plane Polarized Light (PPL) and Cross Polarized Light (XP). This can be observed through sketches and photomicrographs, such as those provided in Fig. 10.4.

Plagioclase

Plagioclase is a sodic-calcic feldspar that ranges in composition from NaAlSi3O8 to CaAl2Si2O8. It is characterized by its pale grey or white color in hand specimen, along with two distinct sets of cleavage at 90 degrees and the presence of striations on crystal faces.

Physical Properties of Plagioclase

• Color: Typically colorless, but may appear cloudy in thin sections
• Pleochroism: Non-pleochroic
• Form: Usually subhedral or anhedral plates or laths
• Cleavage: Two sets of cleavage at 90 degrees; one perfect and one poor
• Relief: Low relief

Optical Properties of Plagioclase

• Under Plane Polarized Light
• 
  
  • Colour: Typically colorless, may appear cloudy
  • Pleochroism: Non-pleochroic
  • Form: Subhedral or anhedral plates or laths
  • Cleavage: Two sets of cleavage at 90 degrees
  • Relief: Low
• Between Cross Polars
• 
  
  • Isotropism/Anisotropism: Anisotropic
  • Interference Colors: First order light yellow
  • Extinction: Oblique extinction with varying angles based on composition

Understanding the optical properties of plagioclase is crucial for its identification under a microscope.

Unit 10: Twinning in Plagioclase

Twinning in Plagioclase

• Polysynthetic twins often result in a striped appearance or occasionally Carlsbad twinning.

Diagnostic Features of Plagioclase

• Plagioclase is colorless and exhibits First-order interference colors.
• It typically displays one or two set cleavages and shows oblique extinction.
• Plagioclase commonly demonstrates polysynthetic twinning, or in some cases, Carlsbad twinning.
• When viewed between crossed polars, plagioclase shows concentric black-grey-white patterns known as zoning, resulting from compositional heterogeneity.

Properties of Plagioclase

• Plagioclase shows two sets of cleavage at right angles.

Illustrative Examples

• In thin section, plagioclase appears with two set cleavages in Plane-Polarized Light (PPL) and Cross-Polarized Light (XP).
• Under XP, polysynthetic twinning in plagioclase can be observed, as depicted in photomicrographs.

Optical Properties of Muscovite

Introduction

• Muscovite is a member of the mica group of minerals.
• It has a chemical composition of KAl2(Si3AlO10)(OH)2.

Physical Characteristics

• Color: Muscovite is usually colorless, but it may exhibit a pale green pleochroism.
• Cleavage: It displays one set of perfect micaceous cleavage.
• Relief: Muscovite shows moderate relief.
• Form: It can be found in tabular crystals, lamellar, and flaky forms.

Special Characteristics

• Muscovite has a sheet-like form with a vitreous and pearly luster.
• It exhibits a special optical phenomenon known as asterism.

Optical Properties

• Under Plane-Polarized Light:
• 
  
  • Color: Generally colorless.
  • Cleavage: Displays one set of perfect micaceous cleavage.
  • Relief: Moderately visible.
  • Pleochroism: Typically non-pleochroic, but may show pale green pleochroism.
  • Form: Found in tabular crystals, lamellar, and flaky forms.

Unit 10: Between Cross Polars

Isotropism/Anisotropism

• Anisotropic minerals exhibit different properties when viewed from different directions.

Interference colours

• Interference colors observed can be up to second order and typically appear yellow or red.

Extinction

• These minerals usually show parallel extinction under cross-polarized light.

Twinning

• Twins may be present in these minerals but are challenging to detect.

Diagnostic features

• Minerals are colorless to pale green in plane-polarized light.
• They have a flaky form with one perfect cleavage.
• They exhibit second-order interference colors and parallel extinction.

Properties of Rock-Forming Minerals

(a) Second order colors

• Minerals may display second-order interference colors.

(b) Muscovite in thin section

Sketch of muscovite with one set cleavage in plane-polarized light and cross-polarized light showing straight or parallel extinction. Muscovite (Musc) in plane-polarized light and cross-polarized light. Note the presence of quartz (Qtz) grain.

Understanding the optical properties of muscovite is essential for its identification under the microscope.

Physical Properties of Biotite:

• Color: Black, dark green, dark brown
• Cleavage: One perfect set
• Cleavage Flakes: Elastic, flexible, sheet-like or lamellar
• Lustre: Vitreous and pearly
• Special Character: Asterism

Optical Properties of Biotite Under Plane Polarized Light:

• Color: Pale green, light brown, red brown, or greenish brown
• Pleochroism: Exhibits strong pleochroism
• Form: Hexagonal plates, tabs, or elongate flakes
• Cleavage: One distinct set
• Relief: Moderate

Optical Properties of Biotite Between Cross Polars:

• Isotropism/Anisotropism: Anisotropic
• Extinction: Parallel extinction
• Twinning: Not easily observable
• Interference Colors: Strong interference colors up to second order

Diagnostic Features of Biotite:

• Color: Brown to yellowish green, reddish brown, or green
• Form: Flaky
• Pleochroism: Strongly pleochroic
• Cleavage: One distinct set with parallel extinction
• Pleochroic Haloes: May be present

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Unit 10: Properties of Rock-Forming Minerals

• Biotite (Btt), Quartz (Qtz), Microcline, and Pleochroic Haloes
• Illustrative Example: Fig.10.8 shows Biotite in thin section with cleavage and pleochroic haloes
• Explanation: Pleochroic haloes are circular areas in minerals like Biotite due to radioactive inclusions
• Radioactive Inclusions: Zircon, Apatite, or Sphene resulting from radioactive disintegration
• Importance: Understanding optical properties of Biotite crucial for microscope identification
• Detailed Description: Fig. 10.9 displays pleochroic haloes in Biotite, highlighting cleavage and pleochroism

```In Unit 10, we delve into the properties of rock-forming minerals such as Biotite, Quartz, Microcline, and Pleochroic Haloes. For instance, in Fig.10.8, we observe Biotite in a thin section, showcasing its cleavage and the presence of pleochroic haloes. These haloes are circular features found in certain minerals like Biotite, a result of the inclusion of radioactive elements like Zircon, Apatite, or Sphene, which stem from radioactive decay processes. To identify Biotite accurately under a microscope, it's essential to grasp its diagnostic optical properties. Additionally, Fig. 10.9 provides a detailed view of pleochroic haloes in Biotite, emphasizing its cleavage characteristics and strong pleochroism, crucial for mineral identification.

Block 3: Optical Mineralogy

Overview

• Discussion on optical properties of various minerals
• Importance of understanding optical properties

Rock-Forming Minerals

• Minerals that are crucial in the formation of rocks

Quartz Mineral

• Diagnostic characteristics of quartz
• Physical properties and significance

Diagnostic Twinning

• Explanation of twinning in orthoclase, microcline, and plagioclase

Optical Properties of Minerals

• Listing optical properties of orthoclase, microcline, and muscovite

Optical Properties of Pyroxene Group Minerals

Introduction to Pyroxenes

• Pyroxenes in igneous and metamorphic rocks
• Differentiation between clinopyroxenes and orthopyroxenes

Augite

• Chemical composition and structure of augite
• Physical properties of augite
• Appearance and cleavage characteristics

Under Plane Polarised Light

• Color and pleochroism of augite
• Form and cleavage under polarized light
• Optical properties like relief and extinction

Between Cross Polars

• Discussion on anisotropism and interference colors
• Details on extinction angles and properties

Unit 10: Twinning in Minerals

• Twinning in Minerals: Types include simple or polysynthetic twins.
• Diagnostic Features of Augite:
  • Color: Normally colorless, pale green, or purplish brown.
  • Physical Characteristics:
    • High relief
    • Middle Second-order interference colors
    • Two distinct cleavages at 87° and 93°
    • Extinction angle at 36° to 45°
• Augite Mineral:
  • Augite has 2 set cleavages.
  • Commonly observed in Plagioclase (Plag).
  • Properties in Rock-Forming Minerals include Second-order colors and distinct cleavages.
  • Augite is characterized by second-order interference colors.
• Fig.10.10: Augite in Thin Section:
  • a) Sketch of augite in Plane-Polarized Light (PPL) and Cross-Polarized Light (XP) showing cleavage.
  • b) Photomicrograph of augite in PPL and XP displaying second-order interference colors.
  • Augite exhibits two set cleavages. Also, observe the presence of plagioclase grain (Plag).
• Importance of Understanding Diagnostic Optical Properties:
  • Essential to identify augite under the microscope.
  • Optical properties aid in accurate mineral identification.

Hypersthene Study Notes

Chemical Composition of Hypersthene

• (Mg,Fe)Si₂O₆

Physical Properties of Hypersthene

• Color: Grey, green, dark yellow, yellow-brown, greenish-brown, brown, black
• Lustre: Waxy, sub-metallic
• Streak: Light brown to greyish-white
• Form: Prismatic with stubby crystals
• Cleavage: Two sets at angles of 87° and 93°

Optical Properties of Hypersthene

Under Plane Polarised Light

• Color: Usually colorless or pale pink to green
• Pleochroism: Strongly pleochroic
• Form: Typically anhedral but may be euhedral, prismatic
• Cleavage: Two sets at right angles
• Relief: Moderately high to high

Between Cross Polars

• Isotropism/Anisotropism: Anisotropic
• Interference Colors: Mid to upper First order, rarely up to second order blue
• Extinction: Parallel extinction
• Diagnostic Features: High relief, light green pleochroic, low birefringence, 90° cleavage, parallel extinction

Schiller Structure in Hypersthene

• Phenomenon due to regular and oriented inclusions of minute platy crystals
• Inclusions arranged along intersecting planes

Fig. 10.11: Hyperthene Characteristics

• Shows 2 set cleavage and pleochroism under Plane Polarised Light
• Parallel extinction seen between Cross Polars

Unit 10: Optical Properties of Hornblende

Introduction

• Hornblende is an inosilicate mineral with the chemical composition CaNa<sub>2-3</sub>(MgFeAl)<sub>5</sub>Si<sub>6</sub>(SiAl)<sub>2</sub>O<sub>22</sub>(OH)<sub>2</sub>.

Physical Properties of Hornblende

• Color: Hornblende can be black, dark green, dark brown, or dark grey.
• Crystal Form: It occurs as prismatic or tabular crystals, sometimes in different shapes like columnar, radiating, acicular, fibrous, and massive.
• Cleavage: Hornblende exhibits two perfect prismatic cleavages at angles of 56° and 124°.

Optical Properties

• Under Plane Polarised Light
  • Color: Shows various shades of green and brown with pleochroism from yellowish-green to dark brown.
  • Form: Prismatic crystals with imperfect diamond-shaped cross sections.
  • Cleavage: Two distinct cleavages at 56° and 124°.
  • Relief: Moderate to high relief.
• Between Cross Polars
  • Isotropism/Anisotropism: Anisotropic behavior.
  • Cleavage: Two sets at 56°-124°.
  • Interference Colors: Mainly second-order colors, potentially masked by the mineral's dark green or brown color.
  • Extinction: Oblique extinction, with the maximum extinction angle ranging from about 12° to 30° depending on composition.
  • Twinning: Simple twins are common.
  • Diagnostic Features: Green color, strong pleochroism, characteristic prismatic form with two-set cleavage (124° to 56°).

Optical Properties of Olivine and Garnet

Olivine

• Chemical composition: Olivine is composed of (Mg Fe)2SiO4.
• Physical properties:
  • In hand specimen, olivine typically appears olive green, but it can range from yellow-green to bright green and occurs in granular masses. It lacks cleavage.
• Optical properties of olivine:
  • Under Plane Polarised Light:
    • Color: Colorless, occasionally very light yellowish or greenish.
    • Pleochroism: Not pleochroic.
    • Form: Usually anhedral, with grains having a six-sided polygonal outline.
    • Cleavage: Absent, but internal fracturing of grains is common.
    • Relief: Moderately high relief.
  • Between Cross Polars:
    • Isotropism/Anisotropism: Anisotropic.
    • Interference colors: Bright second and third-order interference colors.
    • Extinction: Parallel to cleavage and crystal outlines.

Unit 10: Twinning in Minerals

• Twinning - Uncommon and Underdeveloped

  • Diagnostic Characteristics:
  • 
    
    • Colorless appearance
    • Bright second-order interference colors
    • High relief
    • Irregular fracturing
    • Lack of cleavage
    • Rimmed with brownish or greenish alteration products, often serpentine
    • Alters to serpentine along cracks

• Properties of Garnet

  • Chemical Composition: A3B2(SiO4)3
  • Composition breakdown:
  • 
    
    • A can be Ca, Mg, Fe², or Mn²
    • B can be Al, Fe³, Mn³, V³, or Cr³

• Optical Properties of Olivine

  • Diagnostic Importance
  • Crucial for identification under the microscope
  • Fig.10.13: Olivine in thin section
  • 
    
    • Sketch of olivine in PPL and XP with cracks filled with altered products
    • Microphotograph of olivine in PPL and XP

Note: Understanding the optical properties of olivine is essential for accurate identification in microscopic analysis.

Optical Mineralogy - Garnet

Let's revisit the key physical and optical properties of garnet that we studied in Unit 6 of this course:

• Physical Properties of Garnet:

  • Garnet is commonly red, reddish-brown, or black in color.
  • It typically appears as perfect crystal rhomb dodecahedrons or in granular masses.
  • There is no cleavage present in garnet.
  • The hardness of garnet ranges between 7 to 8 on the Mohs scale.

• Optical Properties of Garnet Under Plane Polarised Light:

  • Color: Garnet can appear colorless, pink, light shades of red, brown, green, or other darker hues.
  • Pleochroism: Garnet shows very slight pleochroism.
  • Form: Euhedral crystals have six or eight sides, while irregular polygons or subhedral to anhedral crystals are also common.
  • Cleavage: Absent, but irregular fractures are frequent.
  • Relief: Very high relief is observed.

• Optical Properties of Garnet Between Cross Polars:

  • Isotropism/Anisotropism: Garnet exhibits isotropic behavior.
  • Diagnostic Features: Garnet displays very high relief, is isotropic, usually colorless with a pale tint, irregular fractures, and inclusions.

Refer to Figure 10.14 for a visual aid while studying the optical properties of garnet.

For further clarification, consider the example of garnet's characteristics under different lighting conditions and its crystal structure.

In Figure 10.13, you can observe a sketch and photomicrograph of garnet in thin section, highlighting its properties under various conditions.

Unit 10: Optical Properties of Calcite

Importance of Understanding Garnet's Diagnostic Optical Properties

• Learning the diagnostic optical properties of garnet is crucial for its identification under a microscope.

Recap of Calcite's Physical Properties

• Color: Calcite exhibits a wide range of colors in hand specimens, including white, colorless, and various light shades.
• Cleavage: It possesses three sets of perfect, rhombohedral cleavage.
• Hardness: Calcite has a hardness of 3 on the Mohs scale.

Optical Properties of Calcite

• Color and Appearance Under Plane Polarized Light:
  • Color: Usually colorless and may appear cloudy in thin sections. Pastel hues and twinkling effects may be observed.
  • Pleochroism: Not pleochroic.
  • Form: Typically fine to coarse-grained subhedral to euhedral aggregates.
  • Cleavage: Exhibits three sets of perfect rhombohedral cleavage.
  • Relief: Variable relief ranging from high to low.
• Behavior Between Crossed Polars:
  • Isotropism/Anisotropism: Anisotropic in nature.
  • Interference Colors: Display fourth or fifth-order colors.
  • Extinction: Shows symmetrical extinction to cleavages.
  • Twinning: Polysynthetic twinning may be observed.
  • Twinkling Effect: Prominent in calcite due to rapid changes in relief when rotated under the microscope.
• Characteristics in Plane Polarized Light:
  • Diagnostic Features: Colorless, extreme birefringence, high-order interference colors, pearly appearance, variable relief, rhombohedral cleavage, and polysynthetic twinning.
  • Twinkling Effect: Notable phenomenon caused by rapid changes in relief during stage rotation.

Block 3 - Optical Mineralogy

Calcite in Thin Section

• Sketch of calcite with three set rhombohedral cleavage in PPL and XP
• Microphotograph of calcite showing twinkling and 3 sets of cleavage in PPL and third order interference colors under XP

It is crucial to learn the diagnostic optical properties of calcite to identify it under the microscope.

Progress Check

In the previous sections, we have discussed the optical properties of a few more minerals. Before summarizing what we have learned in this unit, take 5 minutes to evaluate your progress.