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

Introduction to Petrological Microscope

The petrological microscope is an essential tool for studying minerals and their optical properties. It is equipped with lower-power objectives, such as 2.5x or 5x, for a broader view and better illumination. Higher magnifications, like 10x or 40x, are used subsequently for detailed examination.

Examination Methods Using the Polarizing Microscope

The polarizing microscope allows for the study of mineral optical properties in various configurations:

• Plane-polarized light: Analyser in and polariser out.
• Between crossed nicols: Both analyser and polariser in.
• Ordinary light: Both polariser and analyser out (if polariser is removable).

If the polariser cannot be removed, properties under ordinary light may be inferred indirectly.

Table of Optical Properties

Under Ordinary Light

• Form/Habit: Crystalline nature (euhedral, subhedral, anhedral).
• Colour: Natural hues of minerals.
• Transparency: Mineral's ability to transmit light.
• Refractive Index (RI): Degree of light bending.
• Relief: Mineral's contrast with the medium.
• Cleavage: Tendency to break along crystallographic planes.
• Inclusion and Alteration: Internal features and changes.

Under Plane-Polarized Light

• Pleochroism: Color changes with orientation.
• Pleochroic Haloes: Circular features due to radioactive inclusions.
• Twinkling: Rapid changes in brightness.

Between Crossed Nicols

• Isotropism/Anisotropism: Light behavior through the mineral.
• Interference Colors: Colors due to light splitting.
• Extinction and Extinction Angle: Darkness under specific conditions.
• Twinning, Enbu, Zoning, Alteration: Textural and structural phenomena.

Key Optical Properties

Form/Habit

The form reflects a mineral's crystalline structure:

• Euhedral: Complete outlines (e.g., hexagonal, rectangular).
• Subhedral: Partially defined outlines.
• Anhedral: Irregular shapes.

Common habits include:

• Prismatic/Columnar: Length > width (e.g., hornblende).
• Flaky: Thin sheets (e.g., mica).
• Acicular: Needle-like crystals.

Colour

Mineral color varies due to light absorption/reflection.

• Quartz and calcite: Colorless in thin sections.
• Biotite and hornblende: Retain distinct colors.

Relief and Refractive Index (RI)

Relief depends on the difference between the mineral's RI and the surrounding medium.

• Positive Relief: Clear outline (e.g., garnet).
• Negative Relief: Fuzzy outline (e.g., leucite).

Becke’s test can determine RI by observing the movement of the Becke line.

Cleavage

Minerals break along defined crystallographic planes:

• One set: Muscovite, biotite.
• Two sets: Hornblende.
• Three sets: Calcite.

Pleochroism and Pleochroic Haloes

• Pleochroism: Color variation with rotation (e.g., tourmaline).
• Haloes: Circular features caused by radioactive inclusions.

Interference Colors

Anisotropic minerals show interference colors when viewed between crossed nicols:

• First Order: Grey, white, yellow (e.g., quartz).
• Second Order: VIBGYOR (e.g., augite).
• Third Order: Faint rainbow colors (e.g., muscovite).

Extinction

Extinction occurs when vibration directions align with nicols:

• Straight Extinction: No rotation needed (e.g., hypersthene).
• Oblique Extinction: Requires rotation (e.g., hornblende).

Twinning

Crystallographic features include:

• Carlsbad Twinning: Orthoclase.
• Polysynthetic Twinning: Plagioclase.
• Cross Hatch Twinning: Microcline.

Summary

The petrological microscope reveals diagnostic optical properties of minerals such as quartz, feldspar, augite, hornblende, olivine, garnet, and calcite. Observations under different lighting conditions (plane-polarized light, crossed nicols, and interference phenomena) are crucial for identifying mineral composition and structure.