Study Notes: A Color Atlas of Rocks and Minerals in Thin Section

A Color Atlas of Rocks and Minerals in Thin Section

Introduction

  • A Color Atlas of Rocks and Minerals in Thin Section, authored by W. S. MacKenzie (Emeritus Professor of Petrology) and A. E. Adams (Senior Lecturer in Geology) at the University of Manchester, offers an introduction to the identification of minerals and rocks under a polarizing microscope, beneficial for students of earth science, geology, mineralogy, and physical geography.

  • The atlas serves as a comprehensive manual for practical classes, illustrating diagnostic properties of common rock-forming minerals and a variety of igneous, sedimentary, and metamorphic rocks.

  • Aimed at beginners, the content assumes limited knowledge of crystallography or physical optics, encouraging further study of crystal symmetry and optical techniques.

Acknowledgements

  • Most of the thin sections illustrated in the atlas are sourced from the teaching collections of the Manchester University Geology Department.

  • Contributions and suggestions acknowledged from colleagues and aids in text typing.

Overview of Contents

  • The atlas includes a birefringence chart and sections on various topics:

    • Part 1: Optical Mineralogy (Introduction to the polarizing microscope, describing minerals, optical properties such as shape, color, cleavage, relief, birefringence, extinction angles, twinning, and zoning)

    • Part 2: Minerals (Descriptions of minerals such as Olivine, Pyroxene, Amphibole, Biotite, Muscovite, Chlorite, and Quartz)

    • Part 3: Igneous Rocks (Includes Peridotite, Basalt, Gabbro, Rhyolite, Granite, and more)

    • Part 4: Sedimentary Rocks (Examines terrigenous clastic rocks, carbonate rocks, and various sandstone types)

    • Part 5: Metamorphic Rocks (Discusses features of various types of metamorphic rocks and textures)

Preface

  • The primary purpose is to facilitate the teaching of practical classes on rocks and minerals using a polarizing microscope.

  • The atlas aims to provide clear, self-contained routines for identifying minerals and rocks without supervision.

  • Most photographs are taken at low magnification, showing clear interrelationships of constituent minerals.

Birefringence Chart

  • A chart illustrating common mineral birefringence values is provided, including minerals like Quartz, Olivine, and Biotite, enabling users to identify and utilize the properties effectively in study.

Part 1: Optical Mineralogy

The Polarizing Microscope

  • Features:

    • Equipped with a rotating stage and polarizing filters (polarizer and analyzer).

    • Ordinary light vs. polarized light: Ordinary light vibrates in all directions, while polarized light vibrates in a single plane.

  • Operation: Includes focusing, stage centering, and illumination adjustments.

  • Importance of thin section thickness: Standard thickness is 0.03 mm (1/1000 inch).

Description of Minerals

  • Key Descriptive Criteria:

    • Shape and habit of crystals.

    • Color and pleochroism (color change when rotated).

    • Cleavage presence and angles.

    • Differences in refractive indices and their implications.

    • Interference colors observed in crossed polars.

    • Farther investigation into structural features such as twinning and zoning.

Shape and Habit of Crystals

  • Terminology:

    • Euhedral: Well-defined crystal faces in thin section.

    • Anhedral: No recognizable straight edges

    • Subhedral: Some straight edges, some curved.

  • Examples:

    • Phenocrysts in igneous rocks grow larger than the surrounding matrix.

    • Specific terms for habits: Acicular (needle-shaped), prismatic, tabular.

Color and Pleochroism

  • Differential Observation:

    • Many minerals can appear differently under the microscope vs. hand samples; biotite is typically brown in thin section.

    • Observations of pleochroism are essential for correct identification—colors change based upon orientation in polarization.

Cleavage

  • Definition: Breaks along planes defined by atomic structures, where atomic bonds are weaker.

  • Diagnostic Value: Observing the presence of cleavage can lead to the identification of specific minerals.

    • Example: Mica's perfect cleavage allows it to be subdivided into sheets.

    • Angles between cleavages can distinguish mineral groups (e.g., pyroxene vs. amphibole).

Relief

  • Observation of Relief in Thin Sections:

    • Differences in refractive index affects how clearly crystals appear against the background.

    • Becke line technique: Helps indicate which of two adjacent materials has the higher refractive index.

Extinction Angles

  • Extinction is observed when rotating the stage of the microscope; extinction angles indicate how light interacts with crystal structures.

  • Measurement Technique: Setting certain crystal orientations parallel to the eyepiece cross-hairs to read angles accurately.

Twinning and Zoning

  • Twinning refers to crystals having similar orientations, with several types of symmetry observed.

  • Zoning in crystals indicates chemical composition changes during mineral growth.

Part 2: Minerals

  • Descriptions of key minerals:

Olivine

  • Chemical formula: (Mg, Fe)2SiO4; displays high birefringence and often has cracks.

Orthopyroxene

  • Chemical formula: (Mg, Fe)SiO_3; recognized by its straight extinction and one cleavage plane.

Clinopyroxene

  • Chemical formula: Ca(Mg,Fe)SiO_3; generally exhibits zoning and twinning.

Amphibole

  • General composition: (Na,Ca){2}(Mg,Fe,Al)5Si8O{22}(OH)_{2}.

Biotite

  • Defined by perfect cleavage and varies in color based on composition.

Muscovite

  • Colorless in thin section, identified by perfect cleavage.

Chlorite

  • Green in color, recognized with characteristic pleochroism.

Quartz

  • Pure silica mineral; clear in thin section, helpful for assessing section thickness.

Feldspars

  • Series of minerals with compositions alternating between sodium and potassium; significant for rock classification.

Part 3: Igneous Rocks

  • Process of solidification from liquid magma.

  • Types categorized based on grain size and mineralogy:

    • Plutonic Rocks: Coarse grain, formed deep within the Earth.

    • Volcanic Rocks: Fine grain, solidified rapidly after being expelled.

    • Hypabyssal Rocks: Intermediate sized grains.

  • Classification Criteria: Silica saturation, proportions of feldspars, plagioclase content, and average grain size.

Part 4: Sedimentary Rocks

  • Divided into:

    • Terrigenous Clastic Rocks: Comprised of fragments transported from existing rocks.

    • Chemical Rocks: Formed through precipitation in aqueous solutions.

  • Key components and characteristics of sandstones, including quartz arenite and greywacke, processes of diagenesis, and various cement types.

Part 5: Metamorphic Rocks

  • Defined by changes in mineralogy or texture due to pressure, temperature, or stress conditions. Common processes include thermal metamorphism and dynamic metamorphism.