Optical Mineralogy

by

Dave Emley, George Rowbotham and Graham Lees, University of Keele

This courseware module is intended for first and second-year undergraduates and has two main objectives.

It is primarily intended to act as reinforcement to a lecture/practical course, where students can go over points they may not be too clear about. It is not intended to replace a lecture course on the principles of optical mineralogy.
Now that modular courses are becoming more common, it should act as a means by which students, who may not have used a petrological microscope for some time, can quickly refamiliarise themselves with the theory and practice of identifying minerals from their optical properties. It is not intended to replace microscope work in the laboratory.

Description of the module

The module occupies approximately 12 Mb, and is divided into five units selectable from this menu:

The module comprises five units, three of which are subdivided into smaller units.

Unit 1. Introduction

This describes briefly the purpose of the module and explains how to use it.

Unit 2. Basic Concepts

The Nature of Light: Wave theory; refractive index; electromagnetic spectrum; polarization of light.

The Petrological Microscope: Description of the essential components of the petrological or polarizing microscope and their function.

Information is given in a variety of ways, some of which involve user interactions.

Unit 3. Optical studies under plane-polarized light

Shape: Types of shape, terms for minerals in igneous rocks, terms for minerals in metamorphic rocks, with examples.

This page on crystal shapes is from the unit on optical studies under plane-polarized light.

Cleavage: Cleavage quality - perfect to absent, fracture. Relation of cleavage to crystallography.

Refractive Index: Definition; isotropic and anisotroic sections; Snell's Law; dispersion; Becke Line Test; determination in grains and in thin sections; critical angle; determination of refractive index of oils using critical angle; Abbe refractometer.

Relief: Definition - high, medium, low relief, with examples.

Colour: body colour; pleochroism.

Absorption Scheme: Pleochroic absorption scheme. This is dealt with separately as crossed-polar studies need to be covered first.

The theory of optical tests employed is explained in a step-by-step manner.

Unit 4. Cross-polarized Light Studies

Interference of Light: Phase differences; constructive/destructive interference of light.

Extinction: Phenomenon of extinction; types of extinction; measurement of extinction angle.

There are a large number of images of minerals seen through the petrological microscope; this page is from the section on extinction angles.

Twinning: Simple twinning; multiple/polysynthetic twinning; determination of composition using extinction angle.

Birefringence: Two refractive indicies; retardation; behaviour under cross polarized light; interference colours; directions of slow and fast rays and their measurement; measurement of birefringence.

Optical Indicatrix: Definition; isotropic, uniaxial, biaxial indicatrices.

Interference Figures: Theory of interference figures; setting up conoscopic illumination; production of interference figures; use of interference figures; e.g. determination of optic sign.

Unit 5. Mineral Identification

A systematic method for identifying an unknown mineral using all the concepts dealt with in the previous sections, culminating in a determination flow chart. This provides the student with a simulation of a real identification involving interaction at critical points in the sequence.

The procedure for the systematic determination of this unknown mineral includes looking for twinning.

Running the software The different parts of the module are accessed via a main menu and navigation around the module is by means of buttons along the base of the screen.

A glossary is accessed by clicking either on hot words in the body of pages or on the Glossary button along the base of the screen.

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