Crystallography

by

Ann Dray and Trevor Emmett, Anglia Polytechnic University

Crystallography is rarely a popular or easy subject to teach at any level, and yet it provides some of the basic tools used in many disciplines. This interactive courseware module, aimed primarily at first-year geology undergraduates, introduces the user to the basics of crystallography. No prior knowledge is assumed and virtually no use is made of the stereographic projection. Thus, users can use this course to familiarise themselves with the ideas behind crystallography before having to plunge into the details of stereograms and mineral structures.

Description of the module

The module occupies just under 3 Mb, and is divided into eight units fronted by a main menu. Each unit can be run alone, but the main menu provides a link from which users can choose the unit they wish to work on and to which they return on quitting or finishing a unit.

Some units may cover more material than is often required (e.g. mention is made of screw and inversion axes, glide planes and of Miller-Bravais and Weber symbols) but throughout, the emphasis is on providing a good but deliberately sketchy outline of the basics.

The module comprises 8 units accessible from this main menu.

The units are as follows

Introduction

General introduction to crystallography; projections, lattices, motifs, 2D and 3D unit cells, and crystallographic axes.

Most units start with a few pages of background material.

Symmetry I

Introduction to symmetry elements; symmetry and crystal morphology, mirror planes, rotation axes, diads and comparing diads and mirror planes.

Animated sequencies are used extensively to explain concepts.

Symmetry II

Revision and introduction to more symmetry elements; rotation axes, centre of symmetry, non-translational symmetry, inversion axes, translational symmetry.

Various type of user interaction are employed.

Indexing I

Introduction to indexing planes; indexing families of planes, recipe for finding Miller indices, negative indices, parametral plane, planes of atoms, indices and symmetry.

Interactions include users entering numerical data.

Indexing II

Introduction to indexing lines; defining a line, lines in 2D space, recipe for indexing lines, lines in 3D space, lines in a centred lattice, indexing crystallographic axes.

Users interact by clicking to get additional information.

Unit 1. Indexing III

Revision for indexing lattice planes and directions and an introduction to indexing in hexagonal and trigonal systems; indices, forms and equivalent directions, indices for hexagonal and trigonal crystals, 4-figure indices in 3D, recipe for converting [uvw] to [UVTW].

Unit 1. Crystal Systems

Covers combinations of symmetry elements, Bravais lattices, crystal systems and point groups; the five 2D lattices, symmetry of 2D lattices, the 14 Bravais (3D) lattices, crystal systems and point groups, linking definitions together, defining crystallographic axes.

Rotations

Set of seven animations - one for each system. Includes user-controlled rotations, and a 'Guided Tour' that takes users through the main symmetry elements present.

The last unit includes a guided tour of the crystal systems.

Running the software

Navigation around the module is by means of buttons along the base of the screen. Users can work through the module step-by-step and are then required to complete everything on each page before being allowed to proceed. Alternatively, users can opt to be allowed to proceed without having to complete all the material on each page.

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

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