Jewel__A_Celebration_of_Earth_s_Treasures

018 INTRODUCTION

Crystal systems

and habits

A crystal is a solid in which the component atoms are arranged

in a particular, repeating, three-dimensional pattern. When

these internal patterns produce a series of external, flat faces

arranged in geometric forms, this forms a crystal. These

repeating structures are identical structural units of atoms or

molecules, and are called unit cells. The unit cell is reproduced

over and over in three dimensions, meaning that the shape of

the crystal will resemble that of the individual unit cell. The

crystals of different minerals can have unit cells that are the

same shape but are made of different chemical elements.

Because a crystal is built up of repeating geometric patterns, all

crystals exhibit symmetry, depending on the basic geometry of

their unit cells. These fall into seven main groups and are called

crystal systems. The final external form a crystal takes is known

as its habit, and the shape produced by a mass of numerous

identical crystals is a growth habit (see opposite).

Cubic a=b=c The cubic

unit cell has three right angles,

and the lengths of its sides

are all equal. Thus, relative

to the length of its sides,

a equals b equals c.

Monoclinic a≠b≠c The

monoclinic system is created if

the orthorhombic cell is skewed

in one direction, only two right

angles remain, and a does not

equal b does not equal c.

Tetragonal a=b≠c If the cubic

cell is stretched vertically, there

are still three right angles, but

the length of the vertical side is

longer than the other two. Now,

a equals b does not equal c.

Triclinic a≠b≠c To create

the triclinic system, all of

the faces are skewed so that

no right angles remain, and

none of the edges of the

faces are equal.

Hexagonal a=b≠c To create

the hexagonal system, two

of the opposing long edges of

tetragonal cell are squeezed

together, leaving rectangular

sides but lozenge-shaped ends.

Trigonal a=b=c The first

hexagonal cell is further altered

by squeezing two opposing

short edges, so that all of the

faces are lozenge-shaped.

The angle between a and b is

120 degrees. In the USA, the

trigonal is considered a division

of the hexagonal.

Orthorhombic a≠b≠c If

the tetragonal cell is stretched

horizontally, the three right angles

remain, but now none of the sides

are of equal length. Thus, a does

not equal b does not equal c.

Mineralogists and crystallographers

have a complex set of criteria for

determining which mineral belongs

in which crystal system, based on

symmetry. In practical terms, these

systems can be understood as a group

of three-dimensional cells starting with

the basic cube (below). It should be

noted that hexagonal and trigonal

systems (right) are considered to be

one system by some crystallographers.

GREATEST SYMMETRY LEAST SYMMETRY

Minerals and

crystal systems

When two or

more crystals of

the same variety

are intergrown

sy m met r ically,

they are referred

to as twinned

018-019_INTRO_Crystal_habits.indd 18 19/05/2016 16: