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
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