CLASSIFICATION OF CRYSTALS
As you must have read earlier (under elements of symmetry),
the symmetry elements are very important because they have been used to
classify crystals into systems and systems into classes. All together there are 32 classes under 7
symmetry systems.
I.
Classification into
systems:
The classification of crystals into
systems is done on the basis of the following sub categories:
-
The crystallographic
axes,
-
The crystallographic
angles,
-
The type of unit cell
(unit form),
-
The diagnostic symmetry
element.
a) The
crystallographic axes:
In this sub category, note is taken on
-
The number of axes
(whether there are 3 or 4 axes) and
-
The lengths at which
the unit form intersects each crystallographic axis,
b) The
crystallographic angles,
Here, the angles between the
crystallographic axes are considered (that is whether they are orthogonal or
non – orthogonal axes).
c) The
type of unit cell (unit form):
In this sub category, the type of unit
form that makes up the crystal is considered.
d) The
diagnostic symmetry element:
Under this, the symmetry element which
is common to all the classes under the system is considered. For example, the cubic system has 4iii
folds in all its classes, while the tetragonal system has 1iv folds
in all its classes.
II.
Classification into
systems:
The division of crystal systems into
classes is based on the symmetry elements (plane of symmetry, axis of symmetry
and center of symmetry).
Although there are seven (7) crystal
systems which are sub-divided into 32 classes, many of these classes have no
mineral representative, or are represented either by very rare minerals or by
chemical compounds. Most common minerals
fall under one of the first 15 classes.
The Holohedral or Holosymmetric
or Holomorphic
class is the class in a system which contains the highest number of symmetry
elements. Such a class can be used to
represent that system. For example the
Galena class in the Cubic system has 23 symmetry elements.
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