Chemistry Reference
In-Depth Information
When the symmetry of the crystal is lower than the one of the lattice,
one talks of merohedrism (hemihedrism if it contains only half of the faces
of the corresponding, tetartohedrism with a quarter of the faces, etc.). The
most common merohedrism are:
antihemihedrism:
absence of center (and of the associated planes); an
example of antihémihedrism is the (cubic or tetragonal) tetrahedron
that derives from the octahedron; another example is the tourmaline
prisms which have two extremities that are not symmetrical; such min-
erals have piezoelectric properties;
parahemihedrism
: the center of symmetry is present, but the binary axes
are absent; for instance, the pentagonal dodecahedron (a common form
of pyrite, or pyritohedron) derives from an hexatetrahedron;
holoaxial hemihedrism
: no center, no binary axes; trigonal quartz and
scheelite belong to this class.
Any lattice plane can form a face of a crystal. But growth is more or less
rapid according to the directions. The fast growing faces are rapidly elimi-
nated and slow growing faces become dominant. These are the faces that
determine the
crystalline forms
of the mineral. The crystalline forms reflect
the structure of the lattice and the symmetry system to which it belongs.
When a mineral has its own crystal forms it is said to be euhedral, otherwise
we talk of an anhedral mineral. In rocks, minerals such as tourmaline, kyanite
and garnet are frequently euhedral, others, such as quartz, more rarely.
A
cleavage
is a perfect and repetitive separation plane. The cleavage
planes reflect, at the macroscopic scale, one or more zones of weakness:
cleavage typically occurs preferentially parallel to higher density planes. The
cleavages show the same symmetry as the crystal. Some minerals have no
cleavage (quartz, for example), others have one or more.
Figure 1.4
An example of antihemihedral form.
