Chemistry Reference
In-Depth Information
Figure 1.5
An example of parahemihedral form.
Figure 1.6
An example of holoaxial hemihedry.
A
fracture
is a more or less irregular surface, not repeated or repeated a few
times. The appearance of some fractures may be characteristic of a given min-
eral. An example is the tourmaline with fractures transversal on the prism.
A
twin
is an intergrowth of two or several crystals of the same mineral
species, joined by a determined symmetry law. There are two types of twins:
1
Contact twins
: the two individuals are symmetric to a plane, the twin-
ning plane; the surface of accolement is then a plane (example: twinning
of albite in plagioclases);
2
Penetration twins
: the two individuals are symmetric to an axis, the
twinning axis, the accolement surface is then
any
(example: Carlsbad
twinning in feldspars).