TWINNING
Twinning is the symmetrical intergrowth of 2 or more
parts of a crystal which are oriented differently but regularly with respect to
their symmetry elements (twin elements).
In order to distinguish twinned crystals from a
random intergrowth of crystals, the reflective goniometer as well as X – ray
diffraction methods are used.
The individual parts of a twin crystal are related
to each other by the following:
- - A mirror reflection around a mirror plane called the twin plane which most often coincides with a face.
- - A rotation axis (twin axis) which is common to both parts with an angular rotation of 180o.
- - An inversion point or twin center.
Twinning is defined by the Twin law
which indicates whether there is a complete twin center, axis or plane which
gives the crystallographic orientation for the axis or plane. Twin laws are unique to each system.
In the triclinic system, feldspars
twin according to the above law with (0 I0)
being the twin plane (indicating that twinning takes place in a direction
perpendicular to the b crystallographic
axis) and pericline twins in which (0 I0
) is the twin axis.
In
the monoclinic system, gypsum has a swallow-tail twin with ( I0
0) being the twin plane.
Orthoclase shows 3 types of twinning;
-
Manebach twin with ( 0
0 I ) as the twin plane,
-
Baveno twin with ( 0
2 I ) as the twin plane and
-
Carlsbad twin with (0 I0)
which is a clinopinacoid being the twin plane.
(a)
(b)
(c)
Figure
39: Types
of twinning in orthoclase; (a) Maneback, (b) Baveno and (c) Carlsbad twins
respectively.
The surface on which 2 individual parts of a twin
crystal unite is known as the composition surface. If this surface is a plane, then it is called
the composition
plane which often but not always coincides with the twin plane.
| (For web sources, view the post titled Bibligraphy) | |
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