From Wikipedia,
the free encyclopedia
Crystal twinning occurs
when two separate crystals share
some of the same
crystal lattice points in a
symmetrical manner. The result is
an intergrowth of two separate
crystals in a variety of specific
configurations. A
twin boundary or composition
surface separates the two
crystals.
Crystallographers classify
twinned crystals by a number of
twin laws. These twin laws are
specific to the
crystal system. The type of
twinning can be a diagnostic tool
in mineral identification.
Simple twinned crystals may be
contact twins or
penetration twins. Contact
twins share a single composition
surface often appearing as mirror
images across the boundary.
Quartz,
gypsum, and
spinel often exhibit contact
twinning. In penetration twins the
individual crystals have the
appearance of passing through
each other in a symmetrical
manner.
Orthoclase,
staurolite,
pyrite, and
fluorite often show
penetration twinning.
If several twin crystal parts
are aligned by the same twin law
they are referred to as multiple
or repeated twins. If these
multiple twins are aligned in
parallel they are called
polysynthetic twins. When the
multiple twins are not parallel
they are cyclic twins.
Albite,
calcite, and
pyrite often show
polysynthetic twinning. Closely
spaced polysynthetic twinning is
often observed as
striations or fine parallel
lines on the crystal face.
Rutile,
aragonite,
cerussite, and
chrysoberyl often exhibit
cyclic twinning, typically in a
radiating pattern.
There are three modes of
formation of twinned crystals.
Growth twins are the result of
an interruption or change in the
lattice during formation or growth
due to a possible deformation from
a larger substituting ion.
Transformation twins are the
result of a change in crystal
system during cooling as one
form becomes unstable and the
crystal structure must re-organize
or transform into another
more stable form. Deformation
or gliding twins are the
result of stress on the crystal
after the crystal has formed.
Deformation twinning is a common
result of
regional metamorphism.
Of the three common crystal
structures: BCC, FCC, and HCP, the
HCP (hexagonal close-packed)
structure is the most likely to
twin.
Crystals that grow adjacent to
each other may be aligned to
resemble twinning. This
parallel growth simply reduces
system energy and is not twinning.
Reference
- Hurlbut, Cornelius S.;
Klein, Cornelis, 1985, Manual of
Mineralogy, 20th ed.,
ISBN 0471805807
