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Twinning -1.doc


									Rose Hall

Using examples from a number of mineral groups, describe the
mechanisms by which twinning occurs. What is the origin of the
characteristic "tartan" twinning of microcline?

Sometimes during the growth of a crystal, or if the crystal is subjected to stress
or temperature/pressure conditions different from those under which it
originally formed, two or more intergrown crystals can form in a symmetrical
fashion. These symmetrical intergrowths of crystals are called twinned
crystals. Twinning is important to recognise because, when it occurs, it is often
one of the most diagnostic features for the identification of minerals.

A twinned crystal, or twin, consists of 2 or more component crystals with a fixed
orientational relationship between them. The orientational relationship between
components is defined by the twin law. Twin laws are expressed as either form
symbols to define twin plane {hkl}, or zone symbols to define the direction of the
twin axes [hkl].

Twinning can be defined by the symmetry operations that are involved. These
include reflection across a mirror plane (called a twin plane), rotation about an
axis or line in the crystal (called a twin axis), or inversion through a point (called
a twin centre). Reflection twins are found in the albite member of the plagioclase
feldspar series where adjacent components are related to each other by
reflection in the (010) plane. Rotation twins are found in the pericline member of
the plagioclase feldspar series which has a twin axis of [010]. The Carlsbad twin
law in feldspars also produces rotations twins with the twin axis [001]. If the
twin law is a rotation axis the composition surface (the surface along which
lattice points are shared in twinned crystals) will be irregular.

If the twin law can be defined by a simple planar composition surface, the twin
plane is always parallel to a possible crystal face and never parallel to an existing
plane of symmetry. Because twinning adds to the symmetry of the crystal, it
never occurs in relation to the existing symmetry. If the twin law is a rotation
axis the composition surface will be irregular.

Twinning can originate in 3 different ways. Each process produces a different
type of twin:

Growth Twins - during crystal growth 'accidents' (mistakes in the crystal lattice)
can occur and a new crystal can be added to the face of an already existing
crystal. This results in twinning if the new crystal shares lattice points with the
face of the existing crystal, but has an orientation different from the original.
Growth twins can be contact twins (have a planar composition surface) or
penetration twins (have an irregular composition surface).

Glide/Deformation Twins - during deformation atoms in crystals lattices can be
pushed out of their correct place. If this produces a symmetrical arrangement it
results in deformation twins. Calcite can be easily twinned in this way.

Transformation Twins - these occur when a preexisting crystal undergoes a
Rose Hall

transformation due to a change in pressure or temperature. This commonly
happens in minerals that have different crystal structures and different
symmetry at different temperatures or pressures. When the temperature or
pressure is changed to that where a new crystal structure and symmetry is
stable, different parts of the crystal become arranged in different symmetrical
orientations, and thus form an intergrowth of one or more crystals.

The characteristic 'tartan twinning' seen in microcline results from the
combination of 2 types of twinning. The first is Albite Polysynthetic Twinning,
which is very common in plagioclase. Polysynthetic twins have composition
surfaces parallel to one another and in albite twinning the twin plane is {010}.
The second type is Pericline Twinning, which occurs when high temperature
monoclinic orthoclase or sanidine transforms to low temperature triclinic
microcline. The 2 twin laws operating simultaneously result in the prominent
cross hatched twinning seen in microcline which can only be observed under a
microscope when the analyser is in. Observing this pattern in thin section is one
of the most characteristic diagnostic properties for the identification of

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