Types of Twinning inwards Minerals

Smooching Beryl twin photo: Photo By Benjamin DeCamp

Crystal twinning occurs when 2 carve upwards crystals portion exactly about of the same crystal lattice points inwards a symmetrical manner. The number is an intergrowth of 2 carve upwards crystals inwards a diverseness of specific configurations.

A twin boundary or composition surface separates the 2 crystals. Crystallographers variety out twinned crystals past times a number of twin laws. These twin laws are specific to the crystal system. The type of twinning tin endure a diagnostic tool inwards mineral identification.

Twinning tin oftentimes endure a employment inwards X-ray crystallography, equally a twinned crystal does non hit a elementary diffraction pattern.

Pyrite Twin

Types of twinning

Simple twinned crystals may endure contact twins or penetration twins. Contact twins portion a unmarried composition surface oftentimes appearing equally mirror images across the boundary. Plagioclase, quartz, gypsum, together with spinel oftentimes exhibit contact twinning.

Merohedral twinning occurs when the lattices of the contact twins superimpose inwards 3 dimensions, such equally past times relative rotation of i twin from the other. An instance is metazeunerite. In penetration twins the private crystals accept the appearance of passing through each other inwards a symmetrical manner. Orthoclase, staurolite, pyrite, together with fluorite oftentimes demonstrate penetration twinning.

Galvanized surface amongst macroscopic crystalline features. Twin boundaries are visible equally striations within each crystallite, most prominently inwards the bottom-left together with top-right.
If several twin crystal parts are aligned past times the same twin police describe they are referred to equally multiple or repeated twins.

If these multiple twins are aligned inwards parallel they are called polysynthetic twins. When the multiple twins are non parallel they are cyclic twins. Albite, calcite, together with pyrite oftentimes demonstrate polysynthetic twinning. Closely spaced polysynthetic twinning is oftentimes observed equally striations or fine parallel lines on the crystal face. Rutile, aragonite, cerussite, together with chrysoberyl oftentimes exhibit cyclic twinning, typically inwards a radiating pattern.

Twinned crystals (e.g., run into Figure ) may endure described equally follows:

  • Simple twins – composed of alone 2 parts.
  • Multiple twins – composed of to a greater extent than than 2 orientations.
  • Contact twins – this occur if a definite composition airplane is present.
  • Penetration twins – occur if 2 or to a greater extent than parts of a crystal seem to interpenetrate each other amongst the surface betwixt the parts beingness indefinable together with irregular..
  • Polysynthetic twinning – occurs when 3 or to a greater extent than individuals are repeated alternately on the same twinned plane. If the individuals of polysynthetic twins are sparse plates, the twinning is called lamellar e.g. plagioclase feldspars.

Rare Spinel cyclic twin from Pyin-Oo-Lwin District, Mandalay Division, Myanmar.
Photo:: past times a bang-up mineral photographer.

Modes of formation

There are 3 modes of formation of twinned crystals. Growth twins are the number of an suspension or alter inwards the lattice during formation or increase due to a possible deformation from a larger substituting ion. Annealing or transformation twins are the number of a alter inwards crystal organisation during cooling equally i shape becomes unstable together with the crystal construction must re-organize or transform into exactly about other to a greater extent than stable form. Deformation or gliding twins are the number of stress on the crystal subsequently the crystal has formed.

If a FCC metallic similar aluminum experiences extreme stresses, it volition sense twinning equally seen inwards the instance of explosions. Deformation twinning is a mutual number of regional metamorphism.

Crystals that grow following to each other may endure aligned to resemble twinning. This parallel increase but reduces organisation issue energy together with is non twinning.

A rutile aggregate amongst a lot of twins from Simpelveld The Netherlands. Photo :© Fred Kruijen

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