Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. 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. A crystal system is a category of Space groups which characterize Symmetry of structures in three dimensions with Translational symmetry in three directions 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. Plagioclase, Quartz, gypsum, and spinel often exhibit contact twinning. Plagioclase is a very important series of tectosilicate Minerals within the Feldspar family Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in Gypsum is a very soft Mineral composed of Calcium sulfate dihydrate with the Chemical formula Ca[[sulfur S]] O 4·2 The spinels are any of a class of Minerals of general formulation XY2 O 4 which Crystallize in the cubic (isometric 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. Physical properties Orthoclase crystallizes in the Monoclinic crystal system Staurolite is a red brown to black mostly opaque nesosilicate Mineral with a white streak Fluorite (also called fluorspar) is a Mineral composed of Calcium fluoride, Ca[[Fluorine F2]]
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. Albite is a plagioclase Feldspar Mineral. It is the sodium endmember of the Plagioclase Solid solution series Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Striations means a series of Ridges, furrows or Linear marks and are used in several ways Glacial striation Striation (geology Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern. Rutile is a Mineral composed primarily of Titanium dioxide, Ti[[oxygen O]]2 Aragonite is a Carbonate mineral, one of the two common naturally occurring polymorphs of Calcium carbonate, Ca[[carbon C]] O 3 Cerussite (also known as lead carbonate or White lead ore) is a Mineral consisting of Lead Carbonate (PbCO3 The Mineral or Gemstone chrysoberyl, not to be confused with Beryl, is an aluminate of Beryllium with the formula BeAl2O4
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. Annealing or 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. Metamorphism can be defined as the solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids i
Of the three common crystal structures: BCC, FCC, and HCP, the HCP structure is the most likely to twin. The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube. The cubic crystal system (or isometric) is a Crystal system where the Unit cell is in the shape of a Cube.
Crystals that grow adjacent to each other may be aligned to resemble twinning. This parallel growth simply reduces system energy and is not twinning.
Twin boundaries
Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals. This is also a much lower-energy interface than the grain boundaries that form when crystals of arbitrary orientation grow together. A crystallite is a domain of solid-state matter that has the same structure as a single Crystal.
Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. Shock hardening is a process used to strengthen metals and Alloys wherein a Shock wave produces atomic-scale defects in the material's Crystalline Work hardening, strain hardening, or cold work is the strengthening of a material by macroscopically speaking plastic deformation (which has the In Materials science, a dislocation is a Crystallographic defect, or irregularity within a Crystal structure. They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel. Steel 035 water quenchedpng|thumb|200px|035%C Steel water-quenched from 870°C]] Martensite, named after the German metallurgist Adolf Martens (1850–1914 Nickel titanium ( Ni[[Ti]] is a Shape memory alloy also commonly referred to by the name Nitinol, derived from its place of discovery (Nickel Titanium A quench refers to a rapid Cooling. In Polymer chemistry and Materials science, quenching is used to prevent low-temperature processes such as phase Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0
References
- Hurlbut, Cornelius S. ; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed. , ISBN 0-471-80580-7
External links
- Tulane Edu. crystal twinning
- Mineral galleries - twins
- Mathematical and Theoretical Crystallography
- Online Dictionary of Crystallography
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