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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. Image File history File links Please see the file description page for further information. ... Galvanized surface with macroscopic crystalline features. ... In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Galvanized surface with macroscopic crystalline features. ... Crystallography (from the Greek words crystallon = solid and graphein = write) is the experimental science of determining the arrangement of atoms in solids. ... In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ...

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. Quartz is the most abundant mineral in the Earths continental crust. ... Gypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. // Chemical structure Gypsum from New South Wales, Australia Heating gypsum above approximately 150°C (302°F) partially dehydrates the mineral by driving off exactly 75% of the water contained in its chemical structure. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Feldspar (from the German Feld, field, and Spat, a rock that does not contain ore) is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earths crust. ... Staurolite Staurolite is a red brown to black mostly opaque nesosilicate mineral with a white streak. ... The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. ... Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ...

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. Albite, a feldspar mineral, is a member of the plagioclase series. ... Doubly refracting Calcite from Iceberg claim, Dixon, New Mexico. ... The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. ... In bodybuilding, striations are the tiny grooves of muscle across major muscle groups characteristic of a well-developed body. ... Rutile in trellis texture characteristic of secondary rutile. ... Aragonite Aragonite is a polymorph of the mineral calcite, both having the chemical composition CaCO3. ... Sample of cerussite-bearing quartzite Cerussite (also known as Horn silver, Lead carbonate, White lead ore) is a mineral consisting of lead carbonate (PbCO3), and an important ore of lead. ... 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. 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 mineralogical, chemical and crystallographic changes in a solid-state rock, i. ...

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

External links

• Tulane Edu. crystal twinning
• Mineral galleries - twins