Open Adit™ - Laws of Twinning

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Laws of twinning determine the shape of the more common types of twinned crystals. The atomic structures of twins are arranged in specific geometric patterns, which may be as simple as mirroring of structures, or as complex as rotations of atomic structure.

Laws of twinning are usually named after the shape they resemble or the country of origin or species of mineral in which they were first observed.

Common Twin Laws

Triclinic System - The feldspar minerals plagioclase and microcline are the most common triclinic minerals that show twinning. Two common twin laws are observed in these feldspars.

Albite Law - As described above, plagioclase (NaAlSi₃O₈ - CaAl₂Si₂O₈) very commonly shows albite polysynthetic twinning. The twin law indicates that the twining occurs perpendicular to the b crystallographic axis. Albite twinning is so common in plagioclase, that it's presence is a diagnostic property for identification of plagioclase.

Pericline Law - The pericline law has [010] as the twin axis. As stated above, pericline twinning occurs as the result of monoclinic orthoclase or sanidine transforming to microcline (all have the same chemical formula - KAlSi₃O₈). Pericline twinning usually occurs in combination with albite twinning in microcline, but is only observable with the polarizing microscope. The combination of pericline and albite twinning produce a cross-hatched pattern, called tartan twinning, as discussed above, that easily distinguishes microcline from the other feldspars under the microscope.

Monoclinic System - The most common twins in the monoclinic system occur on the planes {100} and {001}. The feldspars - orthoclase and sanidine - are the most commonly twinned minerals in the monoclinic system. Both contact twins and penetration twins occur, and both types result from accidents during growth.

Manebach Law - forms a contact twin commonly observed in the mineral orthoclase. This twinning is very diagnostic of orthoclase when it occurs.

Carlsbad Law - forms a penetration twin in the mineral orthoclase. Crystals twinned under the Carlsbad Law show two intergrown crystals, one rotated 180^o from the other about the [001] axis. Carlsbad twinning is the most common type of twinning in orthoclase, and is thus very diagnostic of orthoclase when it occurs.

Braveno Law - forms a contact twin in the mineral orthoclase.

Swallow Tail Twins - are commonly observed in the mineral gypsum (CaSO₄2H₂O).

Orthorhombic System - Orthorhombic crystals commonly twin on planes parallel to a prism face. The most common is a {110} twin that results in many orthorhombic minerals having cyclical twins.

Cyclical Twins - The mineral aragonite (CaCO₃) , chrysoberyl (BeAl₂O₄), and cerrusite (PbCO₃) commonly develop twinning on {110}. This results in a cyclical twin which gives these minerals a pseudo-hexagonal appearance.

Staurolite Law - The mineral staurolite is really monoclinic, but it has a ß angle very close to 90^o so it has the appearance of an orthorhombic mineral. Two types of interpenetration twins occur in staurolite the {031} twins from a right-angled cross and the {231} twins form a cross at about 60^o.

Tetragonal System - Twinning in the tetragonal system usually occurs on {011} forming cyclical contact twins.

The minerals rutile (TiO₂) and cassiterite (SnO₂) commonly show this type of twinning.

Hexagonal System - The minerals calcite (CaCO₃) and quartz (SiO₂) are the most common hexagonal minerals and both show the types of twinning common in hexagonal minerals.

Calcite Twins - The two most common twin laws that are observed in calcite crystals are {0001} and the rhombohedron. Both are contact twins, but the {012} twins can also occur as polysynthetic twins that result from deformation.

Quartz shows three other hexagonal twins.

Brazil Law - is a penetration twin that results from transformation.

Dauphiné Law - is also a penetration twin that results from transformation.

Japanese Law - is a contact twin that results from accidents during growth.

Isometric System - Three types of twins are common in the isometric system.

Spinel Law - is a twin plane, parallel to an octahedron. It occurs commonly in mineral spinel (MgAl₂O₄).

The twin axis perpendicular to an octahedral face adds three fold rotational symmetry.

Iron Cross - The mineral pyrite (FeS₂) often shows the iron cross made of the interpenetration of two pyritohedrons. Since this occurs in the class 2/m, with no 4-fold rotation axes, the twin axis gives the mineral apparent 4-fold symmetry about 3 perpendicular axes.

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