Common Twinning Laws - Global Epidemic Observational Studies Organization

# Common Twinning Laws

Twin Law is a constant crystallographic expression in terms of twin-plane and twin-axis according to which twins of the same substance are found to be related. Following is a brief outline of the most commonly observed twin laws in different crystallographic systems:

(1). Twinning Laws in Cubic/Isometric Systems:

Spinel Law: It is named after its presence in the minerals of the spinel group. As per this law, the octahedral face (111) is the twinning plane, which is also in most cases, the composition plane. Further, the octahedral axis is the twinning axis. Twins of different types such as simple, contact, multiple contacts, and penetration twins based on this law have been observed in many minerals of the isometric systems, such as magnetite, galena, fluorite, and pyrite, besides spinel.

(2). Twinning Laws of the 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. The Calcite crystals commonly show twinning on three twin laws. The twin plane may be (0001), (1 0-1 1) or (0 1-1 2). Calcite scalenohedron twinned on (0001). Calcite show twinning on three twin laws.

Calcite Twins: Basically there are three twin laws involved in forming calcite twins. Among which the two most common twin laws are {0001} and the rhombohedron {011’2}.  Both are contact twins, but the {011’2} twins can also occur as polysynthetic twins that result from deformation.

Quartz Twins: Quartz shows three other hexagonal twins.

1. Brazil Law: (112’0) – is a penetration twin that results from transformation.
2. Dauphiné Law: (0001)- is also a penetration twin that results from transformation.
3. Japanese Law: (112’2) – is a contact twin that results from accidents during growth.

(3). Twin Laws of Orthorhombic System: In the orthorhombic system the twinning on (110) is most common. For example, the contact twin of aragonite and cyclic twin of cerussite are all twinned on (110).

(4). Twining Laws involved in 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.

(5). Monoclinic System Twin Laws: In the monoclinic system twinning on (100) and (001) is most common. Gypsum frequently shows twinning on (100) and the ‘Manebach twin” of orthoclase is on (001). Orthoclase also forms penetration twins according to the ”Carlsbad Law” in which the twin axis is the ‘c’ crystallographic axis, and the composition surface is roughly parallel to (010). Orthoclase also shows the ”Baveno twin” in which the twin plane is (021).

1. Manebach Law: (001) – forms a contact twin commonly observed in the mineral orthoclase. This twinning is very diagnostic of orthoclase when it occurs.
2. Carlsbad Law: (001) forms a penetration twin in the mineral orthoclase. Crystals twinned under the Carlsbad Law show two intergrown crystals, one rotated 180o 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.
3. Braveno Law: (021) forms a contact twin in the mineral orthoclase.
4. Swallow Tail Twinning Law: (100) are commonly observed in the mineral gypsum (CaSO4.2H2O).

(6). Twin Laws of Triclinic System: In the triclinic system the twinning exhibited by the plagioclase felspars is most important. They are twinned according to the ”Albit Law”, in which the twin plane is (010). Moreover in twins in microcline involves an other law, known as ”Pericline Law”. Let’s see these laws;

1. Albite Law:  As described above, plagioclase (NaAlSi3O8 – CaAl2Si2O8) very commonly shows albite polysynthetic twinning.  The twin law – (010) indicates that the twinning occurs perpendicular to the b crystallographic axis. Albite twinning is so common in plagioclase, that its presence is a diagnostic property for the identification of plagioclase.
2. 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 produces a cross-hatched pattern, called tartan twinning, as discussed above,  that easily distinguishes microcline from the other feldspars under the microscope.

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