Mineral Twinning
by Robby Flood

Mineralogy is the study of minerals and the best place to start any discussion about minerals is to define a mineral. There are six parts to the definition of a mineral: (1) definite chemical composition (2) definite crystalline structure (3) must be naturally occuring (4) homogeneous on a macroscopic scale (5) solid (6) usually of inorganic origin. Thus, minerals are crystalline and this crystallization process occurs when concentrations of chemical elements reach a state of supersaturation. There are several different ways in which crystals can form such as cooling (e.g., water to ice), evaporation (e.g., brine to solid halite), pressure (e.g., fault zone epidote), or chemical processes (e.g., formation of garnet during metamorphism).

Minerals are fascinating to many because they are beautiful works of art created in nature. When a mineral grows, the crystals are formed in a highly symmetrical order. However perfect they may seem, there are accidents in nature too. If an outside force acts upon the mineral during genesis, a new crystal may form in an opposite direction. This results in what is known as a twin or twinned crystal. Twinning is defined by Wenk and Bulakh (2005) as "intergrowths in which the two parts (twin and host) share a lattice plane or a lattice direction" (p. 89).

The mineral specimen shown left is a penetration twin of staurolite formed in a mica schist. The X is the twinned crystal. The image was taken from Exceptional Minerals, www.exceptionalminerals.com/denver2006-11.htm.

There are several ways that a twin can form including growth twins, transformation twins, and deformation twinning. Growth twins, as described by Frost "occur because the energy stage of the twinned crystal is sufficiently close to that of the untwinned crystal for twinning to occur due to random fluctuations in growth" (Frost 514). Nelson explains transformation twinning as "a preexisting crystal undergoing transformation due to a change in pressure or temperature" (Nelson). Deformation twinning occurs when already formed crystals are caused to shift. This can result in what is known as either translation or twin gliding. Many mineral's twinning properties can be associated with only one of the three types. For example, pyrite can only form from growth origins, whereas calcite can form from both growth and deformation.

Two main types of twinning are known. These are contact and penetration. If the twin occurs along a plane, what is known as a contact twin forms. This may or may not be on a mirror plane. The plane at which the twinnig occurs is known as the composition plane. Plagioclase is well known for the specific contact twin it exibits. This is known as polysynthetic twinning. It is shown below in the "albite twin" and is what gives characteristc traits such as labradorite's labradoresence. Penetration twins do not share a plane, but rather an axis or a twin center. These appear more irregular and complex as if one crystal was pushed into another.

The image right is a contact twin in calcite. It was taken from www.dkimages.com/discover/Home/Science/Earth-Sciences/Geology/
Minerals-and-Crystals/Classification/Carbonates/Calcite/index.html.

Below is a twin gallery to illustrate what is meant by twinned mineral crystals. The organization is by crystal system because specific types of twinning are always associated with one of the particular six crystal systems.

Triclinic

Feldspar group minerals twin according the the Albite Law. This contact or repeated twin is called polysynthetic in plagioclase feldspars such as the mineral species albite or labradorite.

These top two images came from www.open-adit.com/mineral/twinning.php and www.yourgemologist.com/labradorite.html.


When twinning is according to the Albite Law and the Pericline Law, a different look results. This pattern is known as a Tartan twin as can be seen by the plaid pattern illustrated in this photomicrograph.

The lower image is from gc.ucsd.edu/Proc/minsPROC.html

Monoclinic

The top two illustrations show twinning according to the Carlsbad Law. This is defined as crystals forming in opposite directions, as shown at the end of these specimens, and creating a penetration twin. Also, twinning is referred to as the Manebach Law...

The lower image is a Swallowtail Law or a contact twin that commonly forms in gypsum as seen here.

Orthorhombic

Staurolite law can occur at either right angles, forming a cross, or not at right angles, forming an X pattern.

Tetragonal

This is an example of a contact twin in the tetragonal crystal system. The illustration shows the contact plane and the mineral used to demonstrate this is rutile. This is referred to as a Knee Twin.

Hexagonal

An example of twinning in the hexagonal crystal system is with vanadinite and this contact twin.

Isometric

This beautiful pyrite specimen shows penetration twinning and is known as the Iron Cross.

Twinning is not the only mistake in mineral growth but it is one that is clearly a visual phenemmon. Polymorphism, pseudomorphism, and isomorphism are also growth phenomenon and somewhat related to crystalline defects.

Polymorphism is when a group of minerals has the same chemical composition but results in varied crystalline forms. Examples include two minerals both composed of carbon but diamond is in the isometric crystal system and graphite in hexagonal. Thus, both are made of carbon, but occur in different crystal systems and this is termed polymorphism or one composition with many crystal structures. Another example would be aragonite (orthorhombic) and calcite (hexagonal). Both minerals are calcium carbonate, but have different structures.

Pseudomorphism means false form and can result in three different ways: (1)substitution (2)encrustation (3) alteration. An example of substitution would be petrified wood, which is actually a microcrystalline variety of quartz replacing the cellular structure and form of a tree after death. Encrustation occurs if a fine layer of a new mineral (crust) is formed on an existing mineral. After the old mineral is removed the resulting crust is said to be a pseudomorph. Alteration results if only parts of a new mineral and old mineral are removed. The new mineral then takes the place of the removed old mineral.

Isomorphism or one form occurs when different chemical compounds come together in one of the basic configurations or elemental structures. An octahedron, a form of minerals in the isometric crystal system, can be composed of the minerals fluorite, spinel, or diamond.

References

For more information email rflood@emporia.edu.

Return to the GO336 Student Projects, www.emporia.edu/earthsci/amber/go336/webpages.htm.