What is mineral twinning?

What is mineral twinning?

Twinning is the symmetric intergrowth of two or more crystals of the same mineral, and commonly is revealed in thin section by the different crystals of the same mineral having different extinction positions or different interference colours.

Which mineral is well known for twinning?

There are several minerals that form classic twins such as chalcocite, fluorite, sanidine, microcline, harmotome, staurolite, gypsum, cinnabar, spinel and rutile to name a few (more are listed below). Some twins have been given colloquial names such as the “fairy cross”, “iron cross” and “cog wheel” twins.

What is twinning in feldspar?

Twinning in feldspars is most obvious between crossed polars. In addition to simple twins, plagioclase feldspar often shows repeated or multiple twinning (Figures 57c and 59a) usually on a microscopic scale, which gives crystals a striped appearance (Figure 59c).

What is twinning and its types?

Twinned crystals (e.g., see Figure ) may be described as follows: Simple twins – composed of only two parts. Multiple twins – composed of more than two orientations. Contact twins – this occur if a definite composition plane is present.

Is twinning a type of dislocation?

Therefore, the theoretical stress required to form a twin is quite high. It is believed that twinning is associated with dislocation motion on a coordinated scale, in contrast to slip, which is caused by independent glide at several locations in the crystal.

Does quartz show 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.

What is anti twinning?

During anti-twinning, a shear displacement of 1/3〈111〉 occurs on every successive {112} plane, in contrast to an opposite shear displacement of 1/6〈ˉ1ˉ1ˉ1〉 by ordinary twinning. This asymmetry in the atomic-scale shear pathway leads to a much higher resistance to anti-twinning than ordinary twinning.

Does quartz have twinning?

Quartz. Keys to identification are:colorless nature, first order white-gray interference colors, and undulatory extinction. It also lacks cleavage, twinning and alteration.

What is twinning in biology?

Twinning refers to the process that leads to the production of more than one offspring at one birth. In humans the most frequent type of twinning results in the birth of two babies (twins) at once, although repetition of twinning may lead to triplets,…

How does twinning happen?

To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information. To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.

What is the difference between slip and twinning?

Summary – Slip vs Twinning The key difference between slip and twinning is that during a slip, all atoms in a block move the same distance whereas, in twinning, the atoms in each successive plane in a block move through different distances that are proportional to their distance from the twinning plane.

What is Carlsbad twinning?

A twinned crystal in which the twinning axis is the c axis, the operation is a rotation of 180 degrees, and the contact surface is parallel to the side pinacoid. It is quite common in the alkali feldspars. An alternative spelling is Karlsbad twin.

What is the significance of twinning in mineral identification?

The type of twinning can be a diagnostic tool in mineral identification. Twinning is an important mechanism for permanent shape changes in a crystal. Twinning can often be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern .

What is crystal twinning and how does it work?

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.

What are the different types of twinning?

Types of twinning Simple twinned crystals may be contact twins or penetration twins. Galvanized surface with macroscopic crystalline features. If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins.

Are there any twinned minerals?

The Twinned Minerals. Twinning is actually rather common in the mineral kingdom, however perfectly formed twins are not. Minerals that commonly grow well formed twins are known to nearly every mineral collector. Twin collecting can be a very enjoyable hobby and most collectors own one or more.