Twinned Crystals: Crystals of many minerals grow in fixed relationships known as Twins. When two or more crystals grow together symmetrically, in other words, they share some crystallographic direction or plane, but other parts are in a reversed orientation they are called twins.
Twinning begins in most instances when crystals are very small. An ion in the growing crystal may take a position which is not geometrically perfect or nearly as good. If rapid growth takes place, it is possible for this ion to serve as a seed attractive to arriving ions which attach themselves and establish a new direction of growth. Gradually, the offset crystal enlarges and becomes half of the twin.
- Since twinning occurs early, it is usual to find twin parts nearly, if not exactly, the same size and exhibiting the same faces, markings, etch pits etc.
- Fast growth promotes twinning and conversely slow growth discourages twinning because ions have more time to shift to correct positions.
- Twin crystals always take the same positions relative to each other, the forces of attraction being very strong along twin junctions.
- As a result true twins do not break apart easily.
- There is also a constancy m the angles between twins very similar to the constancy of angles between faces of normal crystals.
Characteristics & Identification of Twins
- Re-entrant angles: Shallow or steep angles between twins are usually present at one or more points or as deep depressions in the center of some cyclic twins.
- Composition plane: The plane along which two twin halves appear to have been stuck together.
- Reversed surface markings on adjacent faces: Striations meeting in 'vees' (usually indicate twin junctions).
- Change in lustre between adjacent faces: Close examination is necessary to detect differences in etch marks, pits and lustre between twin individuals.
- Position: Twins show unvarying mutual positions; angles between them measure the same from twin to twin.
- Size: Because many twins begin growth at nearly the same instant, individuals tend to be of the same size.
- Occurrence of Twins: Some species twin according to one mode only, others twin in several distinct modes. Very few species do not twin.
Types of twinning: There are two basic types of twins.
- Contact twins - simple contact, lamellar / polysynthetic, cyclic.
- Interpenetrant twins
- The fastest method to detect twins to look for places where 'Vee' - shaped depressions mark the junctions between crystal pairs. Depressions of this sort are commonly called "Re-entrants", and the angles, re-entrant angles.
- A dotted plane passing through the spinel crystal represents the composition plane or the plane on which both halves of the twin meet.
- The twinned crystal can be visualized as a single crystal sliced in half with the front rotated to twin position. Such a twin is called a contact twin.
- This is commonly seen in a number of species e.g. beryl, topaz, diamond, spinel etc.
- In the case of spinel and diamond they are simple contact rotation twins. Such a diamond twin is known as a macle.
- Twinning is also observed as a change in lustre among narrow parallel bands which completely encircle the crystals. The slight differences in reflections indicate that each crystal actually consists of several crystals of repeated twins.
- Repeated twinning is called lamellar or polysynthetic twinning. It is commonly seen in plagioclase feldspar, and one can distinguish between the grains of this mineral and a similar potash feldspar.
- Another form of the repeated twin is the cyclic twin. Cyclic twins develop when crystals twin on several sides at once curving round to form dislike or circular groups, e.g. Chrysoberyl.
- Interpenetrant twins occur in many species. All give the impression that two crystals were pushed into each other as if each had no real substance.
- The twin relationship began when both crystals were very small and merely continued as they grew larger.
- Junctions between such twins are often irregular e.g. Fluorite, Staurolite, Quartz, etc.
- Cyclic and penetration twins produce misleading shapes, sometimes, making one believe that they belong to one class of crystals, when they really belong to another.
- Thus the Chrysoberyl cyclic twins are hexagonal in profile and one could really believe that it is a single crystal belonging to a mineral with a hexagonal structure, whereas they are actually orthorhombic.