ADVERTISEMENTS:
In this article we will discuss about:- 1. Meaning of Twinning in Crystals 2. Some Illustrations of Twinning in Crystals 3. Types 4. Laws.
Meaning of Twinning in Crystals:
Generally crystals grow from the melts as individual specimens, each having its own shape, form and crystallographic characteristic. Sometimes, however, two crystals of the same compound may be formed from the same melt in such a way that both are essentially united, each bearing a definite crystallographic relation to the other.
A group of two crystals mutually united and intimately related are called twins and the phenomenon of their formation is called Twinning.
ADVERTISEMENTS:
Twinning is an interesting field of study in mineralogy because quite a good number of minerals have the habit of occurring in twinned forms. Such mineral twins form valuable pieces of collection in a geological museum.
Some Illustrations of Twinning in Crystals:
Staurolite, a hydrated silicate of aluminum with iron, crystallizes in orthorhombic system, commonly in prismatic form. Quite often two prismatic crystals of this mineral may occur as twins penetrating each other in peculiar fashion. (Fig. 10.38)
Similarly, Gypsum, a hydrated calcium sulphate (CaSO4. 2H2O), commonly occurs in simply flattened or prismatic crystals. Occasionally, two crystals of gypsum may be naturally united as twins popularly known as swallow-tail twins (Fig. 10.39).
Types of Twins:
On the basis of their apparent shape and without reference to their crystallographic description, following types of twins are commonly met within crystals:
ADVERTISEMENTS:
1. Contact Twins:
In this type, the component parts of a twin crystal are held together along a well-defined composition plane. Example- gypsum.
ADVERTISEMENTS:
2. Penetration Twins:
In these twins, the contact plane is not well defined. In fact the two parts of twin crystal may appear to be inter penetrating to each other. Example- Staurolite.
3. Simple Twins:
When a twin crystal has very well defined two-halves held together according to easily understandable relationships, these may be said as simple twins.
4. Multiple Twins:
In these twins, more than two individual crystals are held together in the following manner:
(i) There are a number of individual crystals repeated in the form of twins, but the twin plane is the same for all of them. This type of multiple-twinning is further distinguished into polysynthetic twinning when individuals are well defined crystals and as lamellar twinning when individual crystals occur in the form of thin plates or sheets. Plagiocase Felspars show this type of twinning.
(ii) When in a given composite crystal, there are a number of twin-crystals and the twinning is repeated along different faces (planes), the multiple twinning is called cyclic twinning.
On the basis of their apparent geometric relationship, twins are also often grouped into following three main types:
ADVERTISEMENTS:
(i) Rotation Twins:
Where it appears that twinning has been produced by rotation of one half (i.e. one member) for 90° or 180° about an axis, the twin axis, it is a rotation twin.
(ii) Reflection Twins:
Reflection twins are those where twining is explained by a symmetrical arrangement of one half to the other with respect to a common plane, the twinning plane.
(iii) Inversion Twins:
Inversion twins are those twins in which the arrangement of the two halves is symmetrical with respect to a common point.
Common Twin Laws:
A twin law is that 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. Isometric System:
Spinel Law:
It is so named because of its presence in minerals of spinel group. In this law, octahedral face (111) is the twining plane, which is also in most cases, the composition plane. Further, the octahedral-axis is the twining axis.
Twins of different types such as simple, contact, multiple contact and penetration twins based on this law have been observed in many minerals of isometric system, such as magnetite, galena, fluorite and pyrite, besides spinel. (Fig. 10.40).
2. Tetragonal System:
Rutile Law:
In such twins, the face of a pyramid of Second Order (101) is the twinning plane. This is the most common law for the crystals of the tetragonal system. Twins commonly observed in these crystals are of repeated type. Example- rutile, cassiterite, zircon and scheelite.
3. Hexagonal System:
i. The Brazillian Law:
In this law, the prism of Second Order (112̅ 0) is a twin plane. Quartz (SiO2) shows development of twins according to this law.
ii. The Dauphine Law:
In this law, c-axis is the twinning axis. Twins are generally intergrown. Some Quartz twins are also based on this law.
iii. The Japanese Law:
Contact twins result on this law in which pyramid (112̅ 2) is a twinning-plane. Quartz shows twinning according to this law also.
It may be mentioned here that quartz showing twinning on the above laws becomes a useless material for optical purposes.
4. Orthorhombic System:
In this system, crystals show twinning in a variety of ways of which following are more common.
Prism-Face (110) as the twinning-plane.
There are two variations:
(a) When the prism angle is about 60° and the twinning is repeated, a crystal with pseudo symmetry of higher order is produced. This is seen in the minerals of Aragonite group, which on account of repeated twinning on prism face appear hexagonal in outline.
(b) When the prism angle is 70°. Twins of this type are also common.
Staurolite Twinning:
This mineral shows cruciform twins of two types:
(a) Right-angled Cross – These result when face (031) is a twinning-plane.
(b) Sea-horse twin in which the face (231) is a twin plane.
In both the cases the twins are of penetration type:
5. Monoclinic System:
This system includes the largest number of minerals and also shows a great variety of laws on which twinning occurs in minerals.
The following five laws are the most common and in no case exclusive:
i. Carlsbad Law:
In this case, the c (or vertical) axis is the twinning-axis. The minerals commonly show inter-penetration or contact type of twinning.
ii. Baveno Law:
In this law, the mineral shows twinning with clino-dome (021) as the twinning plane. Twins may be of simple contact or repeated contact type.
iii. Manbach Law:
Here, the basal pinacoid (001) is a twinning plane.
Felspar orthoclase shows twinning according to one or other of the above mentioned laws.
iv. Gypsum Twins:
The mineral gypsum, a hydrated calcium sulphate (CaSO4.2H2O) shows twinning of simple contact type in which the a-pinacoid is the twinning plane. When the rotation is complete along this plane, the resulting twins resemble a swallow-tail. Hence, twins are also referred as swallow-tail twins.
V. Pyroxene Twins:
In this group of monoclinic minerals, twinning is generally according to Manbach Law where basal pinacoid (001) is the twinning plane. The twins are, however, generally of repeated polysynthetic type.
In other cases of pyroxenes, Orthodome (101) is the twinning plane.
6. Triclinic System:
The group of silicate minerals called the Plagioclase felspars having Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8) as the end members show interesting types of twinning.
Following are common examples:
i. Albite Law:
In this case, a plane parallel to fr-pinacoid is the twinning plane. Twinning is most commonly of repeated lamellar type involving very thin lamellae of the mineral.
ii. Pericline Law:
Here, the twinning axis is easily defined as the one parallel to b-axis. The twins may be repeated polysynthetic lamellar type.
The Felspar Plagioclase minerals also show twinning on laws found in Monoclinic Felspars such as Carlsbad, Manbach and Baveno laws. The twinning in these felspars being generally of repeated lamellar type is difficult to recognize in hand specimens and requires preparation of thin sections for study under microscope.