Repeated twinning, twinning of more than two crystals, or parts of crystals.
A contact twin is one in which the parts are united by a plane surface, called the composition face, which is usually the same as the twinning plane.
Defn: Lamellar separation in a crystallized mineral, due to some other cause than cleavage, as to the presence of twinning lamellæ.
Polysynthetic twinning, repeated twinningof crystal lamellæ, as that of the triclinic feldspars.
Defn: Imitative; characterized by resemblance to other forms; -- applied to crystals which by twinning resemble simple forms of a higher grade of symmetry.
This kind of twinning is especially frequent in crystals of spinel, and is consequently often referred to as the "spinel twin-law.
Another curious result of twinning is the production of forms which apparently display a higher degree of symmetry than that actually possessed by the substance.
In calcite and some other substances this lamellar twinning may be produced artificially by pressure (see below, Sect.
Twinning may be several times repeated on the same plane or on other similar planes of the crystal, giving rise to triplets, quartets and other complex groupings.
When often repeated on the same plane, the twinning is said to be "polysynthetic," and gives rise to a laminated structure in the crystal.
The characteristic transverse striae, invariably present on the cleavage surfaces of stibnite and cyanite are due to secondary twinning along glide-planes, and have resulted from the bending of the crystals.
This secondary twinning is often to be observed as a repeated lamination in the grains of calcite composing a crystalline limestone, or marble, which has been subjected to earth movements.
Twinning according to the second law can only be explained by reflection across the plane (101), not by rotation about an axis; chalcopyrite affords an excellent example of this comparatively rare type of symmetric twinning.
Twinning according to the first law is effected by rotation about an axis normal to the sphenoidal face (111), the resulting form resembling the twins of blende and spinel.
Twinning also takes place according to two other laws, giving rise to interpenetrating crystals with the basal planes (s) of the two individuals inclined at angles of 69 deg.
Milton, in the following lines, has applied the name to some twinning plant, perhaps the honeysuckle.
Lamellar separation in a crystallized mineral, due to some other cause than cleavage, as to the presence of twinning lamell\'91.
Imitative; characterized by resemblance to other forms; -- applied to crystals which by twinning resemble simple forms of a higher grade of symmetry.
This lamellar twinning is of secondary origin; it may be readily produced artificially by pressure, for example, by pressing a knife into the edge of a cleavage rhombohedron.
Repeated twinningon this plane is very common, and the twin-lamellae (fig.
Although much of their original material has often disappeared, they still display the lamellar twinning of medium andesine (15° to 20°).
Lamellar twinning is rare, the extinctions being those of oligoclase andesine (10° to 20°).
They rarely display lamellar twinning and give extinctions measured from the long axis of oligoclase and oligoclase andesine (2°-8°).
The twinning machine was invented, about twenty years ago, by a Mr. Thomas, of Philadelphia; but it has been successfully improved by several individuals since that time.
This twinning may be produced by pressure acting either during the crystallization of the rock or at a later period.
Usually they are twinned on a prism plane, M, producing pentagonal stellate groups of five crystals; twinning on the plain g, in which the crystals intercross at angles of nearly 60 deg.
Twinned crystals are not common, but the presence of polysynthetic twinning is sometimes shown by fine striations running diagonally or obliquely across the cleavage surfaces.
Albite twinning is very frequent, but in these rocks it is often accompanied by pericline twinning by which the broad or narrow albite plates are cut transversely by many thin, bright and dark bars as seen in polarized light.
It is of some interest on account of the twinning and the beautiful development of its crystals.
The repeated twinning gives rise to twin-lamellae, which may be detected on the fractured surfaces, even of the massive material.
By twinning we mean the production of equivalent structures by division.
The opinion that twinning may descend through the male directly has been lately expressed by Dr.
The phenomena of twinning provide a convincing demonstration that this is so.
There is another phenomenon bytwinning which, if we could understand it, might help.
The behaviour of homologous twinningin heredity has been little studied.
Besides twinning of the whole body a comparable duplicity of various parts of the same body may occur.
It is possible that in Man twinning is due to a peculiarity of the mother, not of the father.
This twinning is usually often repeated on the same plane (fig.
