Twinning repeated

Figure 7.5. (a) Twin with re-entrant corners in three directions (shown by arrows) and (b) its expected re-entrant corner effect, (c) Morphological change is expected for repeated twinning. 

BOURNONITE. An antimony-copper-lead sulfide corresponding to the formula PbCuSbS3. It is orthorhombic, and repeated twinning often produces crosses or wheel-shaped crystals. It is brittle fracture, subconchoidal hardness, 2.5-3 specific gravity, 5.83 luster, metallic color and streak, dark gray to black opaque. 

Figure 8.15 (a) A stack of unit cells of NiTi in the (110) projection shear displacement represented by arrows is apphed to each cell in sequence, starting at A and ending at B, to produce a martensitic structure, (b) The deformed structure resulting from shear, (c) The overall shape of the original stack is more nearly maintained by repeated twinning... 

Gr. neos, new, and didymos, twin) In 1841, Mosander, extracted from cerite a new rose-colored oxide, which he believed contained a new element. He named the element didymium, as it was an inseparable twin brother of lanthanum. In 1885 von Welsbach separated didymium into two new elemental components, neodymia and praseodymia, by repeated fractionation of ammonium didymium nitrate. While the free metal is in misch metal, long known and used as a pyrophoric alloy for light flints, the element was not isolated in relatively pure form until 1925. Neodymium is present in misch metal to the extent of about 18%. It is present in the minerals monazite and bastnasite, which are principal sources of rare-earth metals. 

Twins are intergrown crystals such that the crystallographic directions in one part are related to those in another part by reflection, rotation, or inversion through a center of symmetry across a twin boundary. Twinned crystals are often prized mineralogical specimens. When twins are in contact across a well-defined plane (which is not always so), the boundary is generally called the composition plane. The only twins that are considered here will be reflection twins, where the two related parts of the crystal are mirror images (Fig. 3.22). The mirror plane that relates the two components is called the twin plane. This is frequently, but not always, identical to the plane along which the two mirror-related parts of the crystal join, that is, the composition plane. Repeated parallel composition planes make up a polysynthetic twin (Fig. 3.23). 

The term chemical twinning (CT) is used to refer to repeated ordered twin planes that change the stoichiometry of the bulk significantly. New coordination polyhedra that do not occur in the parent structure are a feature of chemical twinning, and the occupation of these generates a number of new stuctures. Both aspects are illustrated by the PbS-Bi2S3 system. 

In 1984, it was discovered that human genes contain short, repeating sequence of noncoding DNA, called short tandem repeats (STRs). The STR loci are slightly different for every individual except identical twins. By sequencing these loci, a unique pattern for each individual can be obtained. On the basis of this fundamental discovery, the technique of DNA fingerprinting was developed. 

CHRYSOBERYL. The mineral chrysoberyl. an aluminaie of hery Ilium corresponds to the formula BeALO. , crystallizes in the orthorhombic system with both contact and penetration twins common, often repeated resulting in ro.setted structures. Hardness. 8.5 specific gravity. 3.75 luster vitreous color various shades of green sometimes yellow. A variety which is red by transmitted light is known as alexandrite. Streak colorless transparent to translucent, occasionally opalescent. Chrysoberyl also is known as cymopbane and golden beryl. 

---