Polysynthetic twin

DOia/TigAl. These materials possess a lamellar structure consisting of layers of twin-related TtAl and layers of TigAl in single crystalline form they are called "polysynthetically twinned (PST) crystals" (for a recent review see Yamaguchi, et al. 1995). 

If the stacking faults occur only rarely (say, every 105 layers on average), the result is a polysynthetic twinned crystal (cf. Fig. 18.8, p. 223). Depending on the frequency of the stacking faults, there is a smooth transition between crystals with stacking faults and poly synthetic twinning. 

The occurrence of twinned crystals is a widespread phenomenon. They may consist of individuals that can be depicted macroscopically as in the case of the dovetail twins of gypsum, where the two components are mirror-inverted (Fig. 18.8). There may also be numerous alternating components which sometimes cause a streaky appearance of the crystals (polysynthetic twin). One of the twin components is converted to the other by some symmetry operation (twinning operation), for example by a reflection in the case of the dovetail twins. Another example is the Dauphine twins of quartz which are intercon-verted by a twofold rotation axis (Fig. 18.8). Threefold or fourfold axes can also occur as symmetry elements between the components the domains then have three or four orientations. The twinning operation is not a symmetry operation of the space group of the structure, but it must be compatible with the given structural facts. 

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). 

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

Play of color is the term used to describe the internal spectral colors that appear inside some specimens of materials like opal and some synthetics. The colors seem to move when the specimen is turned or the light source is moved. Labradorescence is a broad play of colors common in labradorite and other minerals (mostly feldspars) having polysynthetic twinning. 

Examples of such materials have been known for a number of years in the mineralogical field, where the terms intergrowth phases , polysynthetic twinned phases , polysomatic phases and tropochemical cell twinning have been applied to describe the stmctures. In recent years, the application of electron microscopy has revealed the defect stmctnres of vast numbers of phases containing extended defects. In this section, some well-characterized examples chosen are chosen for illustration. 

Structure (i) was originally assigned to all three compounds, but as the result of later studies the following structures have been proposed CoAsS, (ii), NiAsS, (iii), and NiSbS, (i). It would seem that the greatest reliance may be placed on the later work on CoAsS, which shows that the apparently cubic structure is a polysynthetic twin of a monoclinic structure, and it is not impossible that other structures in this family are in fact superstructures of lower symmetry. As a result of the difference between Ni-Sb, 2-57 A, and Ni-S, 2-34 A, the symmetry of NiSbS has dropped to the enantiomorphic crystal class 23 the absolute structure has been determined. ... 

A projection of Py2AsF6 is shown in Figure 9. It consists of two independent stacks with different setting angles but identical overlap (type 1). Because the crystal exhibits polysynthetic twinning, a full crystal structural analysis has not been undertaken. Preliminary data indicate that in a phase transition, both stacks become identical that is, the structure is face-centered in the low-temperature phase. This conclusion is supported by electron spin resonance (ESR) data. 

Different 38 phases, in the case of low content of belite in clinker, is most convenient to identify using light microscopy. Among others, Metzger [161] applying this method found the presence of phase a. The striations of belite crystals always observed in industrial ehnkers due to polysynthetic twinning, are distinctly differentiate this phase from alite crystals and are formed in the polymorphic transformation process. 

Figure 43. Hot-pressed B-doped boron carbide with strain-induced polysynthetic twinning.

Beta dicalcium silicate is metastable below 670°C and monoclinic, with an n ofl.717,an n ofl.722,and of 1.736 and parallel to b birefringence is 0.019. The (h-) 2 Vis 64° to 69°, with the axial plane (010). The beta form exhibits very close polysynthetic twinning (Chromy, 1970). It comprises most of belite in clinker. Cleavage is poor on (100) and (010). Colors range from colorless to yellow or brown. Density is 3.28 Mg/m. Boikova (1980) reported a typical yellow color and spotty surface (not striated). 

The cross striations of Type I belite have been called polysynthetic twinning. However, this structure is not polysynthetic twinning but a skeleton structure, consisting of beta and alpha forms of belite. 

If the clinker is burned below the temperature of alpha prime inversion, and belite is cooled from the temperature region of alpha prime, belite has only the alpha prime to beta inversion texture. This texture is ob served as one set of parallel striations, which is polysynthetic twinning and parting on (100) and (010) after the crystal axes of the alpha prime. Rarely occurring are polysythetic twins on (110), (oil), and (011). 

Ono (1975) stated that the structure of Type I belite is skeletal instead of polysynthetically twinned. Thus, confusion, and perhaps some disagreement, in the literature exists as to the description and origin of the microstructure of Types I and II belites. The above pullout presents Ono s argument (1975), with only slight editing, in support of a largely skeletal interpretation of belite and a brief statement of polymorph origins. 

A different interpretation of alite has been given by Fundal (1982), who stated that alite crystals are twinned individuals of a lower symmetry which imitate a higher symmetry (a mimetic relationship). The monoclinic form of alite was said to have an index of refraction of 1.72 and a birefringence of approximately 0.0050. Tri clinic alite was reported to be polysynthetically twinned in a fused clinker and have a birefringence of 0.0020. Alite birefringence, however, exhibits two average val ues in Fundal s Table 1 (1982) 0.0043 (monoclinic) and... 

Polysynthetic twinning (very minute) in alite Triclinic alite due to failure to absorb enough impurities at high temperatures (Ono, 1975)... 

Polysynthetic twinning in pure CjS Develops as a result of monoclinic to triclinic transition at approximately 980°C during cooling (Hofmanner, 1973)... 

Belite with polysynthetic twinning Normal cooling rate (Kramer, 1960)... 

Photograph 7-41 Slowly cooled kiln buildup from the transition zone containing, for the most part, polysynthetically twinned belite (Type II, Insley) in lower left and slightly splintery gamma belite (upper right) in matrix of coarsely microcrystalline ferrite and aluminate. (S A6661)... 

Photograph 7-44 Polysynthetically twinned belite (Type II, Insley) in slowly cooled kiln buildup. (S A6664)... 

Keiviite-(Yb) and -(Y) were described as new minerals by Voloshin et al. (1983a, 1985). Yakubovich et al. (1986) determined the crystal structure of keiviite-(Yb) using a polysynthetically twinned sample, which consisted of keiviite-(Yb) and another phase of R2Si2 07, i.e., Er2Si2 07 having a different structure from that of keiviite-(Yb). The crystal structure of keiviite-(Yb) (fig. 37) is isostructural with that of thortveitite (fig. 36). 

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