Negative crystal

Optical signs of micas are negative, crystal system is monoclinic, and the streak is colorless. 

Wilson (28) noted the presence of etch pits (crystallographically controlled voids or features of negative relief, or "negative crystals") on some soil feldspars, and reviewed similar observations from earlier studies. Some examples of etch pits on naturally weathered feldspars are shown in Figure 3- Etch pits... 

Figure 12.6. Polarization photomicrograph showing dislocation Awith Burgers vector parallel to the c-axis, which is generated from a tube-like liquid inclusion (U in (b)) formed behind foreign mineral grains (arrows in (a)), which were precipitated on a growing surface. The X symbols in (b) denote the banding of successive stages of formation of a negative crystal [1].

The observation of convergent light figures may often provide confirmation of the orientation of the indicatrix. The plane of the optic axes is the ay plane, while the normal to this plane is the j8 vibration direction. For a positive crystal the acute bisectrix is the y vibration direction, while for a negative crystal it is the a vibration direction. 

LiI03 is called a negative crystal, because the extraordinary refractive index is smaller than the ordinary one ne - n0 = 1.78 - 1.93 < 0 (values for 514.5 nm). When the scattering geometry is chosen in such a way that the incident light is an extraordinary ray, a greater polariton shift is obtained because smaller k values can be realized, as in the case of ZnO. 

Figure 4.1 Sections of uniaxial ray velocity surfaces, (a) Optically positive crystal and (b) optically negative crystal.

Figure 5. Physical origin of magnetic anisotropy (a) compass-needle analogy of shape anisotropy and (b-c) magnetocrystalline anisotropy. In (b) and (c), the anisotropy energy is given by the electrostatic repulsion between the tripositive rare-earth ions and the negative crystal-field charges.

The trace of the planar central zone of the microcrack-ladder is marked by the presence of negative crystals and inclusions. This characteristic indicates that microcrack-ladders must be related to healed longitudinal fractures. The formation of unhealed microcracks normal to the loading direction (i.e., the rungs of the ladders) can be understood as follows. Undulatory extinction and dislocation microstructures observed within the microcrack-ladders indicate that plasticity is restricted to a narrow zone ( 100 / m wide) within the otherwise-undeformed host crystal. Therefore, the host crystal responded elastically while the load was applied the elastic strain A/// being about 0.5 percent. Now, in the plastic zone of the... 

Gratz A. J. and Bird P. (1993) Quartz dissolution negative crystal experiments and a rate law. Geochim. Cosmochim. Acta 57, 965-976. 

FIGURE 5.10. Some crystals, each with their optical indicatrices drawn within them, (a) A uniaxial negative crystal (calcite) showing orientation of the indicatrix. (b) A uniaxial positive crystal (quartz) showing orientation of the indicatrix. 

Calorimetric measurements of the heat evolved from the reactions of sodium polyacene dianion salts with water afforded the determination of thermodynamic parameters 70). For all the polyacene dianion salts, the heats of formation were found to be negative. Crystal lattice energies between 400 and 440 kcal/mol were found. These studies add a very important insight into the properties of 4nrc conjugated polycyclic dianions (Sect. 6.4). The measurements were carried out on solvent free dianions in a colimeter... 

A very negative crystal lattice energy indicates very strong attractions within the solid. 

For large negative crystal field at low temperatures the stable stmcture is the ideal two-sublattice in-plane herringbone phase—that is, the 2-in phase in Fig. 4a. The two sublattices which can be oriented in three different ways relative to the triangular lattice lead to six equivalent ground states. The excitation spectrum of this phase in general has a gap. In this phase the molecular wave functions are localized in the substrate plane, and classically the molecular axes are parallel to the surface see Appendix A of Ref. 141 for an interpretation of the order parameters. Thus, the orientational degeneracy of the pararotational phase is broken by the quadrupolar interactions. A closely related structure was already proposed based on atomistic Lennard-... 

A number of terms have been used to describe inclusions, some of which are self-explanatory, such as bubbles, Ijords (parallel channels), veils (thin sheets of small bubbles), clouds (random clusters of small bubbles), negative crystals (faceted inclusions) and so on. Most frequently inclusions are distributed randomly throughout the crystal, but sometimes they show a remarkable regularity, e.g. as in hexamine (Denbigh and White, 1966 Bourne and Davey, 1977) and ammonium perchlorate (Williams, 1981). Sometimes hour-glass or Maltese cross patterns may appear, e.g. as in sucrose (Powers, 1969/70 Man-tovani et al., 1985). Several examples of different types of inclusion in crystals are illustrated in Figure 6.46. 

The dipotassium salt is stable in solution or as a crystalline solid but it explodes on the least contact with oxygen or moisture and turns brown even in an atmosphere of nitrogen or argon [28,30]. The salts are thermodynamically very stable this is attributed partly to the ten ir-electron system but more especially to the large negative crystal lattice energy [33]. 

Negative crystals n. A uniaxial crystal is optically negative if < to. A biaxial crystal is said to be optically negative if... 

The clarity of an emerald is reduced by the presence of inclusions-guest crystals, negative crystals, healing fissures, structural features, and open fissures. Open fissures are especially... 

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