Mechanical properties of crystals

Atmospheric conditions, particularly humidity and temperature, may have a profound effect on the appearance of some crystals that contain water (or other solvents) in their crystal structures. Efflorescence (from the Latin to blossom) is the change of a crystal to a powder upon ex- 

FIGURE 5.1. Crystal structure of potassium isocitric lactone, showing the direction of cleavage. Potassium ions large black circles, oxygen atoms small filled circles, carbon atoms open circles, hydrogen atoms omitted. K+ - 0 dotted lines, hydrogen bonds dashed lines. 

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In a similar fashion, the line and planar defects described above are all, strictly speaking, volume defects. For the sake of convenience it is often easiest to ignore this point of view, but it is of importance in real structures, and dislocation tangles, for instance, which certainly affect the mechanical properties of crystals, should be viewed in terms of volume defects. 

Hardness determination methods find wide uses in basic research on the mechanical properties of minerals and their deformation. In the face of the rapid development of industrial uses of natural minerals, as well as manmade, in monocrystal or grain form or as polymineral materials, there is a definite need for more comprehensive crystallomechanical investigations. Apart from the above aspects, hardness determination should furnish valuable information on the genesis of minerals. These authors consider that it would well serve the purpose to examine the mechanical properties of all minerals so as to obtain their allround crystallomechanical characterization and to investigate into their anisotropy and relationship to the structure and composition of minerals. By determining the typomorphism of the mechanical parameters of minerals and its involvement in the conditions of their formation, and also by investigating the specificity of occurrence of deformations in minerals under natural conditions and of the deformative mechanism, it should be possible to develop a general theory of mechanical properties of crystals. 

FIGURE 22.4 Naturally occurring muscovite mica. The mechanical properties of crystals of mica are quite anisotropic. Thin sheets can be peeled off a crystal of mica by hand, but the sheets resist stresses in other directions more strongly. Transparent, thin sheets of mica, sometimes called isinglass, have been used for heat-resistant windows in stoves or in place of window glass. 

Seeger, A., Dislocations and Mechanical Properties of Crystals, p. 243. Wiley, New York, 1957. 

Figure 12.9a Reprinted from Dash W.C. (1957) The observation of dislocations in silicon, in Dislocations and Mechanical Properties of Crystals, Eds Fisher, J.C., Johnston, W.G., Thomson, R. and Vreeland, T. Wiley, New York, pp. 57-68.

Amelinckx S (1957) Dislocations and mechanical properties of crystals. John Wiley Sons Inc, New York... 

The most important imperfections, so far as the mechanical properties of crystals are concerned, are the various imperfections called dislocations. The ease with which dislocations move through a crystal determine to a large extent its ranges of elastic and plastic deformation under an applied stress and its ultimate yield point —that is, the stress under which the crystal fractures. One kind of dislocation, called an edge dislocation, is shown in Figure 17-11. An edge dislocation can be described as involving removal of one-half of a plane of atoms from the crystal. 

E. M. Nadgornyi Dislocation dynamics and mechanical properties of crystals, volume 31 of Progress in materials science. Pergamon Press, Oxford, 1988. 

The presence of imperfections essentially influences the mechanical properties of crystals. Experience shows that theoretically calculated strength properties appear, as a rule, to be appreciably higher than experimental ones this is caused by neglecting crystal imperfections. Table 9.3 lists the physical methods that allow the investigation of the real structure of crystals. 

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