PHYSICAL CRYSTALLOGRAPHY treats of the physical properties of crystals and their determination. In general any measurable property will yield results varying with the di rection and alike only in directions which are structurally equivalent. If these results are represented by straight lines of proportionate length in the given directions they determine a closed surface which in symmetry and dimen sions represents that property in that crystal.
The physical characters may be subdivided into those which depend upon cohesion and elasticity, and upon the action of radiant energy (light, heat, magnetism and electricity).
The Properties Dependent upon Cohesion. —The resistance opposed by a crystal to forces tending to separate its particles is shown by various important properties, as cleavage, glid ing, parting, percussion-figures, etch-figures, corrosion and hardness. Of these cleavage has been described, see CLEAVAGE, and hardness is discussed under the article MINERALOGY.
Gliding is the tendency of the particles to slip or glide without separation parallel to a so-called gliding plane, which is always a pos sible crystal face, when pressure is exerted a certain direction. Thus if a slender crystal of stibnite supported at the ends is pressed downward with a dull knife edge, it is bent at this point without affecting the other parts. The gliding may be accompanied by a tion of 180 degrees bringing a part of the crys tal into twinning position. Unlike cleavage planes, which are at right angles to directions in which the particles are furthest apart, gliding planes are at right angles to directions in which the particles are very close together.
Parting.—The planes along which gliding has taken place often become planes of easy sepa ration and are then called Part ing resembles cleavage, but is distinguished by the fact that it takes place only in certain deh nite planes, those of the molecular disturbance, while cleavage may be obtained in any part of the crystal in the given direction.
Etch figures.— When a crystal is attacked by any solvent the action proceeds with differ ent velocities in crystallographically different directions, and if stopped before the solution has proceeded far, the crystal faces are often pitted with little cavities of definite shape usually bounded by minute faces.
The figures, whatever their shape, conform in symmetry to the class to which the crystal belongs and as the faces rarely belong to limit forms common to several classes, etch figures are very useful in determining the true•symme try of the crystal.
Etch figures are alike on faces of the same crystal form and generally unlike on faces of different forms.
Corrosion.—The unequal rapidity of solu tion in different crystallographic directions is sometimes studied by immersing a crystal in a solvent. This may result in the development of new faces conforming in position to the sym metry of the crystal. Sometimes a sphere is cut from the crystal and submitted to the ac tion of the solvent. Thus a sphere of quartz subjected to the prolonged action of hydro fluoric acid was attacked rapidly in the direction of the vertical axis but scarcely at all in the direction of the horizontal axes, the resulting form being a subtriangular lens.
Percussion Cracks parallel to crystallographic directions are sometimes devel oped by pressing or tapping a conical point against a face of a crystal. Examples are mica and common salt.
The Properties Dependent upon Elasticity.
—If rods of the same length, breadth and thickness are cut from different parts of the same crystal and subjected to equal strains, they are found to undergo different extensions, and if, from the resulting coefficients of extension for all directions, the extension surface is con structed, this, while often a very complex shape, is found always to be symmetrical to the axes and planes of symmetry of the crystal.
Pressure in different directions will modify the optical characters of a crystal. For in stance, isotropic crystals may be made to yield uniaxial or biaxial phenomena according to the direction in which the pressure is applied.