CLEAVAGE. Many crystallized substances when sharply struck or subjected to pressure or heat or other disturbing forces split into fragments boimded by smooth plane surfaces which are patalbel to crystal faces of the sub stance.
If the separation can be obtained with equal ease in any part of any crystal and if there is only a mechanical limit to the thinness of the resulting plates this tendency to brealc parallel to crystal faces is called cleavage. If, however, the separation can be obtained only at irregular intervals, and only in some crystals, the character is called parung, and has a dif ferent origin.
Cleavage occurs with equal ease parallel to all corresponding crystal faces, thus, for in stance, a crystal of galena will cleave equally well parallel to all the faces bf the cube. Cleavage is due to differences in cohesion, and conforms strictly to the theory of a regular in ternal structure. (See CRYSTAL). The brealcing in one direction more easily than another im plies differences of cohesion in different direc tions, and the break is a plane because all lines perpendicular to it are directions of equal minimum cohesion, and finally the cleavage with equal ease parallel to corresponding faces implies equal cohesion at right angles to all of these.
A structure model shows not only that among the different sets of paralkl molecular planes those parallel to the common crystal faces contain the largest number of molecules but also that the consecutive planes of such a set are furthest apart from those of other sets, and being further apart there is weaker cohesion.
The ease of cleavage is not a measure of the absolute strength of cohesion but a proof of marked differences in cohesion in different directions. Diamond and fluorite illustrate this. The property is shown equally well by the polyhedral and the faceless crystal independent of the external form.
Cleavage is in general limited to directions parallel to symmetry planes or perpendicular to axes of symmetry or parallel to the faces of prominent crystal forms. It may occur in the same substance parallel to the faces of two or tnore crystal forms; for instance, gypsurn splits easily into plates parallel to the faces of one form and these plates again break parallel to the faces of two others.
Directions of cleavage are sometimes indi cated by a pearly lustre, due to repeated light reflections from cleavage rifts, or distinct cracks may occur. The absence of these is not proof that cleavage cannot be obtained, but only that the. separation has not been started.
Cleavage is usually obtained by resting the crystal on a firm support, placing the edge of a knife or small chisel parallel to the supposed direction of cleavage and striking a quick, sharp blow upon it with a hammer. In some instances the cleavage is only obtainable .by such methods as heating and suddenly plunging into cold water.
Cleavage is a valuable aid in classification and identification, the number of directions,. the relative ease and above all the characteristic angles leading to rapid and safe conclusions.