Rock-forming minerals

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1888 edition. Excerpt: ... and repulsions which the molecules of the crystals exercise upon one another; while, in the latter, forces external to the crystal are brought to bear upon it, and their action is only modified by the resistance which the molecular spacing of the crystal (i.e., the repulsions exercised between its molecules) and the relative inclination of its developed faces to the directions of compression may be able to offer. In such a case, therefore, the results of compression may be very unequal and irregular; and the same may be said in instances where contraction instead of compression has troubled a crystal during or after its birth, especially if the latter have occurred at a high temperature, since it may be readily imagined that the unequal thermal conductivity along different directions in the different minerals which frequently surround a rock-forming crystal, would more or less affect the readiness with which that crystal will cool in given directions afc a given time. Under such circumstances, a crystal may undergo irregular changes in its molecular spacing sufficient to produce an appreciable change in its angles, so that a cubic crystal may ultimately resemble one of lower symmetry. In the hexagonal system, as in all the others of lower symmetry than that of the cubic, the crystals are anisotropic. Those of the hexagonal system are optically uniaxial, as already mentioned. The optic axis coincides with, or is parallel to, the principal crystallographic axis; and it may also be the axis of greater or of lesser optical elasticity. In the former case, the double-refraction is negative; in the latter, positive. Crystals belonging to this system, where they possess colour, exhibit phenomena of dichroism. In convergent polarized light basal...