Crystallographic Mineralogy and Crystallography

The great majority of our minerals, when the conditions of formation are favorable, occur in defined and characteristic geometrical forms which are known as crystals. to gain a comprehensive knowledge of the laws which govern the shape and character o crystals is a very important part o the study o mineralogy. This division of he subject is called crystallography. it forms almost a separate science in itself, and to adequately and exhaustively discuss it would require a volume much larger than the present one in the following section, however, the attempt will be made to present the elements o crystallography in a brief and simple manner and at least to introduce the reader to the more essential facts and principles of the subject.

A crystal has been defined as follows: A crystal is a body which by the operation o molecular affinity has assumed a definite internal structure with the form o a regular solid enclosed by a certain number of plane surfaces arranged according to the laws of symmetry. This is a very compact definition and several pages will be devoted to its discussion.

A better idea of the fundamental laws of crystallography will be obtained by first considering the three prominent modes of crystallization. Crystals are formed by crystallization either (1) from a solution, is the most familiar to our ordinary experience. take for example a water solution containing sodium chloride (common salt). Suppose that by containing sodium chloride (common salt). Suppose that by evaporation the water is slowly driven off. The solution will, under these conditions, gradually contain more and more salt per unit volume, and ultimately the point will be reached where the amount o water present can no longer hold all of the salt in solution, and this must begin to precipitate out. In other words, part of the sodium chloride, which has up to this point been held in a of the sodium chloride, which has up to this point been held a state o solution by the water, now assumes a solid form if the conditions are so arranged that the evaporation of the water goes on very slowly, the separation of the salt in solid form will progress equally slowly and definite crystals will result. The particles of sodium chloride as they separate from the solution will by the laws of molecular attraction group themselves together and gradually build up a definitely shaped solid which we call a crystal. Crystals can also be formed from solution by lowering the temperature or pressure of the solution. Hot water will dissolve much more salt, for instance, than cold, and if a hot solution is allowed to cool a point will be reached where the solution becomes supersaturated for its temperature and salt will crystallize out. Again, the higher the pressure to which water is subjected the more salt it can hold in solution. So with the lowering of the pressure of a saturated solution super saturation will result and crystals form. Therefore, in general, crystals may form from a solution by the evaporation of the solvent, by the lowering of the temperature or by a decrease in pressure.