By these various methods it has been possible in many cases, particularly of the simpler compounds, to definitely determine the crystal structure. How to determine crystal structures is an example for the case of halite may be cited. We now know that the sodium and chlorine atoms are arranged in the crystal net-work as shown in Fig. 2. We also know with precision the distances between the atomic layers of its crystals.
The Outward Crystal Form May be Varied with the Same Internal Crystalline Structure. There may be several different limiting forms possible upon crystals of the same mineral. Galena, PbS, for example, usually crystallizes in the form of a cube, but it also at times shows octahedral crystals. The internal structure of galena is constant, but both the cube and octahedron are forms that conform to that structure. The models shown on Plate I illustrate this point. Both are built up of similar particles and their arrangement is the same in each case. In one, however, the planes of a cube, and in the other planes of an octahedron, limit the figure.
With the same internal structure there are, however, only a certain number of possible planes which can serve to limit a crystal. And it is to be noted, moreover, that of these possible planes there are only a comparatively of a crystal are determined by those directions in which on account of the internal structure a large number of the individual mineral units lie. And those planes which include the greater number of units are the ones most commonly found as faces upon the crystals. Consider Fig. 3, which might represent one layer of units in a certain crystal network. These units are equally spaced from each other and have a rectilinear arrangement. It will be observed that there are several possible lines through this network that include a greater or less number of units. These lines would represent the cutting direction through this network of certain possible crystal planes; and it would be found that of these possible planes those which include the larger number of units, like those cutting along the lines A-B and A-C, would be the more common in occurrence.