SYSTEMS OF CRYSTALLIZATION. 



29 



Occasionally, as in the diamond, the planes of crystals art 

 convex ; and then, of course, the angles will differ from the 

 true angle. It is important, in order to meet the difficulties in 

 the way of recognizing isometric crystals, to have clearly in the 

 mind the precise aspect of an equilateral triangle, which is the 

 shape of a face of an octahedron ; the form of the rhombic face 

 of the dodecahedron ; and the form of the trapezoidal face of a 

 trapezohedron. With these distinctly remembered, isometric 

 crystalline forms that are much obscured by distortion, or which 

 show only two or tlnee planes of the whole number, will often 

 be easily recognized. 



Crystals in this system, as well as in the others, often have 

 their faces striated, or else rough with points. This is gener- 

 ally owing to a tendency in the forming crystal to make two 

 different planes at the same time, or rather an 

 oscillation between the condition necessary for 

 m aking one plane and that for making another. 

 Fig. 63 represents a cube of pyrite w T ith stri- 

 ated faces. As the faces of a cube are equal, 

 the striations are alike on all. It will be noted 

 that the striations of adjoining faces are at right 

 angles to one another. The little ridges of the 

 striated surfaces are made up of planes of the pentagonal dode- 

 cahedron (fig. 49, p. 26), and they arise from an oscillation in 

 the crystallizing conditions between that which, if acting alone, 

 would make a cube, and that which would make this hemihe- 

 dral dodecahedron. Again, in magnetite, oscillations between the 

 octahedron and dodecahedron produce the striations in fig. 61. 



65. 



MAGNETITE. 



COMMON SALT. 



Octahedral crystals of fluorite often occur with the faces 

 made up of evenly projecting solid angles of a cube, giving 



