﻿426 E. S. Dana — Crystallization of Native Copper, 



given ; the rather close approximation to a tetragonal crystal 

 is obvious at a glance (o n d z — 90°). 



The same specimen showed other crystals perplexingly like 

 those just described, especially in that they were terminated by 

 a pair of planes looking very similar to the pair of cubic planes 

 in fig. 44. The measured angle between them was, however, 

 about 39° (38° 56') instead of 70° 32' (the angle between two 

 cubic faces in twinning position), and they were thus seen to 

 be octahedral faces in twinning position to each other. These 

 forms proved to be the complementary half of the crystal just 

 described, as is shown in fig. 45, which is a projection upon the 

 twinning plane ; here both parts, not in fact observed together 

 on the same crystal, are represented together. In fig. 45 the 

 octahedral edge is placed above, to correspond with the sym- 

 bols given on p. 423. The forms shown in figs. 43-45 are 

 spoken of further in a later paragraph, where the method of 

 grouping (see figs. 46 and 54) is described. The understanding 

 of this type of twin will be facilitated by reference to the simple 

 forms in figs. 39 to 42, all drawn with the twinning-plane par- 

 allel to the plane of the paper. Figure 39 is a simple octahe- 

 dral twin of the spinel type; fig. 40 the trigonal cubic twin 

 already fully described. In fig. 41 the octahedral twin is 

 shown, but much flattened and with the cubic faces on the 

 angles; this is also a copper form. Finally, fig. 42 is the 

 cubic twin with the octahedral faces. 



Still another type of twin is represented in fig. 47, which, 

 like those already described, very rarely shows a re-entrant 

 angle. The figure is a projection upon the twinning-plane. 

 The crystals of this type are elongated prisms, one edge formed 

 by two octahedral planes (o / ^ o_ t — 38° 56') and the other by the 



pseudo-planes # (# a # = 90° 54'). The figure is placed so as 



to show the prismatic development, but to understand it, it 

 must be turned so that the plane o x is parallel to the same plane 

 in fig. 45 ; then h and h IU correspond to the tetrahexahedron k 

 and Jc UI present there, etc. The pseudo-planes # and #' are 

 formed by the oscillatory combination of two adjacent tetra- 

 hexahedral planes h and h UI , etc., and are in fact only a series 

 of fine ridges. Were they real planes the symbol for # would 

 be 81 1 (S-8) in the isometric system, or 7'10*7 in the ortho- 

 rhombic position. Twins of this type are sometimes very reg- 

 ular and symmetrical and again highly irregular, and varying 

 much from the simple prismatic form. 



Methods of grouping. — The specimens of native copper very 

 commonly show, instead of an irregular combination of crys- 

 tals, a grouping in parallel position with sharply defined lines 

 of growth. Two methods of grouping are most common. In 



