﻿420 E. S. Dana — Crystallization of Native Copper. 



tion of a trigonal axis of the trisoctahedron (3-3), the other 

 planes of the trisoctahedron forming a pyramid £-2 and obtuse 

 rhombohedron \R. 



Frequent examples of this tendency have been observed 

 among the specimens of copper under examination, though 

 none so marked as that just mentioned. The following list 

 includes some of the common forms of the isometric system 

 with the symbols that belong to them when placed with an 

 octahedral axis (normal to 111) vertical, taking the cube with 

 a rhombohedral angle of 90° as the unit form. The vertical 



axis is then c=l*22474. 



Cube 

 Octahedron 



Dodecahedron 



Isometric. 

 a 0, 100, 



1, 111, 

 111, 



Rhombohedral. 

 lOll, R 



110, d 



101, d, 



211, p 



112, p, 



2H, P. 



311, m 



311, m, 



311, m„ 



Tetrahexahedrons i-2, 210, e 

 201, e. 



Trisoctahedrons 2-2, 



3-3, 



530, 

 503, 



502, 



a-4 410, 



401, 



520, k 



k, 



0001, 

 0221, 



0112, 

 1120, 



1014, 

 1010, 

 1232, 



2025, 

 2243, 

 4041, 



1123, 

 2131, 



2358, 



5382, 



3257, 



5273, 



3145, 



4153, 



O 

 -2R 



-\R 



i-2 



>R 



t-2 

 ±R 



3 ~ & 3 



34 or 1 



4-f 1 



6 



7-3 or t 

 44 i" 



1* 



Among the specimens of native copper two showed rather 

 distinctly the acute rhombohedral form (-2R) of fig. 17. The 

 rhombohedral angle is here 70° 32' and the plane facial angle 

 60° at the summit. The basal plane which is needed to fill 

 out the eight faces of the normal octahedron was not distinctly 

 present, and in fact the crystals were all lacking in sharpness, 

 though there seems to be no doubt about the interpretation of 

 the form. 



Another specimen (6g. 18) showed a long hexagonal prism 

 i-2{d : ) terminated by an obtuse rhombohedron — ±R(d) 3 with 

 also subordinate planes of the rhombohedrons R(a), — 2R(o / ); 

 in a normal isometric crystal this would be simply a combina- 

 tion of dodecahedron, cube and octahedron. A basal projection 

 (fig. 23) shows the rhombohedral symmetry very clearly. The 

 tetrahexahedron e (210, i-2) is frequently developed according 



