﻿422 E. 8. Dana — Crystallization of Native Copper. 



appears as a nearly symmetrical cruciform twin, while looking 

 down upon the central octahedral face (as in fig. 26) the habit 

 is rhombohedral. It appears at first sight to be a compound 

 twin but the three octahedral faces in twinning position may 

 all be referred to the same individual ; the symmetry of the 

 crystal is nearly equal to that of the drawing. 



Figure 22 shows a simple contact twin of the cubo-tetrahexa- 

 hedron with the usual re-entering angles; this is similar to the 

 forms figured ,by Levy. Contact twins of simple habit and 

 normal development are very rare. Octahedral twins of the 

 spinel type have been noted only in a complex growth similar 

 to that represented in fig. 54, and described later. 



The spinel law applied to the cube yields the form repre- 

 sented in fig. 25. This is the common type of twins among the 

 Siberian specimens as noted by Rose. Among those from 

 Lake Superior, however, crystals of exactly this form are very 

 rare. Almost invariably, these cubic twins are shortened in 

 the direction of the twinning axis, thus yielding a form con- 

 sisting of two similar triangular pyramids placed base to base. 

 This form, noted by Sadebeck,* on the native silver from 

 Kongsberg, is represented in fig. 24, and again in fig. 27, the 

 latter in the orthorhombic position as mentioned below. These 

 triangular twin crystals have a most anomalous appearance 

 when they occur alone, suggesting a tetrahedron, or a hemi- 

 tetragonal trisoctahedron, at first sight. One specimen from 

 the cabinet of Professor Brush shows a broad surface of cop- 

 per thickly sprinkled with very minute forms (l mm to i mm ), 

 some of them cubes, others tetrahexahedrons ; and not a few 

 are these triangular twins, like fig. 24 or fig. 31, and usually 

 implanted by an acute trihedral angle. 



Cases are rare, however, in which these cubic forms appear 

 with this normal development. Generally they are elongated 

 in a direction of a diagonal of the octahedral twinning-face and 

 thus assume the symmetry of a (hemimorphic) orthorhombic 

 crystal. Figure 27 shows this twin in the position correspond- 

 ing to the latter symmetry. As this method of development 

 is so common it is interesting to note the symbols of the com- 

 mon isometric forms when so placed. Accepting as the funda- 

 mental or unit pyramid, two of the cubic faces (100 and 010), 

 the angles of this pyramid are 90° (brachydiagonal edge), and 

 70° 32 7 (front or macro-edge formed by twinning). The plane 

 angle at the summit is 45°. The orthorhombic axes are 

 accordingly: 



a:5:c = 0-8165: 1: 1-4142 



The direction of the vertical axis coincides with a. diagonal 

 of the octahedral twinning-plane, that of the brachydiagonal 

 * Min. petr. Mitth., i, 295, 1883. 



