﻿E. S* Dana — Crystallization of Native Copper. 421 



to rhombohedral symmetry, as was early noted by Haiiy (see 

 above, p. 413), Haidinger, Levy and others. As shown in the 

 above list it then corresponds to a hexagonal pyramid of the 



3 



second or diametral series (f-2) and a scalenohedron 1 . The 

 twelve planes of the latter form are often subordinate or nearly 

 absent, and the result is then the pyramid shown in fig. 21 ; it 

 is this pyramid which, as remarked before, so often covers the 

 surface of broad octahedral plates. In such a form as fig. 21 

 there is nothing to indicate whether we are dealing with a 

 simple crystal or a twin. If two opposite pairs of these planes 

 are extended the result is an orthorhombic prism terminated by 

 an obtuse pyramid. 



Various more or less complex rhombohedral forms occur 

 according as the planes just mentioned are modified by the 

 faces of the cube, dodecahedron or octahedron. Thus in fig. 

 20 we have the planes of three of these forms together; this 

 represents a simple crystal such as has been repeatedly 

 observed, although twins of similar habit also occur. 



One interesting crystal was observed which was a rhombohe- 

 dral penetration-twin of the tetrahexahedron z-f (530) and cube. 

 Of the faces of the first mentioned form the twelve, comprising 



5 



in the rhombohedral position the negative scalenohedron — j, 

 were prominent, those of the other scalenohedron indistinct; 

 the cubic faces were all present, six above ana six below. The 

 twinning plane was the basal (octahedral) plane. The other 

 tetrahexahedrons f-4 and t-f are also developed at times, more 

 or less distinctly, after the rhombohedral type, especially the 



2 



former. The scalenohedron § forms common six-sided eleva- 

 tions on the octahedral plates, as shown in fig. 14 already 

 alluded to. 



Twin crystals. — The specimens of native copper from Lake 

 Superior occur very frequently in twin crystals, although the 

 remark of Kose in regard to the Siberian specimens that simple 

 crystals are rare would not be true of those under examination. 

 The law of twinning is always the same — the twinning-plane a 

 face of the octahedron — but the variety and complexity of 

 these twinned crystals is truly remarkable. The twins are 

 with very rare exceptions contact twins ; a few penetration 

 twins have been observed, as also a few cases of repeated twin- 

 ning. 



A penetration-twin of rhombohedral type, with I (530, &~j[) 

 as the predominating form has been spoken of in a preceding 

 paragraph. Another interesting case is shown in fig. 26. The 

 predominating form is the tetrahexahedron &, though the allied 

 form h occurs subordinate, and the octahedron and dodecahe- 

 dron are both present. From one point of view the crystal 



