﻿E. 8. Dana — Crystallization of Gold. 133 



1. Gold from Oregon. 



The delicate crystalline threads and arborescent forms of 

 the gold from the White Bull mine in Oregon have long been 

 a prominent ornament of collections, especially in America, 

 but, so far as the writer is informed, no attempt has been made 

 to describe them. A brief sentence is devoted to the subject 

 in his Text-Book of Mineralogy, but the statement there is only 

 partially correct. 



The beauty and delicacy of these forms are perhaps unri- 

 valed in the species, but to decipher them crystallographically 

 does not seem at first to be especially easy. If examined 

 superficially the threads appear to be made up of acute rhom- 

 bohedral forms, closely crowded upon each other. The ter- 

 minal crystal, which is in many cases much the largest of the 

 series, usually shows also the presence of a six-sided pyramid 

 at the apex, a form often only faintly indicated in the other 

 crystals. In rare cases, where this terminal crystal is unusu- 

 ally well developed, close examination reveals also the presence 

 of three other minute planes forming an obtuse termination to 

 the hexagonal pyramid. The position of these planes at once 

 suggests an explanation of the form, and a few measurements 

 show that this explanation is the correct one. The planes 

 observed are all those of the tetragonal trisoctahedron 3-3 (311), 

 common in the species, and the peculiar appearance and appar- 

 ent rhombohedral symmetry are due to the fact that the crystals 

 are uniformly elongated in the direction of the octahedral or 

 trigonal axis. 



This is then simply a case of pseudo-symmetry, analogous to 

 that by which a regular octahedron may be transformed into a 

 rhombohedron with prominent basal plane, or into a simple 

 acute rhombohedron if these two planes are suppressed ; or 

 again, to that in which a rhombic dodecahedron becomes a 

 hexagonal prism terminated by rhombohedral planes ; or, still 

 again, like that which turns the trisoctahedron 2-2 (211) into a 

 combination of an obtuse rhomhohedron, a scalenohedron and 

 a hexagonal prism. These are cases that have been long 

 recognized. The case now described is remarkable for the 

 regularity of the resulting forms and the way in which they are 

 combined. This tendency in nature to thus imitate the sym- 

 metry of one system by the development of crystals belonging 

 to another system, is in part explained by the fact that the 

 planes whether referred to the one system or the other, have in 

 either case rational symbols. Thus if we place a cube with its 

 trigonal axis vertical, and regard it as the fundamental rhom- 

 bohedron with a rhombohedral angle of 90°, the planes of the 

 trisoctahedron 3-3 group themselves as follows (figs. 1, 2 and 3). 



