﻿378 W. G. Brown — Crystallographic Notes. 



The crystals were in all probability accidentally formed by 

 electrolysis, a method used by Grolcling Bird (and before him*), 

 who obtained by means of a modified Daniell cell crystals of 

 copper mixed with crystals of cuprite, a like result to that in 

 the present occurrence, but of the forms present he makes no 

 mention. Kossmannf mentions some electrolytic copper which 

 had the appearance of hexagonal crystals, but were in reality 

 twins, twinning plane the octahedral plane, and cutting each 

 other at angWof 60° and 120°. 



The crystals, found under the circumstances described in 

 this note, are in the form of a mass of stout stalactites, with a 

 drusy surface, about 30 mm long by 25 mm by Ih™* and weigh 

 nearly 20 grams. 



The upper part of the mass, that first deposited in contact 

 with the zinc, consists of spongy dendritic copper of a loose 

 filamentous texture, of a generally dull chocolate-red color 

 with a metallic sheen. This portion possesses little coherence, 

 readily separates into threads, and is very friable. 



Forming a concentric layer round this upper portion are 

 crystals of cuprous oxide (cuprite) mixed with distinctly crys- 

 tallized compact copper, which forms the outer layer. 



The lower and larger portion of the mass consists of definite 

 copper crystals possessing its ordinary color and luster. The 

 forms of the crystal facets, these not exceeding -2 mm at their 

 greatest breadth, cannot be made out without the use of a mag- 

 1 nifying glass. When examined in this way the forms present 

 are seen to be those of the octahedron and cube ; usually the 

 octahedron alone, but at times with its solid angles replaced by 

 cubic planes. Rarely the cube with the solid angles replaced 

 by the faces of the octahedron elongated into an apparently 

 square prism. Doubtful is the octahedron as a right rhombic 

 prism terminated by a dome, and the rhombic dodecahedron 

 appearing as a hexagonal prism terminated by a rhombohe- 

 dron. With these are other forms, apparently those of a hex- 

 agonal pyramid, due, however, to twinning. By far the greater 

 number of crystals are twins, either according to the spinel 

 law, twinning plane an octahedral face, or the twinning plane 

 that of a trapt-zohedron forming polysynthetic twins. The 

 most frequent form is one which appears to be a combination 

 of a pentagonal dodecahedron and octahedron, as in pyrite, but 

 this is probably a twin composed of five individuals, the form 

 being that figured by A. von Lasaulx in his paper, " Ueber 

 Zwillingskrystalle von gediegen Kupfer."^: In many of these 

 crystals four of the components are nearly equally developed, 



* See his paper, Phil. Trans., 1S3Y, p. 37. 



f Abstract in Chemisches Centralblatt, xvii, 432, 1886. 



% Zeitscbrift fiir Krystallograpbie und Mineralogie, viii, 302, fig. 6. 



