Washington — Copper Crystals in Aventurine Class. 415 



As will be seen presently twinned forms are common among 

 the smaller crystals. 



The copper crystals making up the second group are smaller 

 than the preceding, varying from 0'01-O03 mm in diameter, and 

 are not only quite different in habit but much more diverse in 

 crystalline form. They may be divided into three distinct 

 types, each being about equally abundant ; cubo-octahedral 

 forms, octahedra, and twins. 



To the first of these generally belong the largest individuals 

 and their form is that of the combination of the cube and 

 octahedron, with both about equally developed, or more rarely 

 the combination of the dodecahedron with these, the indi- 

 viduals being evenly developed and showing no signs of distor- 

 tion or flattening. In addition to these an occasional trisocta- 

 hedral plane with other indeterminate forms are to be seen as 

 replacements of the angles of the combinations above. The 

 symbols of the trisoctahedra could not be determined by 

 measurement, but they are tetragonal and are presumably 211 

 and 311. These crystals though, rarely show sharp edges, the 

 forms being much rounded and only the octahedral and cubic 

 planes remaining flat in most cases; this rounding being occa- 

 sionally carried so far that the crystals assume an almost 

 spherical shape. The crystals of this type are occasionally 

 partially surrounded by irregular trichitic growths. 



The crystals of the second type of this group are rather 

 smaller than the preceding, varying in diameter from 0*01- 

 0"02 mm . They are in almost every case perfectly sharp and 

 ideally symmetrical octahedra, cubic and dodecahedral replace- 

 ments of the angles and edges being rare, and when present 

 very small. 



The third type, that of twinned forms, while of about the 

 same size as those just described offer much more variety. The 

 twinning plane is in every case the usual one, an octahedral 

 face, and the twins are either simple or repeated. The simple 

 forms are composed of trigonal cubic twins (fig. 9), and octa- 

 hedral twins with cubic planes on the angles (fig. 10), both of 

 which are much flattened parallel to the twinning plane. 

 These forms are identical with some described and figured by 

 Dana in the paper already referred to.* 



The polysynthetic twins are either fourlings or fivelings 

 (figs. 11 and 12), formed of four or five octahedra grouped 

 about a common center, the first showing a reentrant angle at 

 one side, and the last forming almost symmetrical pentagonal 

 bi-pyramids, since the octahedral angle (70° 32') is almost one- 



*Loc. cit., pp. 424 and 426, PI. 11, fig. 28, PI. 12, fig. 41. 



