162 



Field Columbian Museum — Geology, Vol. III. 



Fig. 5. Sphalerite. 



ber of planes. Moreover, the character of their planes is distinctive, 

 the tetrahedrons being always more or less rough from etching and pit- 

 ting and the cubical faces usually smooth and often more brilliant 



than the other planes. The re- 

 maining planes show an angular 

 measurement upon the cube and 

 tetrahedron corresponding to that 

 of planes of the hemitetragonal 

 tristetrahedron p (221), but the 

 full number of planes of this form 

 is never present. As a rule a 

 single plane of the form occurs in 

 three quadrants and two in the 

 fourth. One crystal, however, 

 exhibits two planes of the form in 

 each quadrant. It is of interest 

 to note that in the pyrite de- 

 scribed by Penfield from French Creek, Pennsylvania * a somewhat 

 similar lack of planes occurs. Owing to the etched character of the 

 tetrahedral faces on the sphalerite it is probable that the tetrahedron 

 present is the positive one and the tristetrahedron is therefore 

 to be regarded as negative. Fig. 5 illustrates the development 

 exhibited by the majority of the crystals. The crystals with 

 rounded planes have as a whole more nearly the appearance of 

 the tetragonal tristetrahedron than those which are more fully 

 developed. The tristetrahedron may therefore be regarded as in a 

 sense the fundamental form which is modified in the more fully 

 developed crystals by the cube and tetrahedron. A list of the forms 

 and angles follows: 



a (100) o (111) p (221) 



Observed Calculated 



a a o = (100) a (111) = 55 12' (average of 11 measurements) 54 44' 

 a A ? = (loo) a (221) =48° 14' " " 15 " 48°n / 



a a p = (001) a (221) =70° 00' " " 6 70 31' 



* Am. Jour. Sci., 1889 (3). 37, p. 209. 



