546 Kraus mid Goldsberry — Chemical Composition of 



respect to the various members of this series, but from what is 

 known the crystallization of the various members may be inter- 

 preted as being either cubic or pseudo-cubic. The members, 

 commonly called bornite, from Cn 6 Fe 2 S 6 to Cu ]2 Fe 2 S„ are known 

 to belong to the cubic system. Chalcopyrite, Cu 2 Fe 2 S 4 , is tetra- 

 gonal, but pseudo-cubic, as is clearly indicated by its elements 

 of crystallization : a : c = 1 : 0*9856. Chalcocite, Cu„S, is 

 recognized as orthorhombic, a : b : c = 0*5822 : 1 : 0*9702, 

 being the axial ratio given. If, however, the length of the a 

 axis be doubled, and this does not cause serious difficulty with 

 the resulting indices, we obtain a : b : c = 1*1644 : 0*9702, 

 clearly indicating a pseudo-cubic character. Generally chal- 

 cocite is interpreted as pseudo-hexagonal on account of the 

 value of the prism angle being 60°25 / . It should, however, be 

 pointed out that the rhombic dodecahedron of the cubic system 

 also possesses angles of 60° and may accordingly be held so as 

 to show an hexagonal development. In fact, beginners in 

 crystallography often make this mistake when endeavoring to 

 orient the rhombic dodecahedron. Furthermore, Groth* 

 points out that Cu 2 S may also be cubic. According to him the 

 cubic modification is formed at high temperatures and on cool- 

 ing passes over into the orthorhombic. 



It, therefore, appears that from the crystallographic stand- 

 point the relationships between the various members of this 

 series are in accord with the concejjtion of a regular progres- 

 sion in the chemical composition. In order to test this, the 

 meager data concerning specific gravities and crystallization of 

 the members of the series have been made use of in calculating 

 the topical axes. In doing this barnhardtite has been assumed 

 to be cubic. The specific gravity of chalcocite is that of some 

 crystallized material from Cornwall, England, and was recently 

 determined in this laboratory. The elements of crystallization 

 used for chalcopyrite and chalcocite are those given above. 



In determining the values of M, the molecular weight, and 

 Y, the equivalent volume, Fe 2 S 3 and Cu 2 S were interpreted as 

 the end members of the series, and the various intermediate 

 members as transition products. Hence, if the regular mole- 

 cular weights be assigned to Fe„S, and Cu„S, those for chal- 



O C5 2d 2 7 



co pyrite, barnhardtite, and so forth, are to be obtained by assum- 

 ing the composition of chalcopyrite, Cu 2 Fe 2 S 4 , to be Cu 2 S.Fe 2 S 3 , 

 that is the molecular weight of chalcopyrite may be placed 

 equal to the sum of one-half of the molecular weights of each 

 of the end members, namely 183*56. In a similar manner the 

 weights of the other intermediate members of the series were 

 obtained. 



* Cheraisclie Krystallograpliie, i, 135, 1906. 



