488 wherry: sulfide minerals 



their end-members, if these are well known as individual species. 



Classification. — With the sulfides proper are ranged all other 

 compounds of analogous character, comprising not only the 

 selenides, tellurides, arsenides, antimonides and bismuthides, 

 usually so treated, but also the oxysulfides, nitrides, phosphides, 

 carbides, and silicides, which are not as a rule assigned any definite 

 status. These are separated first on the basis of the metallic or 

 non-metallic character of the more basic element concerned, and 

 next into chemical divisions, depending on the ratios of the basic 

 to the acidic elements present. The divisions are finally sub- 

 divided into groups on a crystallographic basis, as was done with 

 the elements. As before, the order in which groups are taken 

 up is that of decreasing crystallographic symmetry (trigonal, 

 however, preceding tetragonal), the final group in most divisions 

 including amorphous, colloidal, and cryptocrystalline, meta-col- 

 loidal, members, together with those of which the crystallization 

 is as yet unknown; and the order of the individual minerals 

 within the groups is based on the positions of the constituent 

 elements in the Periodic System. 



While there is nothing particularly novel about these rules, 

 they do not appear to have been applied consistently heretofore. 

 In the present work exceptions are admitted only for especially 

 urgent reasons, and the tabulation, which follows, is accordingly 

 uniform and systematic to an unusual degree. Discussion lead- 

 ing to its further improvement is, however, invited. 



The first column of the table contains the names of the species 

 and varieties recognized, worked out in accordance with the 

 above principles. No new names are proposed in this paper, 

 although a few old ones are redefined. The second column con- 

 tains the composition of each; isomorphous replacement^ is repre- 



* Isomorphism was formerly considered to exist between single bivalent elements 

 and pairs of univalent ones (for instance, Pb and Ag2) but recent studies have 

 shown supposed instances of this to be mixtures. Only isomorphous replacement 

 of elements of like valence is here admitted. It may also be noted that the 

 argentite and galena groups are here widely separated, since their structures must 

 be entirely different. 



