wherry: native element minerals 449 



classification 



In Dana's System the native elements are first split up into 

 three divisions, non-metals, semi-metals, and metals. The first 

 two divisions, however, show no pronounced differences, and 

 various inconsistencies arise in apportioning the elements between 

 them. For instance, graphite, which is classed as a non-metal, 

 has a more metallic luster and is a better conductor of electricity 

 than arsenic, which is classed as a semi-metal. Selenium is re- 

 garded as a non-metal when monoclinic, and as a semi-metal 

 when rhombohedral ; and the same would have to be done with 

 sulfur, arsenic, and antimony, were all of their forms included, 

 for these three crystallize in one or more forms with so-called 

 non-metallic, and others with so-called semi-metallic properties. 

 But the illogical character of this arrangement, which assumes 

 that the element changes its fundamental nature on appearing 

 in an unusual crystallized condition, is evident. The distinc- 

 tion between the non- and semi-metals appears, then, to be 

 rather artificial, and both convenience and accuracy suggest that 

 it be abandoned. 



The major divisions are in turn subdivided into groups. 

 Throughout the greater part of Dana's System, the groups are 

 composed of minerals of close chemical and crystallographic re- 

 lationships. Among the elements, however, this arrangement is 

 to some extent departed from, resulting in the establishment of 

 a " carbon group," in which crystallogi-aphic similarity is lacking, 

 and a "platinum group," which includes two types of crystal- 

 lization. As no good reason for this departure appears to exist, 

 it is recommended that these groups be placed on a crystallo- 

 graphic basis. The carbon group then yields an isometric diamond 

 group, into which several recently discovered non-metallic ele- 

 ments fall, and a hexagonal-trigonal graphite group, which may 

 for simplicity be united with the crystallographically identical 

 arsenic group; in graphite c = 1.386, which lies between the 

 value for arsenic, 1.401, and that for antimony, 1.324; all three 

 elements have basal cleavage. 



Similarly, the platinum group may be split into an isometric 

 and a hexagonal-trigonal group, while tin, which crystallizes in 



