GUIDE TO THE STUDY OF THE COLLECTIONS IN THE 

 SECTION OF APPLIED GEOLOGY. 



THE NONMETALLIC MINERALS. 



By GEORGE P. MERRILL, 

 Curator, Division of Physical and Chemical Geology and Head Curator of the Department. 



I. ELEMENTS. 

 1. CARBON. 



The numerous compounds of which carbon forms the chief constit- 

 uent are widely variable in their physical properties and origin. As 

 occurring in nature few of its members possess a definite chemical 

 composition such as would constitute a true mineral species, and they 

 must for the most part be looked upon as indefinite admixtures in 

 which carbon, hydrogen, and oxygen play the more important roles. 

 For present purposes the entire group may be best considered under 

 the heads of (1) The Pure Carbon series; (2) The Coal series, and (3) 

 The Bitumen series, the distinctions being based mainly on the gradu- 

 ally increasing amounts of volatile hydrocarbons, a change which is 

 accompanied by a variation in physical condition from the hardest of 

 known substances through plastic and liquid to gaseous forms. Here 

 will be considered only the members of the pure carbon series, the 

 others being discussed under the head of hydrocarbon compounds. 



DIAMOND. This mineral crystallizes in the isometric system, with 

 a tendency toward octahedral forms, the crystals showing curved and 

 striated surfaces. (Specimen No. 53558, U.S.N.M.) The hardness is 

 great, 10 of Dana's scale; the specific gravity varies from 3.1 in the 

 carbonados to 3.5 in good clear crystals. The luster is adamantine; 

 the colors, white or colorless, through yellow, red, orange, green, 

 brown to black. The transparent and highly refractive forms are of 

 value as gems, and can best be discussed in works upon this subject. 

 We have to do here rather with the rough, confused crystalline aggre- 

 gates or rounded forms, translucent to opaque, which, though of 

 no value as gems, are of the greatest utility in the arts. To such 



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