190 REPORT OF NATIONAL MUSEUM, 1899. 



6. PYRITES. 



Two forms of the disulphide of iron are common in nature. The 

 first, known simply as pyrite or iron pyrites, occurs in sharply denned 

 cubes and their crystallographic modifications (Specimen No. 51740, 

 U.S.N.M.). or in granular masses of a brassy -yellow color (Specimen 

 No. 62152, 'u.S.N.M.). 



The second, identical in composition, crystallizes in the othorhombic 

 system (Specimens Nos. 17124, 55206, and 73613,U.S.N.M.), but is more 

 common in concretionary (Specimen No. 62976, U.S.N.M.), botryoidal 

 (Specimen No. 30772, U.S.N.M.), and stalactitic (Specimens Nos. 62800 

 and 67761, U.S.N.M.) forms, which are of a dull grayish-yellow color. 

 This form is known as the gray iron pyrites. Both forms have the 

 chemical composition, FeS 2? =iron 46.6 per cent and sulphur 53.4 per 

 cent. 



The ore as mined is. however, never chemically pure, but contains 

 admixtures of other metallic sulphides, besides, at times, considerable 

 quantities of the precious metals. The following analyses l of materials 

 from well-known sources will serve to show the general variation: 



I. Milan, Coos County, New Hampshire; II. Ro we, Massachusetts; 

 III. Louisa County. Virginia; IV. Sherbrooke, Canada; V. Rio Tinto, 

 Spain; VI. near Lyons, France: VII. Westphalia, Germany. 



Pyrite is sufficiently hard to scratch glass, and this, together with its 

 color, crystalline form, and irregular fracture, is sufficient for its ready 

 determination in most cases. Once known, it is thereafter readily rec- 

 ognized. Owing to its yellow color, the mineral has by ignorant per- 

 sons been mistaken not infrequently for gold which, however, it does 

 not at all resemble and has hence earned the not very flattering but 

 quite appropriate name of fool's gold." In certain cases, however, 

 it carries the precious metals, and in many regions is sufficiently rich 

 in gold to form a valuable ore. 



Jfode of occurrence. Pyrite is one of the most widely disseminated 

 of minerals, both geologically and geographically, occurring in rocks of 

 all kinds and of all ages the world over. It is found in the form of 



1 Mineral Resources of the United States, 1883-1884, p. 877. 



