114 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



It crystallizes in the rhombohedral system, showing mostly planes 

 of rhombohedrons; crystals, are usually curved. Its most common oc- 

 currence is iu crystalline masses, showing the characteristic rhombohe- 

 dral cleavage with curved faces. It frequently occurs granular. It 

 varies in hardness from 3.5 to 4.5, and in specific gravity from 3.7 to 

 to 3.9, is generally gray to brown in color, and has a vitreous luster. 



Siderite is the chief constituent of several varieties of iron ores, 

 which are generally more or less impure. To illustrate the occurrence 

 of siderite two specimens are shown : 



(1) Crystalline, massive, showing characteristic cleavage. Shepang Iron Com- 

 pany's mine, Roxbury, Litchfield County, Connecticut. (17966.) 



(2) Massive, fine granular. Muirkirk, Prince George County, Maryland. (653"^:^.) 



PYRITE. (MUNDIC.) 



Composition: FeS^. Bisulphide of iron, containing iron 46.7 per 

 cent., and sulphur 53.3 per cent. Occasionally other metals replace 

 some of the iron. 



It crystallizes in the isometric system, forming very beautiful crystals. 

 The most common form is the cube, while the pyritohedron and related 

 forms are quite common, and combinations of these two forms are fre- 

 quent. It also occurs in imitative forms with crystalline structure. 

 Much of it occurs massive. 



It varies in hardness from 6 to 6.5, and iu specific gravity from 4.83 

 to 5.2. It h^s a full metallic luster, and pale brass-yellow color. On 

 being struck with a hard substance, like steel, it throws oft" sparks, 

 whence the name. 



Pyrite is never used directly as a source of iron, and its presence in 

 any large amounts in iron ores is a great injury, as it causes the metal 

 to be weak at high temperature or "red short." Small amounts of it 

 are, however, common in nearly all iron ores, and considerable amounts 

 can be removed by a suitable roasting operation. 



Considerable amounts of it are used in the manufacture of sulphuric 

 acid, and after the small amount of copper frequently present has been 

 extracted, the remaining, nearly pure oxide of iron, known as "blue- 

 billy," is sometimes used in the puddling process of making wrought 

 iron. To illustrate the occurrence of pyrite the following specimen is 

 shown here. Others will be found in the sulphur collection. 



(I) Crystallized, on magnetite. Annie Mine, Lake County, Colorado. (41885.) 



Iron Ores. 



The ores of iron are classified in several different ways, the most im- 

 portant being according to the chief mineral they contain, as magnet- 

 ite, hematite, limonite, and carbonate ores. With the exception of 

 magnetite, each one of these varieties is frequently subdivided accord- 

 ing to some peculiar pliysieal character; thus under hematite Ave hav«' 

 specular, micaceous, fossil; under limonite we have bog, needle, pii)e ; 

 under siderite we have clay ironstone, and blackband ore. Many 



