74 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



Mauy other illustrations of the Lake Superior copper will be found 

 in the ore series. 



CHALCOPYKITE. (COPPER PYRITE.) 



Composition : CuFeS^. Sulphide of copper and iron, containing 

 copper 34.6 per cent., iron 30.5 per cent., and sulphur 34.9 per cent. 



It crystallizes in the tetragonal system, showing- especially octahe- 

 dral forms, but crystals are not common. Its most general occurrence 

 is massive. It varies in hardness from 3.5 to 4, and in specific gravity 

 from 4.1 to 4.3. It has a brass-yellow color and full metallic luster, 

 but is inclined to tarnish and sometimes becomes iridescent. 



Chalcopyrite is by far the most common and abundant copper min- 

 eral, being in a large majority of cases the chief product of a mine and 

 in many instances the only copper mineral found below water level. It 

 is therefore regarded by many as being the original source of nearly 

 all the copper. It furnishes by far the greater part of the copper 

 produced. It is always associated with iron pyrite, and in many cases 

 so intimately tnat it is impossible to separate the two minerals; in 

 such cases the color becomes paler with the decrease of the amount 

 of copper present, until it closely approaches the color of irou pyrite. 

 On exposure to air and moisture it is oxydized to sulphates, which may 

 be removed by solution in water, or may undergo further alteration to 

 the various oxydized copper minerals. It also occurs altered to other 

 sulphides of copper, or copper and iron, especially above the water 

 level. To illustrate the occurrence of chalcopyrite three spciemens are 

 shown: 



(1) Crystallized, a mass of skeleton crystals showing the general octahedral hahit 

 on solid chalcopyrite. French Creek. Chester County, Pennsylvania. (65144.) 



(2) Massive, showing the characteristic yellow color, somewhat darkened by tar- 

 nish. Mineral Hill Mine, Carroll County, Maryland. (17303.) 



(3) With quartz, shows the characteristic iridescent tarnish. Cornwall Mine Ste 

 Genevieve, Missouri. (66429.) ' 



BORNITE. (ERUBESCITE, PURPLE OR HORSE-FLESH COPPER ORE.) 



Composition : Fe CU3S3. Sulphide of copper and iron, containing 

 copper 55.58 per cent., iron 1G.36 per cent., and sulphur 28.06 per cent. 

 The relationship between the copper and iron varies, and sometimes 

 there is an excessive amount of sulphur. 



It crystallizes in the isometric system, but crystals are rare. It gen- 

 erally occurs massive and disseminated. Its hardness is 3 and specific 

 gravity 4.4 to 5.5. The color is reddish -brown, but it soon tarnishes on 

 exposure to air and light and is frequently coated on the outside with 

 carbonate of copper. The luster is bright metallic. 



Bornite is a valuable ore of copper and is frequently found with the 

 other sulphides of copper. lu certain levels it is the chief product 

 of some mines. The following specimen illustrates the occurence of 

 bornite. 



(I) Showing the characteristic tarnish. Gagnon Mine, Butte, Silver Bow County 

 Montana. (66431.) '^' 



