214 REPOKT OF NATIONAL MUSEUM, 1899. 



brown varieties are also known. The mineral is translucent to trans- 

 parent, and of a hardness somewhat greater than calcite (4 of Dana's 

 scale). 



Occurrence. The mineral occurs as a rule in veins, though some- 

 times in beds in gneiss, the schists, limestones, and sandstones. It is 

 also a common gangue of metallic ores, particularly those of lead 

 and tin. 



At Rosiclare, in southern Illinois, the fluorspar veins, according to 

 Emmons, 1 are true fissure veins, varying from 4 to 20 feet in width in 

 limestones immediately underlying the coal measures. He regards 

 the original crevice as formed by dynamic action, as probably com- 

 paratively small and subsequently enlarged by solution by percolat- 

 ing waters. The source of the fluorspar of the veins would seem to 

 be the surrounding limestones. 



The associated minerals are galena and calcite, with smaller quanti- 

 ties of sphalerite and iron and copper pyrites. 



Uses. The material is used mainly as a flux for iron, in the manu- 

 facture of opalescent glass and for the production of hydrofluoric 

 acid. The chief source of supply in the United States is Rosiclare, 

 Illinois, the annual output being some 6,000 to 10,000 tons, valued at 

 about $5 a ton. 



3. CRYOLITE. 



Composition. Na 3 AlF 6 ,= aluminum 12.8 per cent; sodium 32.8 per 

 cent; fluorine 54.4 per cent. The mineral is as a rule of snow-white 

 color, though sometimes reddish or brownish, rarely black, and 

 coarsely crystalline granular, translucent to subtransparent. It has 

 a hardness of 2.5; specific gravity of 2.9 to 3, and in thin splinters 

 may be melted in the flame of a candle. 



The name is from the Greek word /c-p^o?, ice, in allusion to its trans- 

 lucency and ice-like appearance (Specimen No. 17571, U.S.N.M.). 



Mode of occurrence. Cryolite occurs, as a secondary product, in the 

 form of veins. It is rarely found in sufficient abundance to be of 

 commercial value, the supply at present coming almost wholly from 

 Evigtok in South Greenland. The country rock here is said to be 

 granite and the vein as described in 1866 2 was 150 feet in greatest 

 breadth and was exposed for a distance of 600 feet. The principal 

 mineral of the vein was cryolite, but quartz, siderite, galena, and chal- 

 copyrite were constant accompaniments, irregularly distributed 

 through the mass. In 1890 the mine as worked was described as a 

 hole in the ground elliptical in shape, 450 feet long by 150 feet wide, 

 the pit being some 100 feet deep. The drills had penetrated 150 feet 



transactions of the American Institute of Mining Engineers, XXI, 1893, p. 31. 

 2 Paul Quale, Report of Smithsonian Institution, 1866, p. 398. 



