284 ECONOMIC GEOLOGY 



the Vulcano and Lipari Islands, also at Kilauea in the Hawaiian 

 Islands. Selen-tellurium occurs with a gangue of quartz and 

 barite in the silver veins of El Plomo, Honduras. Zorgite occurs 

 in argillaceous schist with galenite and various copper minerals 

 in Thuringia. 



Character of Ore Bodies. Selenium minerals appear in metal- 

 liferous veins with the commoner gangue minerals, and as crusts 

 from volcanic emanation. 



Geographical Distribution. Selenium minerals are rare and 

 not widely distributed. In America they are largely confined 

 to the Cordilleran section. Selenides appear in association 

 with the gold ores of the Camp Bird mine near Ouray, Colorado; 

 in the Tonopah gold ores, Nevada; near Marysville, Utah; and 

 at Clear Lake, California; in the New Zealand gold fields; in 

 Japan ; and in the Lipari Islands. 



Geological Horizon. The selenium minerals are more abun- 

 dant in the terranes associated with the later intrusives of the 

 Cretaceous and Tertiary ages than in the older rocks: 



Method of Extraction. Pyrite containing small quantities of 

 selenium is often used in the manufacture of H 2 SO 4 . In the 

 roasting of the pyrite, the selenium is oxidized to SeO 2 , and is 

 carried off with the sulphur, which is oxidized to S0 2 . The selen- 

 ium dioxide is deposited as a solid partly in the flues and partly 

 in the chambers. These deposits are gathered and boiled with 

 dilute H 2 SO 4 and HN0 3 or KC1O 3 to oxidize the substance 

 completely to H 2 SeC>4. Strong HC1 reduces the selenic acid to 

 selenous acid, H 2 Se0 3 . Then SO 2 passed through the selenous 

 acid precipitates the selenium as a red powder, and the SO 2 is 

 oxidized to H 2 SO 4 . 



Uses. In the light, selenium is a good conductor of electricity, 

 and on account of this peculiarity, it is used in a number of 

 electrical devices. It has been used in telephoning along a ray 

 of light, and in transmitting pictures, photographs, or even 

 sounds to a considerable distance by means of a telephone or 

 telegraph wire. It is used to light and extinguish gas-buoys 

 automatically. This use is dependent upon the fact that selenium 

 is a non-conductor of electricity in the dark and a good conductor 

 in the light. Selenium is used also in measuring the quantity 

 of Rontgen rays in therapeutiapc plications. 



Economics. The production of selenium from year to year 

 is very small. It is sometimes recovered from the anode 



