SULFUR 



607 



Figure 67. — Total sulfur production and production, imports, and exports of elemental sulfur of the United States, 1900- 



70. Data from U.S. Bureau of Mines. 



sulfur and faced increasing competition for its do- 

 mestic markets from Mexican and Canadian suppli- 

 ers. By 1968, U.S. exports and imports were about in 

 balance. 



The present excess of free-world productive ca- 

 pacity probably will be balanced in about a decade by 

 the steady rise in the demand for sulfur. During this 

 period, however, competition for markets will be 

 intense and sulfur prices will be low. After a balance 

 is achieved, sulfur will be recovered in larger 

 amounts from sources now classed as subeconomic, 

 including perhaps those in coal, gypsum, tar sands, 

 and oil shale. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



The average sulfur content of igneous rocks is 520 

 ppm (parts per million), 13th in order of abundance 

 among all elements. In molten rock, sulfur occurs as 

 the sulfide ion and combines most readily with metal- 



lic ions, but it also combines with available hydrogen 

 or oxygen. Sedimentary rocks contain larger 

 amounts of sulfur: sandstone averages 2,000 ppm, 

 shale 2,600 ppm, and limestone 1,100 ppm. In the 

 presence of abundant oxygen, the sulfide ion is oxi- 

 dized to the sulfate ion, in which form it is contained 

 in the hydrosphere. Sulfur is necessary in trace 

 amounts to all plant and animal life and is constant- 

 ly being absorbed and released during biologic 

 cycles. Sulfur thus circulates in both inorganic and 

 organic cycles and is both ubiquitous and mobile in 

 all parts of the lithosphere, hydrosphere, atmos- 

 phere, and biosphere (Rankama and Sahama, 1950). 



THE INORGANIC CYCLE 



Sulfur is brought into the earth's crust by molten 

 rock rising from the mantle and combines near the 

 surface with iron and other metallic elements to 

 form the common sulfide minerals or, if oxygen is 

 available, to form sulfates. Where molten rock 

 reaches the surface to erupt volcanically, its sulfur 



