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Yao Linbo\ Gao Zhenmin\ Long Hongbo (Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002)  
This ppaper presents the physical and chemical properties of selenium, as well as its species, occurrence, geochemical cycle and enrichment. In the natural environment, four principal inorganic species exist: selenide (-2), element(0), selenite (+4), and selenate (+6). These species in a particular environment depend largely on the pH and Eh conditions. The natural, overall geochemical cycle of selenium mainly involves crustal rock sources and oceanic sediment sinks; in addition, the overall crustal cycle (including lithosphere, biosphere, hydrosphere and atmosphere) also is strongly influenced by biochemical reactions that mobilize or fix selenium in organic compounds. Human beings considerably modify this natural geochemical cycle by adding sources of selenium to the environment, principally by coal combustion, sulfide ore processing, and waste processing from the industrial use of selenium. Selenium was enriched mainly in the hydrothermal stage. Large amounts of dispersed forms (i.e., substituted forms in sulfides or adsorbed on carbonaceous matter) and independent selenium minerals occurred in this period. Selenium was accumulated to a higher extent in the products of volcanic activity and volcanic exhalation. In addition, its enrichment in shales, especially in oceanic black shales is chiefly related to biotic processes.\;
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