The thermodynamics of selenium minerals in near-surface environments

Abstract

Selenium compounds are relatively rare as minerals; there are presently only 118 known mineral species. This work is intended to codify and systematize the data of mineral systems and the thermodynamics of selenium minerals, which are unstable (selenides) or formed in near-surface environments (selenites), where the behavior of selenium is controlled by variations of the redox potential and the acidity of solutions at low temperatures and pressures. These parameters determine the migration of selenium and its precipitation as various solid phases. All selenium minerals are divided into four groups—native selenium, oxide, selenides, and oxysalts—anhydrous selenites (I) and hydrous selenites and selenates (II). Within each of the groups, minerals are codified according to the minimum number of independent elements necessary to define the composition of the mineral system. Eh-pH diagrams were calculated and plotted using the Geochemist’s Workbench (GMB 9.0) software package. The Eh-pH diagrams of the Me-Se-H2O systems (where Me = Co, Ni, Fe, Cu, Pb, Zn, Cd, Hg, Ag, Bi, As, Sb, Al and Ca) were plotted for the average contents of these elements in acidic waters in the oxidation zones of sulfide deposits. The possibility of the formation of Zn, Cd, Ag and Hg selenites under natural oxidation conditions in near surface environments is discussed.