Selenide, Natural

any of a group of mineral compounds that is a combination of selenium and metals. Approximately 30 selenide minerals are known. They are analogues of sulfides, with which they form common structural types of continuous or limited series of solid solutions. Unlike S, which forms minerals with more than 40 elements, Se combines with a relatively small number of elements: Pb [clausthalite (PbSe)], Hg [tiemannite (HgSe)], Bi [guanajuatite (Bi₂ Se₃)], Ag [naumannite (Ag₂Se), aguilarite (Ag₄SeS)], Cu [klockmannite (CuSe), berzelianite (Cu₂Se), umangite (Cu₃Se₂)], Co [freboldite (CoSe)], Fe [ferroselite (FeSe₂), achawalite (FeSe)], Ni [blockite (NiSe₂)], Zn [shtilleite (ZnSe)], Cd [cadmoselite (CdSe)], and Tl. Examples of complex selenides are crookesite [(Cu₃TlAg)₂Se] and eucairite (CuAgSe).

Most selenides are rare. The largest commercial Se reserves, in relatively low concentrations, are associated with sulfide deposits, in which Se isomorphically replaces S in sulfide minerals. Selenide minerals proper are formed during hydrothermal processes under conditions marked by a sharply reduced potential of S. Hydrothermal selenide deposits are usually small but have a very high Se content (Pajacana in Bolivia and deposits in Argentina and the German Democratic Republic). There are also subvolcanic deposits, usually of gold ore, and deposits formed through hypergenesis with selenium mineralization.