Understanding the mechanisms of selenium behavior under near-surface conditions is a topical problem of modern mineralogy and geochemistry that is very important in solving some environmental problems. The objective of this study is to develop techniques of synthesizing a chalcomenite analog and to study its speciation and properties. The synthesis was performed by boiling-dry aqueous Cu₂(CO₃)(OH)₂ solutions and selenium acid H₂SeO₃. The obtained samples were identified by X-ray diffraction and IR spectroscopy. The Eh-pH diagrams were calculated using the Geochemist's Workbench (GMB 7. 0) software package. The database comprises the thermodynamic parameters of 46 elements, 47 main particles, 48 redox pairs, 551 particles in solution, 624 solid phases, and 10 gases. The Eh-pH diagrams have been calculated for the Cu-Se-CO₂-H₂O system for the average content of these elements in underground waters and their contents in acidic waters in the oxidation zones of sulfide deposits. The formation of chalcomenite and malachite under near-surface conditions is discussed.