The phase equilibria in the system Ag–Pd–Se were studied by the evacuated-silica glass tube method at 350, 430, and 530 °C. In the system we synthesized four ternary phases: Ag2Pd3Se4 (chrisstanleyite) and the new phases AgPd3Se, (Ag,Pd)22Se6, and Ag6Pd74Se20. The AgPd3Se phase forms a Ag1−xPd3+xSe (x = 0–0.15) solid-solution series. The (Ag,Pd)22Se6 phase forms a (Ag11±xPd11±x)22Se6 (0.2 ≥ x ≥ 3.9) solid-solution series, the solubility of Ag decreasing with increasing temperature. Palladseite (Pd17Se15) forms a limited solid solution and dissolves up to 7 wt.% Ag, and the phase Pd9Se2 dissolves up to 3 wt.% Ag. The Ag2Pd3Se4 phase, an analogue of the mineral chrisstanleyite, forms stable associations with naumannite (Ag2Se) and veerbekite (PdSe2); it also coexists with palladseite (dissolving 7 wt.% Ag). The phase is stable up to 430 ºC. Phase relations determined the mineral assemblages that can be expected to occur in nature. Ternary AgPd3Se and (Ag,Pd)22Se6 phases and the binary PdSe, Pd7Se4, and Pd34Se11 phases can be expected in associations with the mineral palladseite, among other selenides. Finding these phases in nature is highly probable in telethermal selenide veins and unconformity-related uranium deposits, at conditions of high Se/S fugacity ratio, an oxidizing environment, and Se-rich fluids, forming at low temperatures. The Ag6Pd74Se20 phase forms stable associations with AgPd3Se and Pd7Se2. It also coexists with Pd4Se and Pd9Se2. The Ag6Pd74Se20 phase is stable above 430 ºC. Under natural conditions the Ag6Pd74Se20 and Pd9Se2 phases can be expected to occur in less traditional environments than are usual for selenides, forming at higher temperatures (above 430 ºC).

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