Cyanide is used extensively for the extraction of gold, and mine waste may contain significant quantities of aqueous cyanide complexes and solid cyanide phases. The geochemistry of dissolved cyanide has been well studied, but solid phases have rarely been identified from mine waste. Our objective in this study was to charaterize a solid cyanide phase from a spent gold heap-leach pad. Blue pseudocubic crystals, 2–25 μm in diameter, were found with feldspar, mica, sphalerite, chalcopyrite and pyrite on a syenite sample from Leach Pad 91, Landusky gold mine, in Montana. Single-crystal XRD indicates a rhombohedral unit-cell [ a 12.4980(15), c 32.2618(41) A] with space group R 3 c . The solid belongs to a group of zeolitic cyanide phases with a framework of [(ZnN 4 ) 3 (FeC 6 ) 2 ] 2− , and cavities are occupied by two alkali cations (K + , Na + or Cs + ) and H 2 O. Electron-microprobe analysis gave average values (wt.%) of 1.0 Na, 1.6 K, 25.8 Zn, 14.3 Fe, 0.8 Cu, 22.3 C, 26.1 N, and 8.1 H 2 O by difference. The 1:1 atom ratio for Na:K, and 3:2 for Zn:(Fe 2+ + Cu 2+ ) fits the zeolitic cyanide group; the formula calculated from EMPA data and a structure refinement is NaKZn 3 (Fe 1.9 Cu 0.1 ) ∑ 2 (CN) 12 ·3.7H 2 O. This solid phase was formed by reaction of CN − and Na + in the processing solution with Fe 2+ , Zn 2+ , and Cu 2+ from weathered pyrite, sphalerite, chalcopyrite, and K + from K-feldspar. Thermodynamic calculations indicate that K 2 Zn 3 [Fe(CN) 6 ] 2 ·n(H 2 O) is stable between pH 5.6 and 10 at the concentrations of K, Zn, Fe and CN − in water draining the leach pad. Further oxidation of sulfides in the pad would result in a decrease of pH below 5.6 and promote the dissolution of the cyanide phase.