Summary: Microprobe and X-ray investigation of a mineral occurring with breithauptite, nickeline, and other arsenides in the naujaite at Igdlúnguaq, Ilímaussaq alkaline massif, S. Greenland, shows that this mineral, which was formerly misidentified as ‘maucherite’, is a westerveldite (Fe_0·85Ni_0·15)As, of much iron-richer composition than the nickeloan westerveldites from the only two other known occurrences of this mineral. Westerveldite from Igdlúnguaq was presumably formed by reaction of nickeline with iron-rich solutions under conditions of reheating at temperatures of about 450° C and under partial pressures of arsenic below those necessary to stabilize nickeline. Sulphur-bearing löllingite occurs as exsolution blades in the westerveldite. Iron-rich nickeline, up to a composition (Ni_0·82Fe_0·18)As_0·95Sb_0·05, was presumably formed by recrystallization of nickeline in the transition stage just before this mineral became unstable and was replaced by westerveldite.