Phases on the Fe-tetrahedrite–goldfieldite and Fe-tennantite–goldfieldite joins were synthesized from high purity elements in evacuated silica glass tubes at 450 °C and analyzed using electron microprobe analysis and X-ray powder diffraction. The synthetic studies confirm that the compositions on these joins are determined by a (formal) interplay of the Fe3+ + Cu+ ↔ 2 Fe2+, Cu+ + Te4+ ↔ Fe2+ + (Sb,As)3+, Cu+ + 2Te4+ ↔ Fe3+ + 2(Sb,As)3+, and Cu+ + Te4+ ↔ Cu2+ + (Sb,As)3+ substitutions, and also for the region close to Te = 2 apfu by □ + Te4+ ↔ Cu+ + Sb3+. The latter exchange determines the Te-tennantite/tetrahedrite–goldfieldite join. For the region of goldfieldite just above and below 2 Te atoms pfu, the latter exchange is modified by the Fen+ ↔ nCu+ exchange. The 2Fe2+ ↔ Fe3+ + Cu+ exchange causes an arc-like retreat of the composition field away from the linear Cu+ + Te4+ ↔ Fe2+ + (Sb,As)3+ boundary. Incomplete elimination of Fe, compensated by early onset of Cu vacancies, results in a gradual transition between the region of tellurian td/tn and goldfieldite sensu stricto. This variety of substitution and omission mechanisms determines the proportions of alternative endmembers and intermediate members: Cu10Fe2+2(Sb,As)4S13, Cu11Fe3+(Sb,As)4S13, Cu12(Sb,As)4S13, Cu12Te2(Sb,As)2S13, and Cu10Te4S13, plus a measure of the Fen+ ↔ nCu+ exchange. Natural goldfieldite conforms with the synthesis results in its compositional extent. The natural data are mostly not good or complete enough to examine the compositional details suggested here because the calculations described above are very sensitive to the quality of the analyses.