AbstractSynthesis of Ni and Zn substituted nano-greigite, Fe3S4, is achieved from single source diethyldithiocarbamato precursor compounds, producing particles typically 50–100 nm in diameter with plate-like pseudohexagonal morphologies. Up to 12 wt.% Ni is incorporated into the greigite structure, and there is evidence that Zn is also incorporated but Co is not substituted into the lattice. The FeL3X-ray absorption spectra for these materials have a narrow single peak at 707.7 eV and the resulting main X-ray magnetic circular dichroism (XMCD) has the same sign at 708.75 eV. All XMCD spectra also have a broad positive feature at 711 eV, a characteristic of covalent mixing. The greigite XMCD spectra contrast with the three clearly defined XMCD site specific peaks found in the ferrite spinel, magnetite. The FeL2,3X-ray absorption spectra and XMCD spectra of the greigite reflect and reveal the high conductivity of greigite and the very strong covalency of the Fe–S bonding. The electron hopping between Fe3+and Fe2+on octahedral sites results in an intermediate oxidation state of the Fe in the Ohsite of Fe2.5+producing an effective formula of [Fe3+↑]A-site[2Fe2.5+↓]B-siteS42–]. The NiL2,3X-ray absorption spectra and XMCD reveal substitution on the Ohsite with a strongly covalent character and an oxidation state <Ni1.5+in a representative formula [Fe3+↑]A[[(2 – x)Fe2.5+↓][Nix1.5+]]BS42–.