Hydrothermal precipitation of artificial violarite

The nonstoichiometric nickel-ore mineral, violarite, (Ni,Fe)3S4 was prepared as a phase-pure fine powder by a comparatively quick hydrothermal method from an aqueous solution of iron(II) acetate, nickel(II) acetate and DL-penicillamine in an autoclave at 130 °C for 45 h. Powder-XRD showed that the violarite crystallised with the thiospinel structure, a = 9.478(3) Å, Z = 8; SEM revealed platy crystals 10 × 10 × 1 μm; EMPA gave a mean composition, Fe0.31Ni2.36S4. Violarite compositions with a higher Fe/Ni ratio than this were concluded to be metastable in this aqueous system. When DL-penicillamine was used in excess, nickeliferous pyrite formed in addition to violarite. This is the first successful method for preparing violarite directly from aqueous solution, and demonstrates that it is feasible for nickel-rich violarite to precipitate from aqueous media in a geochemical environment. The mechanism for the pseudomorphic replacement of pentlandite, (Fe,Ni)9S8 by violarite is discussed in the light of these new results. The Fe-free end-member, polydymite, Ni3S4 was also prepared by this method.