THE PYROLYTIC DECOMPOSITION OF SYNTHETIC VIOLARITES OF VARIOUS STOICHIOMETRY

Four samples of violarite of different stoichiometry were synthesised and characterised. The pyrolytic decomposition of Fe0.97+/-0.01Ni1.94/-0.02S4.0+/-0.03 of particle size 45-63 mu m was examined at a heatin g rate of 10 degrees C min(-1) in a nitrogen atmosphere using a simult aneous thermogravimetric-differential thermal analyser (TG-DTA). The f irst reaction observed was the loss of sulfur between 420 degrees and 525 degrees C, giving a mass loss of 2.25% with the formation of a mon osulfide solid solution (mss), violarite of a different stoichiometry, and a minor amount of vaesite. An endothermic peak at 495 degrees C w as assigned to a phase transition from a thiospinel structure to mss. Further sulfur losses of 7.65 and 4.10% took place in the 525-615 degr ees C and 615-795 degrees C ranges, and the products were mss of incre asing metal richness, with (Fe,Ni)(3+/-x)S-2 also existing in the high er temperature range. This last phase melted at 835 degrees C. Finally , more sulfur was lost between 845 degrees and 900 degrees C from the molten material. Products were taken at different temperatures for cha racterisation by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The TG-DTA curves for all four samples were obtained under similar conditions. The samples were all found to decompose by the same reaction sequence. The endothe rmic events associated with the thiospinel transition, and the melting of (Fe,Ni)(3+/-x)S-2, shifted to lower temperatures as the nickel con centration of the original violarite increased. (C) 1998 Elsevier Scie nce B.V.