A sample of violarite was synthesized from pure components and characterized. Samples of particle size 45-75 mu m were oxidized in a TG-DTA apparatus at a heating rate of 10 degrees C min(-1) and the products characterized at various temperatures by X-ray diffraction (XRD), backscattered electron (BSE) images obtained on a Scanning Electron Microscope (SEM), Fourier transform infrared (FT-IR) spectroscopy and electronprobe microanalysis (EPMA). Only minor sulfation reactions occurred up to 405 degrees C, but above this temperature the violarite decomposed to form a monosulfide solid solution (mss, (Fe,Ni)(1-x)S) and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur produced an exotherm. From 470 degrees C, there was a mass gain due to the continued formation of sulfate species and an exotherm caused by the conversion of iron sulfide to hematite. From 585 degrees C, a mass loss occurred as the iron sulfates decomposed. At 670 degrees C decomposition of the mss took place, with the formation of heazlewoodite, (Ni, Fe)(3+/-x)S-2, and sulfur. The first reaction produced a mass loss, and oxidation of the evolved sulfur gave an exotherm. Trevorite, (Fe, Ni)(3)O-4, was also formed above this temperature, from the oxidation of either mss or heazlewoodite, and this reaction also contributed to the mass loss. Above 725 degrees C a mass loss and sulfur dioxide evolution was associated with the decomposition of nickel sulfate. This was followed by an endotherm at 785 degrees C caused by the melting of the heazlewoodite, which immediately oxidized to produce a mass loss and exotherm. The final product above 860 degrees C contained trevorite, nickel oxide and hematite. Extensive migration of iron occurred during the oxidation process.