Thermal behaviour of hydrous nickel–magnesium silicates when heating up to 750°C

The thermal decomposition sequence of hydrous nickel–magnesium silicates from Colombia and Brazil was studied under air/Ar atmosphere from room temperature up to 750°C by differential scanning calorimetry–thermogravimetry followed by X-ray diffraction and scanning electron microscopy analyses. Differential scanning calorimetry curves of the samples obtained showed three endothermic peaks at 100, 250 and 600°C due to the release of free water, the dissociation of goethite and the release of crystalline water respectively. To determine the mineral species and microtexture, the ores were studied by scanning electron microscopy. Scanning electron microscopy–energy dispersive spectroscopy analyses showed that the ores are rich in Mg and Mg–Fe silicates, Cr spinel, Mn oxide, goethite and silica and exhibit complex alteration texture. X-ray diffraction analyses of Colombia-2 and Mirabela (Brazil) after the experiments showed that the dehydroxylation produces an amorphous intermediate phase, which is supposed to be due to the exsolution of silica. However, Colombia-1 sample, which was confirmed to contain antigorite mineral, was observed to undergo dehydration and recrystallisation simultaneously.