The phase transformation of laterite minerals reduced at temperatures up to 800 degreesC and the phase reversibility with cooling were investigated in this study. The phases were studied by in situ synchrotron radiation based X-ray powder diffraction with temperature resolved studies. The degrees of reduction were confirmed by a leaching of reduced ore with ammoniacal solution. Key features, which were observed in this study, include the role of dehydroxylation in releasing nickel and cobalt from the main gangue minerals in which these metals are incorporated. The transformation of hydrated gangue, goethite in limonite to magnetite and the serpentine minerals to the olivine phases in the saprolite and control of these phase transformations, in particular the recrystallisation of magnesium silicate to the forsterite phase were also investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.