This technical note summarizes the results from a mineralogical analysis of limonite and the solids produced during sulfuric acid pressure leaching, using transmission electron microscopy. The elemental composition was revealed by means of X-ray analysis while the crystallographic characterization was obtained using electron diffraction. Iron in the form of goethite (alpha-FeOOH) was the dominant phase of limonite forming needle-shape particles rich in nickel while aluminum existed mainly in gibbsite (Al(OH)(3)) particles. Cobalt was found to be associated with a manganese phase rich also in nickel. No structural changes were observed during heating the limonite slurry up to 270 degreesC and before acid injection. Both Ni and Co were highly leachable in the manganese phase during sulfuric acid pressure leaching of the limonite. Ni was also highly leachable in the magnesium silicate structures that were present in minor amounts. Hematite was the only precipitation phase of iron forming spherical particles of a size of 0.1-0.3 mum while; alunite was the only precipitation phase of aluminum. (C) 2003 Elsevier Ltd. All rights reserved.