X-ray diffraction line profile analysis of acid-resistant goethite in Western Australian nickel laterite ore

Abstract

The goethite peaks in synchrotron and laboratory X-ray powder diffraction (XRPD) patterns of an acid-resistant nickel laterite ore sample from a site in Western Australia exhibit a 'super-Lorentzian' shape. The method for extracting the coherently scattering domain size distribution published by Leoni & Scardi [J. Appl. Cryst. (2004), 37, 629-634] is adapted to fit the asymmetric goethite peak profiles, allowing the refinement of lattice parameters for multiple goethite structural models while maintaining their relationships. The anisotropic peak broadening due to the acicular shape of the goethite crystals is addressed using spherical harmonics predefined from an XRPD pattern of a synthetic goethite sample. A bimodal coherently scattering domain size distribution of goethite crystals is predicted from the goethite profile fitting and agrees with previous transmission electron microscopy findings that two goethite populations with different domain sizes and metal substitutions exist in the ore sample. The small goethite size fraction dissolved slowly during atmospheric acid leaching, while the large goethite fraction barely dissolved. Caustic pre-treatment by KOH digestion significantly enhanced the acid-leaching performance of the small goethite fraction, but had no effect on the large goethite fraction. This study demonstrates that quantitative phase analysis on designated goethite size fractions can successfully fit the super-Lorentzian shaped line profiles of natural goethite crystals with a confirmed bimodal domain size distribution.