Nickel is an important metal for modern society and nickel-laterite deposits constitute a major Ni resource. In these deposits, Ni occurs intimately associated with the mineral goethite. In these systems, Ni may substitute for Fe in the goethite crystal lattice, but not much is known about the mechanisms associated with the intake or the release of Ni from those sites. An improved understanding of goethite crystallography will provide new insights into Ni-laterite geology and may lead to the improvement in Ni metallurgical recovery. To study the crystallographic effects of Ni substitution for Fe in goethite, I synthesized a series of pure and metal-doped goethites under various environmental conditions (temperature, pH, solution composition), and characterized crystallography of the synthetic phases by synchrotron x-ray diffractometry (SXRD) followed by Rietveld refinement. Also investigated, by SXRD, were natural goethites from the Ravensthorpe Ni-laterite deposit in Western Australia. Selected natural samples were also analysed by electron microprobe and dated by (U-Th)/He geochronology.
High resolution synchrotron SXRD data collected at the Australian Synchrotron, Melbourne, Australia (synthetic goethites), and at the Photon Factory, Tsukuba, Japan (natural goethites) were refit by Rietveld refinement. Results for the synthetic samples reveal that goethite unit-cell volumes (UCV) vary systematically with temperature – as precipitation temperature increases, UCV decreases. Unit-cell volumes also depend on pH, where goethites precipitated under alkaline conditions have smaller UCVs than goethites precipitated under acid conditions. Ni and Al readily substitute for Fe in goethites precipitated under alkaline conditions, but these metals are excluded from the goethite lattice when the mineral precipitates under acid conditions. From the experimental studies on synthetic samples, an experimental curve was developed that relates UCVs to environmental parameters. I apply this curve to the UCVs measured for natural goethites from Ravensthorpe and show that goethite crystallography can be used as a reliable environmental indicator. The combination of goethite crystallography with (U-Th)/He geochronology for the Ravensthorpe deposit also shows that the combined approach permits extracting environmental information through time. It is also shown that further calibration for the effect of Cr, Mn, Co and Cu and for a larger range of temperature and pH can improve the precision and accuracy of the environmental record obtained from goethite crystallography. Some of the conclusions from this study may benefit the metallurgical processing industry. Current technology requires Fe in low-pH leach liquors to be removed by raising the pH of the system, a step that often results in Ni losses. Understanding the precise environmental conditions at which Ni becomes soluble in the goethite structure may lead to an alternative approach of removing Fe from the leach liquor.