Fundamental studies have been undertaken to determine the microstructural and phase transformations occurring during the reduction roasting of saprolite ores. Laboratory studies have been undertaken to simulate the conditions occurring during the reduction roast step of the Caron Process. Selected serpentine samples have been treated at temperatures between 500°C and 800°C in H 2 /N 2 gas mixtures, and leaching tests on the reduced samples have been undertaken. Phase and microstructural changes have been characterised using Xray Powder Diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) techniques.A series of complex physical, microstructure and phase changes has been shown to occur during reduction roasting involving: (i) the dehydration of serpentine, (ii) the formation of a high-silica amorphous phase, (iii) the formation of Ni-Fe nanoparticles, and (iv) the nucleation, growth and recrystallisation of the forsterite (olivine) phase. The principal mechanism of removal of nickel from the reduced ore has been shown to occur through the selective leaching of the Ni-Fe alloy nano-particles from the surfaces in the product oxide.

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule $$\left( {{\text{lnD}}_{{{\text{grt}}/{\text{olv}}}}^{{{\text{Ni}}}} ; \frac{{{\text{Ni}}_{{{\text{grt}}}} }}{{{\text{Ni}}_{{{\text{olv}}}} }}} \right)$$ lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ($${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}};$$ X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ($${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}};$$ X grt Cr ; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:$$T \left(^\circ{\rm C} \right)=\frac{-8254.568}{\left(\left( {\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}} \times 3.023 \right)+\left({\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}} \times 2.307 \right)+\left({\mathrm{lnD}}_{\frac{\mathrm{grt}}{\mathrm{olv}}}^{\mathrm{Ni}} - 2.639 \right)\right)}-273\pm 55$$ T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where $${\mathrm{D}}_{\mathrm{grt}/\mathrm{olv}}^{\mathrm{Ni}}= \frac{{\mathrm{Ni}}_{\mathrm{grt}}}{{\mathrm{Ni}}_{\mathrm{olv}}},$$ D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, $${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Ca}}$$ X grt Ca  = Ca/(Ca + Fe + Mg) in garnet, and $${\mathrm{X}}_{\mathrm{grt}}^{\mathrm{Cr}}$$ X grt Cr = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.

The sub-continental lithospheric mantle (SCLM) underlying the South Australian Craton has received considerable research interest over the past decade due to its potential role in formation of the world-class iron-oxide copper gold (IOCG) deposits (

Laboratory studies have been undertaken to investigate the mechanisms and kinetics of reactions occurring during the reduction roasting of saprolite ores. Reduction was undertaken using a H 2 /N 2 gas mixture at temperatures between 400 and 800°C. The focus of the study was to determine the effect of dehydration, reduction, sintering and olivine recrystallisation reactions on nickel recovery. The effects of thermal history, temperature and time on the rates and extents of these processes have been investigated. The results provide valuable insights into the relative contributions of the elementary processes taking place during the reduction roast process, and help to explain some of the behaviour observed in industrial practice.