Composition and dissolution kinetics of garnierite from the Loma de Hierro Ni-laterite deposit, Venezuela

The composition of a garnierite sample (Mg-Ni silicate) from the Loma de Hierro Ni-laterite deposit (Venezuela) has been determined by means of X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HR-TEM) and electron microprobe (EM) analyses. This garnierite was collected from a vein in the parent rock of the laterite (serpentinized peridotite). XRD showed peaks at 7 angstrom and 10 angstrom, characteristic of the structures of serpentine and talc, respectively. HR-TEM revealed different areas characterized by either a 7 angstrom or 10 angstrom spacing. EM showed a homogeneous sample composition, with a (Mg+Ni)/Si ratio equal to 0.92. The stoichiometric formula for this garnierite is given by Mg2.91Ni0.09Si3.27O8.17(OH)(2.74), which corresponds to a mixture of Ni-containing serpentine (37 mol%) and talc (63 mol%). EM analyses were also performed on six additional samples from the main Ni enrichment zone of the laterite. The results show a clear correlation between Ni content and mole fraction of talc, indicating the affinity of Ni for the talc phase in the garnierite. Quantitative XRD fitting of one of these additional samples indicated that the talc belongs to the variety known as kerolite. Flow-through experiments were performed to measure far-from-equilibrium dissolution rates. At steady state, dissolution of garnierite was stoichiometric at pH greater than about 5 and non-stoichiometric at more acidic conditions. Since these solutions were undersaturated with respect to possible secondary precipitates, the results seem to indicate different contributions of Ni-containing serpentine and talc to the total rate. For pH below 5, the contribution from serpentine to the total rate increases with decreasing pH. The overall dissolution rate is proportional to a(H+)(0.28), although pH dependence could also be described with a model based on non-linear sorption of H+ on garnierite. (C) 2007 Elsevier B.V. All rights reserved.