Ni Enrichment and Stability of Al-Free Garnierite Solid-Solutions: A Thermodynamic Approach

Abstract

Abstract-Garnierites represent significant Ni ore minerals in the many Ni-laterite deposits worldwide. The occurrence of a variety of garnierite minerals with variable Ni content poses questions about the conditions of their formation. From an aqueous-solution equilibrium thermodynamic point of view, the present study examines the conditions that favor the precipitation of a particular garnierite phase and the mechanism of Ni-enrichment, and gives an explanation to the temporal and spatial succession of different garnierite minerals in Ni-laterite deposits. The chemical and structural characterization of garnierite minerals from many nickel laterite deposits around the world show that this group of minerals is formed essentially by an intimate intermixing of three Mg-Ni phyllosilicate solid solutions: serpentine-népouite, kerolite-pimelite, and sepiolite-falcondoite, without or with very small amounts of Al in their composition. The present study deals with garnierites which are essentially Al-free. The published experimental dissolution constants for Mg end-members of the above solid solutions and the calculated constants for pure Ni end-members were used to calculate Lippmann diagrams for the three solid solutions, on the assumption that they are ideal. With the help of these diagrams, congruent dissolution of Ni-poor primary minerals, followed by equilibrium precipitation of Ni-rich secondary phyllosilicates, is proposed as an efficient mechanism for Ni supergene enrichment in the laterite profile. The stability fields of the solid solutions were constructed using [log a SiO 2 (aq) , log ((a Mg 2+ + a Ni 2+)/(a H +) 2 )] (predominance) diagrams. These, combined with Lippmann diagrams, give an almost complete chemical characterization of the solution and the precipitating phase(s) in equilibrium. The temporal and spatial succession of hydrous MgNi phyllosilicates encountered in Ni-laterite deposits is explained by the small mobility of silica and the increase in its activity.

Nickel dispersion and enrichment at the bottom of the regolith: formation of pimelite target-like ores in rock block joints (Koniambo Ni deposit, New Caledonia)

“…It is suggested that the last zone to dry is in the centre of the joint, and there, water precipitates the remaining Si and Mg once Ni is already precipitated as pimelite near the joint boundary, or alternately, Mg-rich silicate could also have precipitated earlier. Pimelite is much more insoluble than Mg-kerolite, thereby explaining why this mineral precipitates first despite a Ni/Mg ratio in water close to 10 −3 ; this process was already described by Galí et al (2012). The process is probably recurrent and repeated several times before reaching a significant volume of precipitated minerals but is limited to joints with a sufficiently low permeability, preventing rapid dissolution and the release of elements into the percolating waters.…”

Pseudomorphic replacement of diopside during interaction with (Ni,Mg)Cl2 aqueous solutions: Implications for the Ni-enrichment mechanism in talc- and serpentine-type phases

“…6) were constructed following the procedure of Galí et al (2012) to understand the relative solubility of different phases with respect to the interfacial fluid composition. The figure also lists the respective dissolution reactions shown on the diagrams and corresponding log K values.…”

“…For Ni-olivine (Ni 2 SiO 4 ), the thermodynamic data was used as reported in Robie et al (1984). In order to calculate the Gibbs free energy change during the Ni-endmember phase dissolution reaction, the Gibbs free energies of formation for dissolved aqueous species were taken from Galí et al (2012) and references therein. The thermodynamic data for Ni-end-member phases are not available for high temperatures and pressures and hence, each diagram was calculated at standard conditions (25°C and 0.1 MPa pressure).…”

“…6a and b, each diagram defines regions in the log In our experiments, sequential formation of compositionally homogeneous rims indicates that only one solid solution composition was actually in equilibrium with specific solutions during precipitation. For a more detailed understanding of different stability fields of these types of solid solution series compositions in similar systems, the reader is referred to Galí et al (2012). At the temperatures relevant to our experiments, diopside dissolves at a SiO 2 ;aq higher than 10 −5 at 300-600°C (extrapolated from Fig.…”

Controles mineralógicos e geoquímicos do níquel em rochas ultramáficas no escudo Sul-Riograndense

“…Os dados de química mineral de olivinas e serpentinas compilados da bibliografia internacional mostram que as concentrações de NiO apresentados por diversos autores são condizentes com as concentrações destes minerais no ESrg, obtidos nesta pesquisa. Demonstram também que as análises de olivinas realizadas em amostras de depósitos lateríticos de níquel (Gleeson, 2004;Galí, 2012) atingem valores superiores, chegando a níveis de 0,50%, compará-veis ao determinado para as olivinas da Sequência Cerro Mantiqueiras no ESrg.…”