Ni enrichment and stability of Al-free garnierite solid-solutions : A thermodynamic approach

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 , log ((a + a )/(a ) )] (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 Mg-Ni phyllosilicates encountered in Ni-laterite deposits is explained by the small mobility of silica and the increase in its activity