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|Title:||ALS Seminar: Josep Roque Rosell|
|When:||10/25/2012 2:00 PM - 3:00 PM|
“Crystallochemistry of Ni in lateritic profiles - Relationship between the Ni geochemical processes and the hosting minerals microtextures”
Presented by: Josep Roque
Date: Thursday October 25, 2pm
Location: 2-400F Conf Room
Ni laterite deposits al located predominantly between the 22º N and 22º S latitudes. They are the end products of the laterization process of ultramafic rocks rich in Mg with Ni contents between the 0.2 and 0.4 % Wt and they do represent the 72% world resource on land of Ni. Despite of its economic interest the comprehension of the mechanisms by which Ni is retained by and liberated from the lateritic minerals is still scarce (e.g. Singh et al., 2002; Carvalho-e-Silva et al., 2003). These mechanisms have important implications for the mobility of the metal, the subsequent development of Ni-bearing phases in limonitic horizon, and the supergene enrichment in the underlying saprolite horizons of the profile. Samples form the The Moa Bay laterite deposits (Cuba) and Falcondo Mine (Republica Dominicana) have been chosen as representative samples of both limonitic horizon and saprolitic horizons of Ni bearing laterites. These samples have been studied by means of microfocus Raman, micro X-ray diffraction (μXRD), electron probe micro analysis (EPMA), transmission electronic microscopy (TEM) and synchrotron radiation microfocus X-ray absorption spectroscopy (XAS) with the aim to gain structural and chemical information on Ni. The data obtained from the limonitic horizon has revealed that Ni is preferably accumulated in “lithiophorite–asbolane” intermediates. The local environment of Ni shows Ni–Mn distances ∼3.5 Å suggesting that Ni is sorbed mostly in inner-sphere complexes sitting on Mn vacancies and at the edge of the Mn layers. However it is shown that in the presence of Al the Ni is incorporated within the “lithiophorite–asbolane” intermediate by developing brucite-like interlayers (Roqué-Rosell et al., 2011). On the saprolitic horizon the studied sample corresponds to a Ni rich sepiolite, within a cryptocrystalline quartz and amorphous silica matrix. The Ni rich sepiolite fibres do contain Ni within the polysomes substituting Mg in the octrahedra sites. Our prelimary data confirms Ni clustering within the sepiolite structure and thus suggesting the development of coprecipitated domains of Ni-rich and Mg-rich within sepiolite (Manceau and Calas 1985, Manceau 1990 and Galí et al., 2012). For the first time the relationship between the Ni geochemical processes and the hosting minerals microtexture have been shown allowing us to partially explain the diffusion and mobility of Ni across the lateritic profiles.