“Garnierite” ore and Ni-serpentine mineralizations from the Falcondo Ni-laterite deposit (Dominican Republic): An approach from quantitative XR element imaging Cristina Villanova-de-Benavent Joaquín A. Proenza Salvador Galí Esperança Tauler Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia (Universitat de Barcelona). Martí i Franquès, s/n 08028 Barcelona, Catalonia, Spain. Antonio García-Casco Department of Mineralogy and Petrology (Universidad de Granada) and Andalusian Institute of Earth Sciences (IACT, UGR-CSIC). Campus Fuentenueva s/n 18071 Granada, Spain. John F. Lewis Department of Earth and Environmental Sciences, The George Washington University. 20052 Washington D.C., USA. Francisco Longo Falcondo Xstrata Nickel. Box 1343 Santo Domingo, Dominican Republic. Abstract. ”Garnierites” and Ni-serpentines are the main Ni ores in hydrous silicate-type Ni-laterites. In the Falcondo Ni-laterite, “garnierites” are found within the saprolite horizon mainly as fracture-fillings and thin coatings on joints. Different greenish colours are distinguished, which correspond to different mineral phases, according to XRD: talc-like (10 Å-type), serpentine-like (7 Å-type), a mixture of talc- and serpentine-like, and sepiolite-like. Compositional data suggest continuous solid solution along the joints lizardite-népouite/chrysotile-pecoraite, kerolite-pimelite and sepiolite-falcondoite. In general, talc-like “garnierites” display higher Ni contents than serpentine-like. Deviations from serpentine and talc stoichiometric compositions are due to talc- and serpentine-like mixing at the nanoscale. A detailed textural study by means of quantified X-ray element imaging provides a wealth of new information about the relations between textural position, sequence of crystallization and mineral composition of the studied “garnierite” samples. These results indicate several stages of growth with strongly variable Ni content, pointing to recurrent changes in the physical-chemical conditions of “garnierite” dissolution- precipitation in an aqueous medium. Keywords. “garnierite”, Ni-laterite, Dominican Republic, quantitative XR element imaging 1 Introduction Ni-Co laterites are regolith deposits formed by the chemical weathering of ultramafic rocks (e.g. Golightly, 1981). These deposits contain 70% of the world Ni reserves (Gleeson et al., 2003, Kuck, 2011). Also, they are the source of approximately the 40% of world annual Ni production, and there is an increase amount of Ni being extracted from laterite ores (Mudd, 2010). The highest grade Ni-laterites are the hydrous Mg silicate- type, with 1.8-2.5 wt % Ni (Freyssinet et al., 2005). The Falcondo deposit (Dominican Republic) is the main hydrous Mg silicate-type Ni-laterite of the Greater Antilles in terms of Ni content and reserves. The estimated Ni production of this deposit is about 1Mt with grades of 1-2% Ni (Mudd, 2010; Berger et al., 2012). As in many hydrous Mg silicate-type deposits worldwide, the lower part of the lateritic profile contains most of the main ore minerals: Ni-bearing serpentines and “garnierites” (Freyssinet et al., 2005). “Garnierites” are green, fine-grained mixtures of Ni- bearing magnesium phyllosilicates, including serpentine, talc, sepiolite, smectite and chlorite (e.g. Brindley and Hang, 1973; Springer, 1974). In addition, an intermediate phase between talc and serpentine was described by Rukavishnikova (1956), karpinskite (Mg,Ni) 2 Si 2 O 5 (OH) 2 , but it is not yet accepted as a mineral name by the IMA Commission on New Minerals, Nomenclature and Classification. “Garnierites” are supergene minerals precipitated from a Ni-enriched solution during the weathering of the ultramafic protolith. Therefore, they are characterised by having low Fe concentrations (< 1.5% FeO, Manceau & Calas, 1985). In the case of Falcondo, previous results show that “garnierites” consist mainly of the combination of three solid solutions: lizardite-népouite or chrysotile-pecoraite [(Mg,Ni) 3 Si 2 O 5 (OH) 4 ], talc-like or kerolite-pimelite [(Mg,Ni) 3 Si 4 O 10 (OH) 2 ·n(H 2 O)] and sepiolite-falcondoite [(Mg,Ni) 4 Si 6 O 15 (OH) 2 ·6(H 2 O)] (Tauler et al., 2009; Galí et al., 2012). For these minerals, the terminology “serpentine-like” (or 7 Å-type), and “talc-like” (or 10 Å- type) and “sepiolite-like” “garnierites” will be used hereafter (e.g. Brindley & Hang, 1973). Little work on detailed characterisation of “garnierites”, including information of textures and composition has been done, except for sepiolite- falcondoite series (Springer, 1976; Tauler et al., 2009). The aim of this study is to provide new data on textural relations and mineral chemistry of Dominican “garnierites”, focusing on the serpentine- and talc-like phases.