Compositional and structural characterisation of Ni-phyllosilicates in hydrous silicate type Ni-laterite deposits

Abstract

[eng] Ni-bearing Mg-phyllosilicates (commonly known as garnierites) are significant ore minerals in many Ni-laterite deposits worldwide. However, the characterisation of these mineral phases is complex, as well as their classification and nomenclature, due to their fine-grained nature, low crystallinity and frequent occurrence as mixtures. The aim of this study is to shed some light to the nature of the Ni-bearing Mg-phyllosilicates occurring at the Falcondo Ni-laterite. In this deposit, these minerals are found within the saprolite horizon mainly as fracture-fillings, coatings on joints and as breccias. The Falcondo garnierites display easily distinguishable different shades of green and characteristic textures, which correspond to different mineral phases. Five garnierite types were defined by X-ray diffraction (XRD), optical and electron microscopy, and electron microprobe (EMP) analyses: I) Ni-Fe-bearing serpentine-like, II) Ni-(Al)-bearing serpentine-like with minor talc-like, III) Ni-rich mixture of serpentine- and talc-like, IV) talc-like and V) sepiolite-falcondoite. The characterisation was completed with differential thermal analysis and thermogravimetry (DTA-TG), transmission electron microscopy (TEM), Raman spectroscopy, microfocus X-ray absorption spectroscopy (i.tXAS) and dissolution experiments, in order to gain further insight on these mineral phases from different points of view. EMP oxide totals and DTA-TG indicate that talc-like contain higher H2O than talc sensu stricto (about 4.5% mass loss at 200°C, and up to 5% at 650°C), and therefore the names kerolite-pimelite [(Mg,Ni)3Si4010(OH)2• nH20] should be used instead of talc-willemseite [(Mg,1•103Si401o(OH)21. Compositional data showed continuous Mg-Ni solid solution along the joins lizardite-nepouite/chrysotile-pecoraite (serpentine-like), kerolite-pimelite (talc-like) and sepiolite-falcondoite. The phases with larger amounts of talc-like displayed the highest Ni contents (up to 2.2 apfu out of 3 octahedral atoms). In addition, EMP analyses of the mixed phases showed deviations from the stoichiometric Mg-Ni solid solutions of serpentine and talc. This is best explained by mixing at the nanoscale, which was confirmed by XRD and high resolution TEM imaging. Furthermore, a detailed textural study by means of EMP quantified X-ray element imaging was used to explain the relationships between textural position, sequence of crystallization and mineral composition of the studied Ni-phyllosilicates. These results indicate several stages of growth with variable Ni content, pointing to recurrent changes in the physical-chemical conditions during the precipitation of the different Ni-phyllosilicates. HRTEM and low magnification imaging enabled to measure the characteristic basal spacings of these phyllosilicates and to observe a wide variety of nanotextures, respectively: 15-sectored polygonal serpentine, chrysotile tubes, lizardite lamellae, kerolite-pimelite lamellae and sepiolite ribbons. In accordance with EMP results, chemical analyses by TEM showed that Ni is more concentrated in the kerolite-pimelite lamellae than in the serpentine particles. In addition, the HRTEM revealed that kerolite-pimelite lamellae replace the Ni-poor serpentine particles. These observations evidence the processes of progressive Ni-enrichment within some Ni-bearing Mg-phyllosilicates. Characteristic Raman bands were observed for serpentine-, talc- and sepiolite-like phases, and therefore this technique allowed discriminating the different garnierite types. The synchtrotron radiation-based µXAS analyses were conducted to investigate the speciation of Fe (by Fe K-edge X-ray absorption near edge structure, XANES) and the local environment of Ni (by Ni K-edge extended X-ray absorption fine structure, EXAFS). The XANES results indicate that Fe in the weathered saprolite and in the garnierites is mostly oxidised, whereas in the primary serpentine from the ultramafic protolith it is in the ferrous form. The EXAFS spectra indicate that Ni is homogeneously distributed in the unweathered rock, and is accumulated in discrete domains (clusters) in the weathered saprolite and in all the garnierite types. Finally, the dissolution experiments of a set of garnierites which cover a wide range of mineral compositions at acidic pH suggest that the dissolution rates are faster in serpentine- dominated garnierites than in talc (kerolite)-dominated garnierites or sepiolite-falcondoite, and that the dissolution is not congruent, being the serpentine component the first to dissolve in the garnierite mixtures of serpentine and talc (kerolite). In summary, the various analytical techniques applied to the garnierites of this study provide an accurate outlook of their complex mineralogy, structures, textures and chemistry at different scales that provides further details on the formation of these Ni-bearing Mg- phyllosilicates in a lateritic environment.

[cat] Els fil- losilicats de níquel (comunament anomenats "garnierites") són importants menes en molts jaciments laterítics de Ni del món. La seva caracterització, classificació i nomenclatura és complexa, ja que normalment es troben en forma de mescles íntimes de diferents fil- losilicats de gra molt fi i baixa cristal• linitat. L'objectiu d' aquesta tesi és avançar en el coneixement de la mineralogia dels fil- losilicats de Ni del dipòsit de laterites niquelíferes de Falcondo, a la República Dominicana, per mitjà de l'ús d'un ampli espectre de tècniques analítiques: difracció de pols de raig X, anàlisi tèrmica diferencial i termogravimetria, microscòpia òptica, microscòpia electrònica de rastreig, microsonda electrònica, microscòpia electrònica de transmissió, espectroscòpia Raman, espectroscòpia d' absorció de raigs X (EXAFS i XANES, amb radiació sincrotró) i per mitjà d'experiments de dissolució. Les garnierites de Falcondo es localitzen a la part baixa de l'horitzó saprolftic en forma de rebliment de fractures i vetes, i ciments i clasts en bretxes. Els principals resultats indiquen que hi ha cinc tipus de garnierites, segons el seu color, la seva mineralogia i la química mineral. La majoria són barreges de serpentina i talc hidratat (kerolita-pimelita), tot i que també hi ha fases de la sèrie sepiolita-falcondoita. La fase que concentra més Ni és la kerolita-pimelita, mentre que a la serpentina el Ni és sistemàticament molt baix i a la sepiolita-falcondoita és molt variable. Les imatges d' alta resolució obtingudes amb microscopi electrònic de transmissió indiquen que els fil- losilicats que componen les garnierites formen una gran varietat de textures: serpentina poligonal, tubs de crisòtil, lamel• les de lizardita, lamel• les de talc hidratat i cintes de sepiolita. A més, s'ha observat que la kerolita-pimelita (rica en Ni) reemplaça sistemàticament la serpentina (pobra en Ni). L'espectroscòpia Raman permet distingir els cinc tipus descrits de garnierites i per tant, s'ofereix com una tècnica ràpida, senzilla i no destructiva per analitzar aquest tipus de minerals al camp. L' absorció de raigs X per mitjà de radiació sincrotró ha permès identificar que tot el Fe contingut en les garnierites i en les serpentines de la saprolita està en forma de Fe3+, i que tot el Ni es troba acumulat formant clusters, i no pas en forma de solució sòlida homogènia, a la capa octaèdrica. Finalment, els experiments de dissolució suggereixen que les velocitats de dissolució són més ràpides en les garnierites que contenen molta serpentina, i que la dissolució no és congruent, sent el Mg el primer element en ser posat en solució, seguit del Si i del Ni.