Micro-Raman spectroscopy of garnierite minerals: a useful method for phase identification

Garnierites are important Ni-Ores found in worldwide hydrous silicate-type Ni-laterites

The supergene origin of ruthenian hexaferrum in Ni-laterites

For two decades, the nature of Fe-rich, oxygen-bearing, Ru-Os compounds found in the supergene environment has been debated. Ru-Os-Fe-oxides and nano-intergrowths of ruthenium with magnetite have been proposed. We applied FE-SEM, EMPA, mu-Raman spectroscopy and synchrotron tts-lXRD to Ru-Os-Fe compounds recovered from Ni-laterites from the Dominican Republic. The results demonstrate that a significant portion of Fe exists in a common structure with the Ru-Os alloy, that is, ruthenian hexaferrum. This mineral occurs both as nanoparticles and as micrometric patches within a matrix of Fe-oxide(s). Our data suggest that supergene ruthenian hexaferrum with a (Ru-0.4(Os, Ir)(0.1)Fe-0.5)(Sigma 1.0) stoichiometry represents the most advanced weathering product of primary laurite within Ni-laterites from the Dominican Republic

Supergene neoformation of Pt-Ir-Fe-Ni alloys: multistage grains explains nugget formation in Ni-laterites

Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as 'floating chromitites' within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Ir-dominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Ni-laterites from the Dominican Republic

Crystallographic information data of natural occurring zaccariniite (RhNiAs) obtained by means of precession electron diffraction

The crystal structure of naturally occurring zaccariniite (RhNiAs) has been studied in Transmission Electron Microscopy (TEM) with variable angle Precession Electron Diffraction (PED) techniques. The analysis of the data has yielded tetragonal cell parameters of 3.86, 3.86, 6.77 Å and space group of P4/nmm for the basic structure, and its constituent atom positions for Ni, As and Rh were determined as well by ab-initio structure resolution method. The data is related to "Structural characterization and ab-initio resolution of natural occurring zaccariniite (RhNiAs) by means of Precession Electron Diffraction" (Roqué Rosell et al., 2019)

Secondary REE-minerals in the karst bauxites of the Bahoruco Peninsula (Dominican Republic)

Rare earth elements (REE) are essential for numerous technological applications. Currently about 90% of the worldwide REE demand is supplied by China (Hatch, 2012), and therefore REE are defined as critical metals (Dutta et al., 2016). Bauxites, the main source for Al in the world, gained interest since they are capable of concentrating significant amounts of REE (Liu et al., 2016), and have been recently considered unconventional deposits for REE (Goodenough et al., 2017). Two types of bauxitic deposits are defined: a) lateritic bauxites (formed after intense weathering of an Al-bearing protolith), and b) karst bauxites (of controversial origin, hosted in carbonaceous rocks). One hypothesis of karst bauxite formation suggests dissolution of the carbonates, whereas the other implies an external contribution of Al-bearing minerals during or after carbonate deposition (Bárdossy, 1982)

Discovery of Ni-smectite rich saprolite at Loma Ortega, Falcondo mining district (Dominican Republic): geochemistry and mineralogy of an unusual case of 'hybrid hydrous Mg silicate-clay silicate' type Ni-laterite

Hydrous Mg silicate-type Ni-laterite deposits, like those in the Falcondo district, Dominican Republic, are dominated by Ni-enriched serpentine and garnierite. Recently, abundant Ni-smectite in the saprolite zone have been discovered in Loma Ortega, one of the nine Ni-laterite deposits in Falcondo. A first detailed study on these Ni-smectites has been performed (μXRD, SEM, EPMA), in addition to a geochemical and mineralogical characterisation of the Loma Ortega profile (XRF, ICP-MS, XRD). Unlike other smectite occurrences in laterite profiles worldwide, the Loma Ortega smectites are trioctahedral and exhibit high Ni contents never reported before. These Ni-smectites may be formed from weathering of pyroxene and olivine, and their composition can be explained by the mineralogy and the composition of the Al-depleted, olivine-rich parent ultramafic rock. Our study shows that Ni-laterites are mineralogically complex, and that a hydrous Mg silicate ore and a clay silicate ore can be confined to the same horizon in the weathering profile, which has significant implications from a recovery perspective. In accordance, the classification of "hybrid hydrous Mg silicate - clay silicate" type Ni-laterite deposit for Loma Ortega would be more appropriate

REE ultra-rich karst bauxite deposits in the Pedernales Peninsula, Dominican Republic: Mineralogy of REE phosphates and carbonates

Karst bauxites have recently received renewed attention for their potential as non-conventional REE sources. Karst bauxites from the Pedernales Peninsula in the Dominican Republic stand among the world's richest in REE. Bauxite ore from two deposits from this bauxite district, Aceitillar and El Turco, have been selected for this study due to their outstanding REE contents and contrasting mineralogy. REE (La to Lu) contents in Aceitillar, range from 0.07 to 0.16 wt%, and Y from 0.01 to 0.13 wt%, whereas El Turco contains between 0.28 and 1.40 wt% REE, and 0.33 to 1.48 wt% Y. The characterisation of REE mineralisation was performed through powder and monocrystal XRD, SEM-EDS, and EMP analyses. REE phosphates and carbonates reveal textural features that suggest significant REE mobilisation and re-deposition within the bauxite profile. The identified REE minerals can be classified into: i) primary monazite(-Ce) and minor monazite(-La); ii) secondary Y- and Nd-dominant phosphates; and iii) secondary Gd- and Nd-carbonates of the (hydroxyl)bastnäsite group. While monazites are ubiquitous in the two studied deposits, secondary phosphates are predominant in El Turco while secondary carbonates are exclusive of Aceitillar. This contrasting mineralogy is explained by the total concentration of carbonate and/or phosphate in the karst bauxite groundwater solutions. REE phosphates are the most stable phases at [CO32−]total/[PO43−]total ≤ 2; whereas REE carbonates are stable at near neutral pH when the total aqueous carbonate concentration is two orders of magnitude higher than that of phosphate. Results of this investigation contribute to a better understanding of the formation REE minerals in the supergene environment and can be applied in REE separation methods

Fosfatos de tierras raras en las bauxitas kársticas de la Sierra de Bahoruco

S'han estudiat els fosfats d'elements de les terres rares (REE), i la mineralogia associada, de dos dipòsits (Sombrero i Km-30) de bauxites kàrstiques de la Sierra de Bahoruco (República Dominicana), per mitjà de microscopi electrònic de rastreig i de microsonda electrònica. La mineralogia principal està formada per gibbsita, hematites, anatasa, boehmita, goethita, amb traces de zircó, apatita, quars, Cr-espinel·la i asbolana-litioforita. Els fosfats de REE són principalment xenotima-(Y), rhabdòfana-(Y), rhabdòfana-(Nd,Dy,Y) i churchita-(Nd), i es troben en forma de grans o agregats lliures en la matriu de gibbsita

Geological, geochemical and mineralogical characteristics of REE-bearing Las Mercedes bauxite deposit, Dominican Republic

Bauxite deposits, traditionally the main source of 'aluminum, have been recently targeted for their remarkable contents in rare earth elements (REE). With Sigma REE (lanthanoids + Sc + Y) concentrations systematically higher than similar to 1400 ppm (ay. = 1530 ppm), the Las Mercedes karstic bauxites in the Dominican Republic rank as one of the REE-richest deposits of its style.The bauxitic ore in the Las Mercedes deposit is mostly unlithified and has a homogeneous-massive lithostructure, with only local cross-stratification and graded bedding. The dominant arenaceous and round-grained texture is composed of bauxite particles and subordinate ooids, pisoids and carbonate clasts. Mineralogically, the bauxite ore is composed mostly of gibbsite and lesser amounts of kaolinite, hematite, boehmite, anatase, goethite, chromian spinel and zircon. Identified REE-minerals include cerianite and monazite-Ce, whose composition accounts for the steady enrichment in light-relative to medium-and heavy-REE of the studied bauxites.Considering the paleo-geomorphology of the study area, we propose that bauxites in the Las Mercedes deposit are the product of the erosion and deposition of lithified bauxites located at higher elevations in the Bahoruco ranges. Based on the available data, we suggest a mixed lithological source for the bauxite deposits at the district scale: bedrock carbonates and an igneous source of likely mafic composition. (C) 2017 Elsevier B.V. All rights reserved

REE en las bauxitas kársticas de la Sierra de Bahoruco (República Dominicana): geoquímica y mineralogía

Los elementos de las tierras raras (REE, divididas en LREE: La- Gd y HREE: Tb-Lu+Y, a veces distinguiendo las MREE: Sm-Gd) tienen un papel muy relevante en la denominada transición del sector energético hacia fuentes renovables que permitan alcanzar el objetivo de cero emisiones netas de carbono. Esta transición depende de la disponibilidad de metales y en particular de REE (p. ej. en la fabricación aerogeneradores y vehículos eléctricos) (Goodenough et al., 2018; Charles et al., 2021). En la actualidad, las principales fuentes de REE son los yacimientos de carbonatitas y rocas alcalinas (típicamente ricos en LREE±U±Th). Sin embargo, los yacimientos secundarios como los de REE adsorbidos en arcillas, y las bauxitas kársticas, destacan por su potencial de concentrar significativamente estos elementos (Goodenough et al., 2018). En la Sierra de Bahoruco (suroeste de la República Dominicana) existen importantes afloramientos de bauxitas kársticas con contenidos muy altos en REE (Torró et al., 2017). Este trabajo tiene como objetivo estudiar la mineralogía de las fases portadoras de REE en el contexto de los perfiles de bauxitas kársticas de la Sierra de Bahoruco, a partir de muestras representativas de una veintena de depósitos