Co–Mn Mineralisations in the Ni Laterite Deposits of Loma

Cobalt demand is increasing due to its key role in the transition to clean energies. Although the main Co ores are the sediment-hosted stratiform copper deposits of the Democratic Republic of the Congo, Co is also a by-product of Ni–Co laterite deposits, where Co extraction efficiency depends, among other factors, on the correct identification of Co-bearing minerals. In this paper, we reported a detailed study of the Co mineralisation in the Ni–Co laterite profiles of Loma Caribe (Dominican Republic) and Loma de Hierro (Venezuela). Cobalt is mainly associated with Mn-oxyhydroxide minerals, with a composition between Ni asbolane and lithiophorite, although a Co association with phyllosilicates has also been recorded in a Loma de Hierro deposit. In Loma Caribe, Co-bearing Mn-oxyhydroxide minerals mainly developed colloform aggregates, and globular to spherulitic grains, while in Loma de Hierro, they displayed banded colloform, fibrous or tabular textures. Most of the compositional analyses of Mn-oxyhydroxides yielded 20 and 40 wt.% Mn, with Ni and Co up to 16 and 10 wt.%, respectively. In both profiles, Mn-bearing minerals were mainly found in the transition from the oxide horizon to the saprolite, as observed in other laterite profiles in the world, where the precipitation of Mn-bearing minerals is enhanced because of the pore solution saturation and pH increase.


Introduction
In 2020, the European Commission published a new list of critical raw materials considered essential for the economy of Europe, according to their potential supply risk and their economic and industrial importance [1]. This list contains 30 raw materials, compared with the 14, 20 and 27 that were indicated in the previous lists of 2011, 2014 and 2017, respectively. One of these critical materials is Co, which is a transition metal that has been in the list since 2011. The main reason to consider Co as a critical raw material is that worldwide production comes primarily from the sediment-hosted stratiform copper deposits of the Democratic Republic of the Congo (accounting for 70% of the world's production) [1][2][3], which is also the main supplier to Europe. Cobalt is essential in many industries such aerospace, textile, electronics, automotive, renewable energies, and energy-

Geological Setting
The Loma Caribe Ni laterite deposit belongs to the Falcondo mining area, located in the central part of the Dominican Republic, in the northern Caribbean (Figure 1a). This deposit, together with other six other satellite deposits ( [16] and references therein) developed during the Miocene after weathering of the Loma Caribe peridotite, which occurs as a serpentinised belt of ultramafic rocks, approximately 4-5 km wide and 95 km long, NW of Santo Domingo (Figure 1b). The Loma Caribe peridotite represents a harzburgitic oceanic mantle, as part of a dismembered Cretaceous ophiolitic complex [19]. The Loma Caribe deposit is classified as hydrous Mg silicate type, and, based on its geochemical footprint, textures and mineralogy, has been divided into several zones, which are, from bottom to top, the serpentinised peridotite, the saprolite horizon, the oxide horizon and a duricrust zone [16]. The protolith is a serpentinised peridotite rich in olivine and enstatite, crosscut by serpentine minerals (i.e., lizardite). The saprolite horizon consists of Ni-rich lizardite with relicts of olivine and enstatite, and with minor clinochlore, maghemite and goethite. In the oxide horizon, ore samples contain goethite, hematite, gibbsite and chromian spinel [16] (Figure 2).
The Loma de Hierro laterite deposit is located in the northcentral part of Venezuela. This deposit is placed on the Faja Loma de Hierro, which belongs to an elongated, deformed belt in the Caribbean Plate southern margin (Figure 1a), represented by the Cretaceous Dutch and Venezuelan Islands and the Cordilleras of Venezuela [21] (Figure 1c).
The exploited area has a length of 15 km and a width of 1 to 7 km [17]. This deposit formed over the partially serpentinised peridotites of the Loma de Hierro ophiolite, a W-E aligned unit (100 km 2 ) within the Serranía de la Costa in the south Caribbean Plate margin ( Figure 1) [21,22]. The Loma de Hierro ophiolite is made up of harzburgite and dunite (Loma de Níquel mantle peridotite), gabbros (Gabro de Mesia; 127 + 1.9/−4.3 Ma) and basaltic rocks with MORB affinity (Basalto de Tiara) [23][24][25]. Several studies [24,25] have described the Loma de Hierro ophiolite as being a fragment of proto-Caribbean lithosphere.
The Loma de Hierro deposit is also considered as a hydrous Mg silicate type Ni deposit, developed from a partially serpentinised harzburgite peridotite. Over this peridotite, and as in the case of Loma Caribe, several horizons have been defined: lower saprolite, upper saprolite and a poorly developed oxide zone [17]. The protolith is comprised of olivine and orthopyroxene, with minor amounts of lizardite, chrysotile, kerolite and chlorite. The lower saprolite contains large amounts of olivine, orthopyroxene, lizardite and chrysotile. In some samples, the presence of garnierites is observed. The upper saprolite is poorer in olivine and orthopyroxene, and richer in lizardite, chrysotile, kerolite and chlorite. The saprolite zone is enriched in Ni (1.1-2.7 wt.% NiO). The oxide zone is characterised by high contents of goethite, hematite and gibbsite. Ore minerals are secondary Ni serpentine and Ni-rich kerolite-pimelite garnierite mixtures (~22 wt.% NiO).
Economic concentrations of Mn and Co are observed in both profiles. Co-Mn mineralisations mainly occur as fine-grained, black, semi-metallic or dull coatings and open space infillings near the transition between the oxide and the saprolite horizons ( Figure  3).
In Loma Caribe, two Mn-Co-rich levels have been described: (i) at the transition zone between the saprolite and the oxide horizon (8.4 wt.% MnO; 1.23 wt.% Co); (ii) at the lower part of the upper oxide zone (2.2 wt.% MnO; 0.4 wt.% Co) [16]. In Loma de Hierro deposit, the MnO and Co reach grades of up to 1.40 wt.% and 0.14 wt.%, respectively [17].

Sampling and Sample Preparation
A total of seven samples were selected for the present study, two from Loma Caribe, (LC-6 and LC-7) and five from Loma de Hierro (LH-10, LH-11, LH-12, LH-13/14, and LHA) ( Figure 2). All samples except LHA have been studied previously [16,17] but they have been revisited here with a focus on Mn-Co-bearing minerals. Sample LHA from Loma de Hierro was collected from a Mn-Co rich zone, and was divided into two subsamples according to their colour, which were named LHA-1 (whitish sample) and LHA-2 (blackish sample).

Analythical Methods
Samples LC6, LC7, a subsample of LC7 with higher concentration of blackish material (rich in Mn-oxyhydroxides) named LC7_black, and samples LHA-1 and LHA-2 were analysed using X-ray powder diffraction (XRPD). Samples were powdered using an agate mortar and pestle. XRPD analysis was carried out at the Centres Científics i Tecnològics of the Universitat of Barcelona (CCiT-UB) with a PANalytical X'Pert PRO MPD Alpha1 powder diffractometer in Bragg-Brentano θ/2θ geometry of 240 mm of radius, nickel filtered Cu Kα1 radiation (λ = 1.5406 Å), 45 keV and 40 mA. The samples were scanned from 4 to 100° (2θ) with a step size of 0.017° and a measuring time of 150 s per step, using an X'Celerator detector (active length = 2.122°) and a variable divergence slit. Mineral identification was performed by X'Pert Highscore search-match software using the Powder Diffraction File (PDF-2) of the International Centre for Diffraction Data (ICDD), and quantitative mineral phase analyses were obtained by full profile Rietveld refinement using XRPD data and TOPAS V4.2 software [26,27]. Sample LH-10 was also analysed by powder angle-dispersive micro-XRD in the microdiffraction/high-pressure station of the BL04-MSPD beamline at ALBA Synchrotron, Barcelona. This beamline is equipped with a Kirkpatrick-Baez mirror that allows a focus of the monochromatic beam to around 15 µm × 15 µm (full width at half maximum) with a Rayonix SX165 CCD detector.
For the above measurements, a wavelength of 0.4246 Å was selected, as determined from the absorption K-edge of Sn (29.2 keV). The data were acquired by using the synchrotron through-the-substrate X-ray microdiffraction technique [28]. Small amounts of powdered material were placed on top of a Kapton ® tape attached to a glass substrate. The sample-to-detector distance and the beam centre position was calibrated from LaB6 diffraction scans obtained under exactly the same conditions as the sample. The calibration and subsequent integration of the CCD images were performed with Dioptas software [29]. Phase identification on the integrated powder scans was carried out by using the package Diffrac.EVA from Bruker, together with the Powder Diffraction File (PDF-2) and the Crystallography Open Database (COD).
Samples LC6, LC7, LC7_black, LHA-1 and LHA-2 were analysed by differential thermal analysis coupled with thermogravimetry (DTA-TG) using a Netzsch STA 409C/CD instrument. Approximately 150 mg of powdered sample was put in an alumina crucible under a N2(g) atmosphere with a flow rate of 80 mL min −1 , in a range between 25 to 800 °C at 10 °C min −1 using ~80 mg of an alumina standard (Perkin-Elmer 0419-0197). The same powder samples were analysed by Fourier transform infrared spectroscopy (FTIR) using a 2000 FTIR spectrometer (Perkin-Elmer System, Waltham, MA, USA) at a range for vibrational spectra of 400-4000 cm −1 , at the Departament de Mineralogia, Petrologia i Geologia Aplicada de la Facultat de Ciències de la Terra (Universitat de Barcelona).
The major, minor and trace element compositions discussed in this paper were drawn from references [16,17] except data for sample LHA that were obtained by X-ray fluorescence (XRF) and inductively coupled mass spectrometry (ICP-MS; after acid dissolution) at the Actlabs Laboratories (Ontario, Canada) as described in the aforementioned papers.
Polished sections and polished thin sections of the selected samples were examined under a transmitted and reflected light optical microscope at the Departament de Mineralogia, Petrologia i Geologia Aplicada de la Facultat de Ciències de la Terra (Universitat de Barcelona), while samples containing Co and Mn mineralisations were carbon coated and studied in a scanning electron microscope (SEM) Quanta 200 FEI, XTE 325/D8395, with an INCA energy dispersive spectrometer (EDS) microanalysis system, and a field emission SEM (FE-SEM) Jeol JSM-7100 equipped with an INCA Energy 250 EDS, under 20 kV and 5 nA, at the CCiT-UB. Co-Mn-bearing minerals were analysed by electron microprobe analyser (EMPA) at the CCiT-UB by using a JEOL JXA-8230 electron microprobe, equipped with five wavelength-dispersive spectrometers and an energy-dispersive spectrometer. The operating conditions were 20 kV accelerating voltage, 15 nA beam current, 2 µm beam diameter and a counting time of 20 s per element. The calibration standards used were: wollastonite (Si, Ca), corundum (Al), orthoclase (K), hematite (Fe), periclase (Mg), rhodonite (Mn), NiO (Ni), metallic Co (Co), rutile (Ti), albite (Na), Cr2O3 (Cr), V (V), Sc (Sc), and baryte (Ba).
Thermodynamic calculations were carried out with GibbsStudio software [30] based on the PHREEQC code [31] and the thermodynamic data llnl.dat database supplied with PHREEQC code and modified, when specified, to complete Co and Mn aqueous species and solid-phases thermodynamic data.
Although in minor amounts (up to 0.3%), lithiophorite was also identified in the Loma de Hierro samples (Figure 4d,e). The main mineral phases were goethite, gibbsite, quartz, lizardite and smectite. Micro-XRD scans of sample LH-10 ( Figure 4e) were dominated by typical XRD peaks of goethite. However, whereas the LH-10_2 curve exhibited the XRD signature of gibbsite, the LH-10_1 curve showed several features that can be attributed to lithiophorite and/or asbolane ( Figure 4e). Although it is difficult to distinguish these two minerals solely by XRD due to their similar structure and lattice spacings, it should be noted that the PDF-2 pattern for lithiophorite (00-041-1378) provides a better match to the experimental results. Thermogravimetry showed that samples from Loma Caribe experience a mass loss of about 4% before 260-280 °C, probably due to the loss of adsorbed water for temperatures below 110-160 °C [33,34] and to the transformation of gibbsite to boehmite, which is reported to occur at temperatures between 150 and 260 °C [34] (Figure 5a). Between 260 and 320 °C, the mass loss is between 5 and 8%. The latter process can be ascribed to the loss of the OH − groups in the reaction from goethite to hematite (Figure 5a). This reaction (i.e., 2FeOOH  Fe2O3 + H2O) occurs at temperatures between 300 and 344 °C [35] or 270 and 330 °C [36], and it is commonly associated with a mass loss of 10% or less if there are secondary phases. This mass loss coincides with an endothermic peak around 325 °C in the DTA curve ( Figure 5b) that is considered as the result of the breakdown of goethite [34][35][36].
Differential thermal analyses of samples LC7 and LC7_black revealed an additional endothermic peak at T~450 °C, associated with a mass loss of ~6%. This peak can be attributed to the dehydroxylation of AlOOH (AlOOH  Al2O3 + H2O) (even though it was not detected by XRD) or of an amorphous Al-hydroxide, that may occur between 450-600 °C and 400-500 °C, respectively, or to the loss of OH in lithiophorite (430-500 °C) [37]. Llorca and co-authors [38] also measured an endothermic peak around 420-460 °C in samples of asbolane, lithiophorite and asbolane-lithiophorite intermediates of New Caledonia that was attributed to the loss of hydroxyl groups.
Samples LHA-1 and LHA-2 from Loma de Hierro showed distinctive TG and DTA analyses, compared with those of Loma Caribe (Figure 5c,d). Specifically, an endothermic peak at ~130 °C, associated with a mass loss around 10% (sample LHA-1), as well as two minor peaks around 525 and 600 °C could be distinguished. The peak at 130 °C was due to the loss of adsorbed water from clays such as smectite [37], although [39] indicated that it could also be attributed to one of the endothermic reactions shown by asbolane. Peaks at 525 and 600℃ could be associated with asbolane (570-625 °C) [38]. However, research by Medeiros Ribeiro and co-authors [39] suggests that the peak around 600 °C refers to the dehydroxylation of the interlayer hydroxide of chlorite. Depending on the crystallinity of Mn-oxyhydroxides, the characteristic IR frequencies appeared as a single broad band, or in distinct bands in the range of 425-530 cm −1 (associated with Mn-O bonds of the MnO6 octahedra) and in the range of 3100-3500 cm −1 (stretching modes of OH groups) [40,41]. The band around 1600 cm −1 was the water bending mode, that may suggest that at least some of the OH is H2O [41,42]. According to IR spectra ( Figure 6), samples were mostly poorly crystalline, although peaks around 3400, 1600 and 1000 cm −1 could be assigned to lithiophorite, while those around 3600-3500 cm −1 were related to gibbsite [43,44]. The shadowed area (800-600 cm −1 ) in samples LHA-1 and LHA-2 included some peaks related to Mn-oxides and clays [45], and the peak around 1200 cm −1 could be related to kaolinite. It must be noted that mineral contamination can also affect IR spectra.

Mn-Oxyhydroxides
In the Loma Caribe samples, Co-rich Mn-oxyhydroxides were in close association with Fe-Al-oxyhydroxides. Three major textural types were identified.
The first type was characterised by banded and colloform aggregates (Figure 7), which is the most common texture observed in Mn-oxyhydroxides. Some bands within the colloform aggregates appeared to be formed by parallel and/or radial fibrous aggregates (Figure 7a-c). Figure 8 shows the spatial distribution of some elements within these colloform aggregates and fibrous bands. In general, a close spatial association between Mn, Ni, Co and Al was observed, while, conversely, Si had a mirrored distribution, and no significant variations of Fe or Mg were observed. Colloform aggregates locally displayed distinct external rims ( Figure 7a) and in rare occasions, these particles had a uniform core (Figure 7c).
The second Mn-oxyhydroxide type was slightly less abundant, and consisted of globular to spherulitic aggregates, with sizes from some tens to several hundreds of micrometres; locally, larger aggregates appeared to be hollow-cored, filled with goethite and/or gibbsite (Figure 7d,e). These globular to spherulitic aggregates were composed of short, flaky crystals (Figure 7f). The least abundant occurrences were discrete, micrometre-sized, Ba-rich angular grains (with high mean atomic number Z in BSE images, Figure 7d,f) disseminated within some globular-spherulitic aggregates (of lower mean atomic number Z). These last two types were only observed in the saprolite horizon sample.  In Loma de Hierro (Venezuela), Mn-oxyhydroxide grains are a minor component of the mineralogical assemblage. According to their textural features, three types were identified ( Figure 9). Firstly, as in Loma Caribe, they typically displayed banded colloform features, as kidney-shaped (Figure 9a), elongated (Figure 9b), and sub-rounded ( Figure  9c,d) grains, measuring a few hundreds of micrometres in diameter or length, surrounded by goethite and other oxides. Certain rounded grains displayed one or more concentric rims having an homogeneous compositional footprint (Figure 9c,d). Locally, Ni and Co were concentrated in the outer rims, while the Mn grade was higher in the core ( Figure  10).   The second type, less common, were short fibres of a few tens of micrometres in length, in close association with Fe-oxide and within serpentine veinlets (Figure 9e).
Finally, the least common Mn-oxyhydroxide in Loma de Hierro occurred as fragmented tabular aggregates, dispersed in the goethite matrix (Figure 9f).
Most of the compositional analyses of Mn-oxyhydroxides had a Mn content within 20 and 40 wt.% Mn, with contents of Ni and Co up to 16 and 10 wt.%, respectively. Some analyses from the Mn-Co rich zone (sample LHA) were the richest in Mn content (~60 wt.% of Mn) although they showed very low Ni and Co contents ( Figure 11).
In general, samples from Loma de Hierro were richer in Mn than those of Loma Caribe (Figure 11a,b). Both sets of samples showed similar Ni contents but those of Loma Caribe were found to be Co richer. A common feature observed in all samples was a positive correlation between Co and Al contents (Figure 11c).   Table 2. Representative EMP analyses (in weight percent) of the studied Co-Mn-oxyhydroxide types in Loma de Hierro (Venezuela). <d.l.: below detection limit; F-fibres; CA-colloform aggregates; C-core; R-rim; T-tabular.

Co-Mn Bearing Minerals in Loma Caribe and Loma de Hierro
Most of the Loma Caribe and Loma de Hierro Mn-bearing minerals from the oxide horizon had a composition between Ni asbolane ((Ni,Co)xMn(O,OH)4·nH2O) and lithiophorite ((Al,Li)MnO2(OH)2) ( Figure 13). These minerals mainly formed colloform to tabular textures in the case of Loma de Hierro, while in Loma Caribe they were also present as fibrous aggregates. The purest Ni asbolane was found in sample LH-10 of Loma de Hierro. However, in the saprolite horizon of Loma Caribe, Mn-bearing minerals were mainly globular or spherulitic lithiophorites. These observations were consistent with the results of DRX, DTA-TG and FTIR, which clearly indicated the occurrence of lithiophorite in sample LC-7.
The highest Mn contents were found in sample LHA, from Loma de Hierro. They were explained by the abundance of grains with a core rich in Mn (up to 80 wt.% MnO, Table 2), but poor in Co and Ni. Hence, these were not considered lithiophorite nor lithiophorite-asbolane intermediates. These grains were surrounded by rims with alternate Mn-rich or Co/Ni-rich compositions.
Putzolu and and co-authors [15] described two different generations of romanèchite, which despite their close paragenetic association, showed different chemical compositions. Romanèchite I had an average BaO content of 7.64 wt.%, with low amounts of NiO and CoO (average values of 1.21 and 0.72 wt.%, respectively), while romanèchite II had lower BaO (average 2.68 wt.%) and higher NiO and CoO (average values of 2.90 and 1.48 wt.%, respectively). Ba-rich Mn-oxyhydroxides identified in Loma Caribe (Figure 7f) presented slightly higher BaO contents (up to 10.5 wt.%, Table 1, Figure 7f) than those of romanèchite I of [15], although NiO and CoO contents were lower (less than 0.7 and 1.4 wt.%, respectively). Pure romanèchite has the structural formulae Ba0.66Mn5O10·34H2O, in which the content of Ba is around 20 wt.% BaO [48]. In our case, as in [15], Ba content was far below the stoichiometric value.
Furthermore, the association of Mn (and Co) with phyllosilicates (Table 3) is not uncommon, as it has also been observed in serpentine minerals [49] and smectite clays [50]. Figure 13. Ternary plots representing Mn, Ni, Co, Al contents (cation proportions calculated on the basis of 100 oxygens) obtained from EMPA analyses on Mn-oxyhydroxides (lithiophorites, lithiophorite-asbolane intermediates and asbolane) from Loma Caribe and Loma de Hierro (this study), Loma Ortega [46] and Moa Bay [47].

Comparison with other Co-Mn Mineralisations in Lateritic Systems
The Ni and Co of Mn-oxyhydroxides from the Loma Caribe and Loma de Hierro were similar to those of other Ni laterites of the Caribbean region. Their Ni grade was similar to that of Moa Bay, and a bit lower than that measured in Loma Ortega [46,47], however, the Co contents were considerably higher in Loma Caribe and Loma de Hierro when compared with those of Loma Ortega and Moa Bay (Figure 11a,b).
The Al content in Mn-oxyhydroxides object of this study was higher in Loma Caribe and Loma de Hierro than in Loma Ortega and Moa Bay (Figure 11c). In the hybrid hydrous Mg silicate-clay silicate type Ni laterite deposit of Loma Ortega (Falcondo mining district, Dominican Republic), Co-bearing Mn phases were observed as coatings or vein fillings, close to quartz, goethite and serpentine II, replacing pyroxene, olivine and serpentine along grain boundaries and fractures [46]. As shown in Figure 13, they were mostly Ni asbolanes with Mn and Ni contents between 11 and 70 wt.% MnO, and up to 23 wt.% NiO, respectively. In most cases, Co content was below 1 wt.% CoO, although in some cases it graded up to 6.5 wt.% CoO. Al and Fe were highly variable, ranging from 0.1 to 11.5 wt.% Al2O3 and from 0.1 to 21 wt.% FeO. In general, in these samples, the Fe and Al contents were anti-correlated. In Moa Bay, an oxide type laterite deposit in Cuba, Mn-bearing minerals, mostly lithiophorite-asbolane intermediates (Figure 13), are found as veins and coatings along fractures at the lowest part of the oxide horizon [47]. In these minerals, Mn content ranges from 23 to 37 wt.% MnO, and present high amounts of Ni (12%-21% wt. NiO) and Co (1-8 wt.% CoO). In the analysed samples, Al and Fe contents ranged from 4 to 17 wt.% Al2O3 and from 3 to 11.4 wt.% Fe2O3, again inversely correlated.
The chemical footprint of Mn minerals reported in this study was also similar to other lithiophorite, asbolane-lithiophorite intermediates, and cryptomelane recorded in other localities (Table 4). Most data reported in the literature come from studies from New Caledonia, where Co-bearing minerals occur as colloform or fibrous cryptocrystalline to microcrystalline aggregates, intimately intergrown with Mn-Fe-oxides and other poorly crystallised minerals, which appear as spots or coatings, onion peel structures, pseudomorphs of silicates or (rhizo)concretions [51]. The identified Co-bearing minerals in New Caledonia were phyllomanganates (asbolane, lithiophorite and intermediates) with a variable range of compositions between Ni and Co; being lithiophorite, more abundant in those weathering profiles with plagioclase lherzolite as a protolith ( [51] and references therein), although other Co and Mn minerals were also identified (heterogenite, cryptomelane, ramsdellite, todorokite). Several authors [38,[51][52][53][54][55] have investigated the crystal chemistry and composition of several samples of Ni and Co-bearing minerals of Poro, Tiébaghi and Koniambo.
In the Nkamouna deposit (Cameroon), lithiophorite is the main Mn-phase, and is commonly endowed in Co. It was identified as bluish to black cryptocrystalline to finegrained concretions, commonly associated with gibbsite and magnetite [56]. Other phases were pyrolusite, cryptomelane (associated with pyrolusite) and lithiophorite-asbolane intermediates.
In the Wingellina oxide-type laterite deposits (Western Australia), Mn-oxyhydroxides were studied by [15], who identified lithiophorite-asbolane intermediate dominant phases and some other minor phases such as romanèchite, ernienickelite-jianshuiite, manganite, and birnessite. Mn-oxyhydroxides of the Palawan deposit (Philippines) were studied by [57], who identified lithiophorite-asbolane intermediates as the main host of Ni and Co in the oxide horizon, while [58] and [59] studied the asbolane from the products of weathering of the Lipovsk serpentine deposit (Middle Urals, Russia), which occur as platy fragments associated with goethite.

Formation of Co-Mn Bearing Minerals in Laterite Deposits
Mn-bearing minerals in Loma Caribe and Loma de Hierro were mainly found in the transition from the oxide horizon to the saprolite (Figure 2). Their formation at this level within the laterite profile is not uncommon, as it has also been observed in other deposits worldwide [55,60].
The shift of soil solution from acidic to slightly alkaline is the main trigger for the formation of Mn oxides at this interface in the laterite profile ( [55] and references therein). In the oxide zone, Mn is commonly sunk by goethite, hematite or other Fe-oxyhydroxides. Due to weathering, temperature, and humidity, Mn can be leached from Fe-bearing minerals and transported downwards as colloidal complexes or aqueous complexes [55,61,62]. In the transition to the saprolite horizon, due to the increase in pH and solution saturation, Mn-oxides precipitation may occur [61]. Although being thermodynamically favoured, oxidation of Mn(II) is very slow in natural waters, but Fe(III) and especially bacterial and fungal activity can accelerate this process by several orders of magnitude [61,63,64]. The presence of lithiophorite, however, suggests that the oxidation of Mn(II) is a slow process [58], as the structure of this mineral needs time to form. Figure 14 shows predominance diagrams (Eh-pH) of the Mn-Co-H2O system calculated with the Gibbs Studio code [30] based on Phreeqc [31] and the llnl.dat database from [31]. The thermodynamic data for Co aqueous species Co(OH)3and Co(OH) + were taken from the ThermoChimie database [65] and [66]. Due to the lack of thermodynamic data for lithiophorite, asbolane or lithiophorite-asbolane intermediate compounds, birnessite was considered instead, in the construction of predominance diagram. Birnessite, being part of the same phyllomanganate group, can be thus considered representative also for lithiophorite and lithiophorite-asbolane [48]. Furthermore, it has to be emphasised that lithiophorite is considered to form after the alteration of birnessite (and other less mature Mn phases as romanechite) [55]. Calculations were performed allowing only the formation of birnessite, Mn(OH)3(s) and Mn(OH)2(s). Other Mn solid phases such as pyrolusite, todorokite, hausmannite and manganite were not allowed to precipitate since their higher thermodynamic stability inhibits the formation of birnessite. The outcome of the thermodynamic calculation showed that an increase in pH in the oxidised zone allows the formation of Mn-oxyhydroxides.  The type of mineral that precipitates depends on the chemical conditions, water composition, Eh and pH. The authors of [56] also observed rim/core textures in the Nkamouna laterite (Cameroon), with pyrolusite at the centre of cryptomelane-lined cavities. The authors considered that as the presence of other cations inhibits the formation of pure Mnsolid phases, pyrolusite should have formed after the precipitation of cryptomelane, which would have removed other cations from the solution. In Loma Caribe and Loma de Hierro, core grains were richer in Mn than rims. This observation agrees with those of [15,56]. However, textural features suggest that rims overgrew the core instead of being formed first (Figure 9c).
In Loma Caribe and Loma de Hierro, lithiophorite and lithiophorite-asbolane intermediates were the most common minerals observed. Most researchers consider that these minerals are a weathering product of previous Mn-oxide minerals [15,56,67,68], but in other cases there is no direct evidence of a primary mineral [57]. In Loma Caribe and Loma de Hierro, Mn-bearing mineral textures did not indicate alteration from previous bearing minerals, so lithiophorite precipitation from soil solution cannot be discarded [56].
Due to its higher content in Al, it is considered that the stability of lithiophorite is higher than that of other Mn minerals, as Al inhibits electron transfer hindering Mn(III) reduction, and, therefore, mineral dissolution under supergene regime ( [52] and references therein).
Formation of lithiophorite implies the incorporation of Al in the system. In either Loma Caribe or Loma de Hierro, Al-bearing minerals such as gibbsite and Mn-bearing phyllosilicates have been identified, confirming the availability of Al in the system, that ultimately may come from clinopyroxene and plagioclase minerals from the protolith (lherzolite, clinopyroxene-rich harzburgite, gabbro and diorite) [23][24][25]69,70].
Mn-oxyhydroxides, and in particular those of biogenic origin, present a high sorption capacity because of their structure, variable (but low) crystallinity with a large surface area, and a very low point of zero charge that promotes negative charged surface areas under common soil pH values [64,71,72]. Phyllomanganates also present a high cation exchange capacity [73]. Cobalt is easily adsorbed onto Mn-bearing minerals' surfaces, and despite Co(II) being the most thermodynamically stable oxidation state in environmental conditions, it is easily oxidised to Co(III) by reduction of Mn(IV) to Mn(III) under supergene conditions [7,71]. With ageing, Co enrichment increases, and the initially adsorbed Co becomes permanently bound [56,74]. Repetition of this process would have led to an enrichment of Co in relation to Mn. This is consistent with the highest content of Co in the saprolite samples from Loma Caribe (Figure 11b). At higher pH, precipitation of Co hydroxide minerals is enhanced (Figure 14a), which might represent an explanation of the Co enrichment in saprolite [51]. Cobalt behaviour is also affected by Eh conditions. Under reducing conditions, Co is released into solution because Co(III) is directly reduced or by reductive dissolution of Mn and/Fe oxides ( Figure 14) [75]. All these oxidation-reduction processes take place under supergene conditions, implying good drainage and oxygenated conditions, such as those near a fluctuating water table [51,56]. Therefore, Co-Mnbearing minerals can be considered a reliable indicator of the position of the paleo-water table within a laterite profile. Similar to Ni, the primary sources of Mn and Co in Loma Caribe and Loma de Hierro can be found in the protolith [16,17]. Continuous weathering under tropical conditions leaches most of the soluble elements of the protolith, and the least soluble ones, such as Fe and Al, accumulate, forming their own minerals [6,51]. Nickel, Mn and Co are first retained (e.g., sorption processes and/or replacement of Fe) in Fe and Al-bearing minerals [6], and ultimately leached and concentrated in the saprolite-to-oxide zone interface.

Conclusions
The Co mineralisations in the Ni-Co laterite profiles of Loma Caribe and Loma de Hierro were mainly associated with Mn-oxyhydroxide minerals, with a composition between Ni asbolane and lithiophorite, found in the transition between the saprolite and the oxide zone. In Loma Caribe, Co-bearing Mn-oxyhydroxide minerals showed colloform aggregate, and globular to spherulitic granular textures, while in Loma de Hierro, they displayed banded colloform, fibrous or tabular textures. In Loma de Hierro, Mn-Co phyllosilicates were also identified. Most of the compositional analyses of Mn-oxyhydroxides yielded 20 and 40 wt.% Mn, with Ni and Co up to 16 and 10 wt.%, respectively, and only in a few cases were Mn contents of up to 54 wt.% MnO on average, measured.
These type of Co mineralisations have also been identified in other laterite deposits worldwide. Formation of Mn-oxyhydroxides in the transition zone is mainly due to the change of pH and Eh occurring in this area and to the saturation of pore solution that may lead to the formation of these minerals by precipitation. Co is considered to be firstly sorbed onto Mn-bearing minerals, after being oxidised because of Mn reduction under supergene conditions, becoming permanently bound into Mn-minerals. Therefore, these minerals are considered to be an indicator of the water level during laterite profile formation.

Acknowledgments:
We are grateful to Maite Garcia Vallès and to the staff of the Centres Científics i Tecnològics of the Universitat de Barcelona, Barcelona (Spain) (CCiT-UB) for their assistance in measurements. Micro-XRD measurements were performed at BL04-MPSD beamline of ALBA Synchrotron, Cerdanyola del Vallès, Barcelona (experiment AV-2019023550). We would like to thank Oriol Vallcorba and Catalin Popescu for their support during the XRD experiments at ALBA Synchrotron. IDAEA-CSIC is a Severo Ochoa Center of Research Excellence (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). We are also grateful to the reviewers for improving the quality of this manuscript.