Garnierites and garnierites: Textures, mineralogy and geochemistry of garnierites in the Falcondo Ni-laterite deposit, Dominican Republic

Abstract Garnierites (Ni–Mg-bearing phyllosilicates) are significant ore minerals in Ni-laterites of the hydrous silicate-type. In the Falcondo Ni-laterite deposit (Dominican Republic), garnierites are found within the saprolite horizon mainly as fracture-fillings and thin coatings on joints. Field observations indicate an important role of active brittle tectonics during garnierite precipitation. Different greenish colours and textures can be distinguished, which correspond to different mineral phases, defined according to X-ray diffraction (XRD) and electron microprobe (EMP) analyses: a) talc-like (10 A-type), b) serpentine-like (7 A-type), c) a mixture of talc- and serpentine-like, and d) sepiolite-like types. Compositional data indicate continuous Mg–Ni solid solution along the joins lizardite–nepouite (serpentine-like), kerolite–pimelite (talc-like) and sepiolite–falcondoite (sepiolite-like). In general, talc-like garnierite is dominant in Falcondo Ni-laterite and displays higher Ni contents than serpentine-like garnierites. EMP analyses showing deviations from the stoichiometric Mg–Ni solid solutions of serpentine and talc are best explained by 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 relationships between textural position, sequence of crystallization and mineral composition of the studied garnierite samples. These results indicate several stages of growth with variable Ni content, pointing to recurrent changes in the physical–chemical conditions during garnierite precipitation. In addition, our detailed mineralogical study of the Falcondo garnierites revealed that the different types identified have characteristic H 2 O content and SiO 2 /MgO ratios, which play important roles during the pyrometallurgy process.


Introduction
Although Ni (±Co) laterite deposits account only for about 40% of the current world's annual Ni production, they host over 60% of the world land-based Ni resources (Gleeson et al., 2003;Kuck, 2013), and the amount of Ni being extracted from laterite ores is increasing steadily (Mudd, 2010). About 10% of the world's Ni resources are found in the Caribbean region, mostly in the northern part, and include the Moa Bay and Punta Gorda deposits in eastern Cuba and the Falcondo deposit in central Dominican Republic (Dalvi et al., 2004;Lewis et al., 2006;Nelson et al., 2011). Other Ni-laterite deposits in the region include the Gloria deposit in Guatemala (Golightly, 2010) and the Meseta de San Felipe deposit in Camagüey in Central Cuba (Gallardo et al., 2010a). On the other hand, the major Ni laterite resources in the southern Caribbean include the Cerro Matoso in Colombia (Gleeson et al., 2004) and Loma de Hierro in Venezuela (Soler et al., 2008), both presently exploited.
Ni (±Co) laterite deposits are regoliths formed by the chemical weathering of ultramafic rocks, mainly in tropical and subtropical latitudes (e.g. Golightly, 1981;Elias, 2002;Freyssinet et al., 2005). Under the high temperature and intense rainfall, typical of these environments, the most soluble elements (especially Mg and Si) are leached from primary rock-forming ferromagnesian minerals, and the least mobile elements (especially Fe, Al) accumulate in successive horizons of the lateritic profile (e.g. Freyssinet et al., 2005;Golightly, 2010).
Although there is no widely accepted terminology and classification, Ni (±Co) laterites are commonly classified into three categories, according to the main Ni ore assemblage (Brand et al., 1998). These include: i) Oxide laterite deposits in which the ore assemblage is principally Fe oxyhydroxides; ii) Clay silicate deposits dominated by Nirich smectites; and iii) Hydrous Mg silicate deposits in which the ore is mainly Mg-Ni phyllosilicates (including garnierites). The hydrous Mg silicate deposits generally have the highest Ni grade (1.8-2.5 wt.% Ni) and are characterised by a thick serpentinedominated saprolite horizon covered by a relatively thin Fe-oxyhydroxide-dominated limonite horizon (laterite sensu stricto horizon). These deposits are formed under conditions of a low water table and continuous tectonic uplift (Freyssinet et al., 2005;Butt and Cluzel, 2013).

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In terms of production and reserves, the Falcondo deposit is the largest hydrous Mg silicate-type Ni-laterite deposit of the Greater Antilles, with estimated Ni reserves of about 79.2 million dry tonnes at a grade of 1.3 wt.% Ni (Falcondo Annual Report, 2010; http://www.falcondo.com.do/ES/Publicaciones/brochures/Memoria_FALCONDO_2010 .pdf). As other hydrous Mg silicate-type deposits worldwide, Ni-bearing serpentines and particularly garnierites are concentrated in the lowermost part of the saprolite horizon, toward the base of the profile (Freyssinet et al., 2005).
The term garnierite is generally used to refer to the group of green, fine-grained poorly crystallized Ni-bearing magnesium phyllosilicates, which include serpentine, talc, sepiolite, smectite and chlorite; often occurring as mixtures (e.g. Faust, 1966;Brindley and Hang, 1973;Springer, 1974;Brindley, 1978). Although garnierite is not recognized as a mineral species by the International Mineralogical Association (IMA), it is a convenient field term used by mine geologists for all green Ni-silicates when more specific characterization is not possible (Brindley, 1978).
The most common garnierite minerals found in nature are lizardite-népouite and kerolite-pimelite (Brindley, 1978). For these minerals, the terminology "serpentine-like" (or 7 Å-type) and "talc-like" (or 10 Å-type) garnierites, respectively, has been widely used Hang, 1973, Brindley andMaksimović, 1974;Wells et al., 2009;Galí et al., 2012). It is important to note here that the term "talc-like" does not refer to the normal composition and/or structure of talc (Brindley and Hang, 1973). Actually, the characterisation of talc-like garnierites is controversial and both the Mg and Ni talclike end members kerolite and pimelite, respectively, are discredited mineral species by the IMA. Despite being historically described as hydrated talc-like minerals, kerolite

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and pimelite were classified into the smectite group by Faust (1966). In contrast, other authors proved a structure with talc affinity, since kerolite and pimelite do not exhibit intracrystalline swelling (e.g. Slansky, 1955;Kato, 1961;Maksimović, 1973;Brindley and Hang, 1973). In addition, an intermediate phase between talc-like and serpentinelike end members, karpinskite (Mg,Ni) 2 Si 2 O 5 (OH) 2 , was described by Rukavishnikova (1956), however it is not accepted as a mineral species by the IMA.
Several mineralogical studies using different techniques have been published on garnierites since their discovery in 1864, most of them during the late 1960s to early 1980s. The majority of the publications since the 1960's focussed on the composition of garnierites from New Caledonia (Caillère, 1965;Troly et al., 1979;Pelletier, 1983;1996), Indonesia (Golightly, 1979) and Australia (Elias et al., 1981). It is only in the past few years that detailed additional information, including mode of occurrence in the field, petrography and relations between garnierites and their host rocks, is provided (Cluzel and Vigier, 2008;Wells et al., 2009 (Proenza et al., 2008;Tauler et al., 2009;Galí et al., 2012). However, except for the sepiolite-falcondoite series (Springer, 1976;Tauler et al., 2009), little work has been done on the mode of occurrence, textures and composition of these Ni-phyllosilicates.
In this paper we summarize the information on the garnierites from the Falcondo Nilaterite deposit up to the present, and provide detailed descriptions of their occurrence in the field and textural relationships, as well as new results on mineralogy and mineral chemistry. Our study focuses on serpentine-and talc-like garnierite, presents data from the saprolite host and also includes new information on the sepiolite-falcondoite garnierite. The aim of this work is to gain further insight into the origin of garnierites in the Falcondo deposit.

Geological setting
A C C E P T E D M A N U S C R I P T

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The main nickel laterite deposits in the central Dominican Republic occur over the serpentinised Loma Caribe peridotite belt ( Fig. 1), which is about 4-5 km wide and extends NW for 95 km from La Vega to Loma Sierra Prieta, north of Santo Domingo; its south-eastern part is exposed as thin fault slices Jiménez, 1991, Lewis et al., 2006). The peridotite is interpreted, from airborne magnetics and drilling, to extend south-eastward below the surface to the coast (Lewis et al., 2006). The Loma Caribe belt is bounded by reverse faults dipping eastward at 75-80º (Mann et al., 1984). The major faults within the body are shear zones that strike parallel to the northwest trend of the belt and the foliation of the serpentinised peridotite. These faults and other basic structures in the central area of the Loma Caribe peridotite massif have been mapped (Haldemann et al., 1979; Fig. 1), although they have not been systematically studied.
Based on the mineralogy and textural features of the ultramafic rocks, the Loma Caribe peridotite was interpreted by Lewis and Jiménez (1991) as a (serpentinised) harzburgitic oceanic mantle forming part of a dismembered ophiolite complex. The ultramafic body includes podiform chromitite pods within dunites and suggests that most of the peridotite is part of the transition zone of the mantle section of harzburgite-type alpine peridotite complexes.

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The Loma Caribe ultramafic rocks originally formed in the upper mantle below an evolving suprasubduction oceanic lithosphere (Lewis et al., 2006). We infer that the peridotites would have likely been partially serpentinised by the hydrothermal reaction of oceanic water with the mantle rock as they moved upwards through the forearc/arc lithosphere prior to their final emplacement in their current crustal tectonic position.
The tectonic emplacement occurred as early as the late Albian, as a result of the collision of an oceanic plateau (the Duarte plateau terrane) with the primitive Caribbean island-arc (Maimón-Amina terrane) at Aptian time (Lewis et al., 2002). This event resulted in the northward emplacement (obduction) of the peridotite over the Maimón Formation. In the middle to late Eocene, Hispaniola underwent southwest contraction (Mann et al., 1991). Many of the mid-Cretaceous thrust structures were reactivated at this time, resulting in thrusting of the peridotite in a northeast direction over the Peralvillo Fm. (Draper et al., 1996). Uplifting of the northwestern margin of the Cordillera Central occurs along three reverse or thrust mapped faults (Bonao, Hatillo and Hispaniola Fault Zones). The northwesterly striking segment of the Oligocene age strike-slip Hispaniola Fault Zone comprises the Loma Caribe ultramafic body (Mann et al., 1984). Further exhumation of the peridotite in the Oligocene is well-documented in the La Vega area (Lewis and Jiménez, 1991).
Both "fresh" and serpentinised peridotites have been exposed to weathering and erosion since the early Miocene. These processes were followed by uplift, block faulting and subsequent cycles of laterisation (Haldemann et al., 1979). The Miocene land surface was broken into blocks by vertical movements related to transpressional deformation along major faults. At least four physiographic cycles have been recognised corresponding to different surface levels (Lewis et al., 2006). Geomorphological observations indicate that block-faulting postdates the first of the laterisation cycles (Haldemann et al., 1979).
Under supergene conditions, the serpentinised peridotites of Loma Caribe evolved in a similar way to that of the ophiolite-related, hydrous Mg silicate deposits found (and exploited) in the arcs of the West Pacific, e.g. in the Philippines, Indonesia and New Caledonia (Golightly, 2010).

The Ni-laterite profile
A C C E P T E D M A N U S C R I P T

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As other hydrous silicate-type Ni-laterite deposits, the weathering profile in Falcondo is divided into three main horizons: the unweathered protolith at the bottom, a thick saprolite horizon and a limonite horizon at the top (Fig. 2). However, the mine geologists have subdivided profile into six zones (A to F) on the basis of nickel and iron contents, texture, and proportion of rocky fragments. These zones are, from the base to the top: unweathered ultramafic (F), serpentinised peridotite (E), hard serpentine (D), soft serpentine (C), lower limonite (B) and upper limonite (A), which may be covered by a hematitic cap ( Fig. 2; Haldemann et al., 1979;Lithgow, 1993;Lewis et al., 2006).
The zones E, D and C represent the saprolite horizon, whereas the zones B and A correspond to the limonite horizon. The thickness of the laterite profile varies from 1 to 60 meters (Haldemann et al., 1979). Likewise, the thickness of the different zones in the profile varies vertically and laterally in all exposures in the Falcondo deposit ( Fig.6 in Lewis et al., 2006).
The flat summit levels of Guardarraya, Peguera and Taína occur at an elevation of 610 ± 40 m. They are remnants of the Miocene land surface which had undergone laterisation followed by uplifting, block-faulting and subsequent cycles of laterisation (Haldemann et al., 1979).

Mode of occurrence of garnierites in the Falcondo Ni-laterite deposit
Garnierite mineralisation exposed in the mining pit are concentrated in discrete zones from tens to one hundred metres long and up to five metres thick (Fig. 3a, b). They are found mainly within the lowermost part of the saprolite horizon ( Fig. 3c) as well as in unweathered serpentinised peridotite at the base of the profile (Fig. 3d) and in the soft serpentine zone in the upper saprolite horizon (Fig. 3e).

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The garnierites occur mainly as mm-cm veins in fractures ( Fig. 3c-h), as well as thin coatings on joints and fault planes which are oriented in random directions ( Fig. 3i-k).
Garnierites are also found in two different kinds of breccias: i) weathered serpentinised ultramafic rock (saprolite) fragments cemented by garnierite, and ii) silicified Nisepiolite fragments cemented by a later generation of garnierite. In addition, another type of breccia is found in the laterite profile, not containing garnierite, and consisting of saprolite clasts cemented by Ni-free sepiolite and silica/quartz (Fig. 3m, n).
The garnierites occur in various shades of green. Apart from the sepiolite-falcondoite species, with typical white to pale green colour and remarkable schistosity, four types of garnierite have been distinguished according to their colour and will be hereafter named type I to IV: Type I is yellowish pale green, type II is apple green, type III is dark green, and type IV is bluish green. Type IV is the dominant phase and the other phases are less common but quite distinctive. Type I occurs as millimetre-thick infillings in fractures ( Fig. 3c). Types II and III form millimetre to centimetre-scale fracture infillings and coatings ( Fig. 3c, g, i). Finally type IV occurs as thin coatings and fillings in fractures ( Fig. 3g), and coexists with other garnierite types, typically superimposed to the yellowish green and apple green types. In addition, all the above four types of garnierites also occur as cementing material in breccia with saprolite clasts. Finally, the sepiolite-falcondoite (classified here as type V) commonly forms centimetre-thick vein fillings (Fig. 3h).

Sampling
A total of forty samples were collected from the saprolite horizon of different lomas in the Falcondo Ni-laterite deposit. The samples were analysed by X-ray powder diffraction (XRD), optical and scanning electron microscopy (SEM-EDS) and electron microprobe (EMP) for their mineralogical and chemical characterization. From the forty samples, twelve were selected as the most representative of the different types of garnierites described above (Table 1, Fig. 4). All analyses were performed in the Centres Científics i Tecnològics of the Universitat de Barcelona, Spain.

Analytical and computation techniques
A C C E P T E D M A N U S C R I P T

X-ray powder diffraction (XRD)
In order to obtain the most pure phase specimens for XRD, different greenish-coloured garnierite types were carefully separated hand-picking and ground in an agate mortar.
Powder specimens were set in standard cylindrical sample holders (16 mm of diameter and 2.5 mm thick) by manual pressing using a glass plate to get a flat surface.
During the analysis, the sample was spun at 2 revolutions per second. A variable divergence slit kept the area illuminated constant (10 mm) and a mask used to limit the length of the beam (12 mm). Axial divergence Soller slits of 0.04 radians were used.
Samples were scanned from 3 to 80º 2 with step size of 0.017º and measuring time of 50 seconds per step, using a X'Celerator detector (active length = 2.122º).

Electron Microprobe (EMP)
Polished thin sections previously examined in an Environmental Scanning Electron The quantified pixels of the X-ray distribution images, appropriately filtered for minerals and/or voids and polish defects, were represented in ternary diagrams. These diagrams were calculated using DWImager software and are expressed as absolute frequency (colour code). Ternary diagrams with the point analyses of the same scanned area were prepared using CSpace software (Torres-Roldán et al., 2000), and overlain onto the ternary plots obtained with DWImager.

Mineralogy and textures of the Falcondo garnierites and related rocks
The hand specimen descriptions of the twelve representative samples are summarised in Table 1

Saprolite Ni-serpentine
Saprolite, which represents the weakly weathered protolith, is the most common host for garnierites. In the studied samples, saprolite is reddish to dark brown and is crosscut by a close fine mesh of black veinlets (Fig. 4a, b) and consists mainly of serpentine and Fe oxides and oxyhydroxides. Close to the fractures and to the edges of saprolite where the serpentinite is further altered (Fig. 4a, b), the serpentine is greenish grey to black in colour. Under the optical microscope, serpentine forms yellow veins wrapping reddish brown cores of serpentine and Fe oxides and oxyhydroxides, preserving the previous mesh textures of the serpentinised olivine. Serpentine near fractures and edges of fragments is pale yellow and show diffuse cores under the optical microscope. The groundmass of serpentine contains scattered tiny anhedral relict chromite grains, altered to ferrian chromite ( Fig. 6a, b). Relict crystals of olivine and pyroxene are extremely rare. Intergrowths of a Ni-bearing talc-like phase are common near cracks, and are interpreted as replacements of serpentine ( Fig. 6b, lower left).

Type I garnierite
Type I (yellowish pale green) garnierite occurs as millimetre to centimetre-thick veins in the saprolite (Fig. 4a). These veins are often crosscut or superimposed by millimetric veinlets of type IV (bluish green) garnierite and white quartz. Type I garnierite is characterized by a maximum diffraction peak between 7.26 and 7.30 Å, which coincides with the characteristic basal spacing range of the serpentine group minerals (Fig. 5a).
Under the optical microscope this garnierite type consists of a yellowish brown heterogeneous material, intergrown with Fe oxyhydroxides (Fig. 6a, b). In addition, a nickeliferous talc-like phase may be present as seen in the backscattered electron image in the lower left corner of the Figure 6b.

Type II garnierite
Type II (apple green) garnierite occurs as coatings on angular saprolite fragments ( Fig.   4b). The garnierite is in turn superposed by type IV garnierites and millimetre-thick veinlets of white quartz. The Type II garnierite is characterized by a very intense and sharp diffraction peak at 7.27-7.31 Å, and a lower broad peak at 9.95-10.14 Å, corresponding to the basal spacings of serpentine and talc, respectively (Fig. 5b). This suggests that the sample is a serpentine-like phase with talc-like impurities.
Under the optical microscope, this garnierite consists of colourless to yellowish, slightly anisotropic serpentine aggregates up to 0.5 mm in length, frequently enveloped by a very fine grained brownish grey fibrous matrix of a Ni-talc-like phase (Fig. 6c, d). Pores in the garnierite are systematically filled by euhedral elongated quartz crystals.

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Type III (dark green) garnierite shows a characteristic dull to greasy luster, and consists of friable aggregates up to 15 cm in length (Fig. 4c).They also occur as matrix in breccias with: a) millimetre to centimetre-sized fragments, composed of fine-grained, whitish green mixture of Ni-sepiolite and quartz (Fig. 4d), and b) brownish orange, centimetre-scale, angular saprolite clasts (Fig. 4e). This dark green garnierite is characterised by a diffraction peak at 7.30-7.38 Å and a slightly broader peak at 9.86-10.46 Å, both having similar intensities (Fig. 5c) suggesting a mixture of serpentine-like and talc-like phases.
Under the optical microscope, type III garnierite occurs as greenish to intense green homogeneous coatings, frequently presenting botryoidal features, intergrown with quartz crystals (Fig. 6e). The euhedral terminations of the quartz crystals, in addition to the presence of several pores in the matrix, suggest that quartz and garnierites have grown in voids.

Type IV garnierite
Type IV (bluish green) garnierite occurs as millimetre to centimetre-sized coatings superimposed over other garnierite types and saprolite fragments (Fig. 4a, b). It also occurs as cement in breccias with centimetre to millimetre-size, rounded, brownish orange saprolite clasts. The saprolite clasts may contain narrow greenish white quartz veinlets (Fig. 4f), and up to 2 cm long rounded, non-spherical granules embedded in a greenish white, microcrystalline quartz matrix coexisting with millimetric angular, dark brown and/or orange saprolite fragments (Fig. 4g). Type IV garnierite shows a broad diffraction peak at 9.58-10.02 Å suggesting it consists uniquely of a talc-like phase with minor quartz impurities (Fig. 5d).
Under the optical microscope, type IV garnierite consists of yellow to brown, banded, botryoidal aggregates, and is grey to yellow in crossed polars. It is commonly intergrown with, superimposed and/or crosscut by quartz veinlets (Fig. 6d, f). In the samples where type IV forms rounded granules, the saprolite fragments are coated by botryoidal garnierite, and this in turn is enveloped by the microcrystalline quartz matrix (Fig. 6g). This matrix is formed by equigranular rounded quartz grains of about 0.1 mm accompanied by disaggregated, angular, talc-like fragments up to 0.5 mm long, which are dark brown under plane polarised light and grey to orange under crossed polars (Fig.   6g, h). The banded aggregates are often fractured and crosscut by quartz veinlets of up to 0.1 mm thick, and may show microscopic shearing (Fig. 6h). These features suggest that type IV and microcrystalline quartz precipitated from a colloidal silica gel, while the saprolite clasts are interpreted as relicts. The X-ray diffraction patterns and textures of the type IV garnierite are similar to those reported for garnierites from New Caledonia (e.g. sample G6, Figs. 7 and 9 in Wells et al., 2009).

Type V garnierite (Ni-sepiolite-falcondoite)
Ni-sepiolite and falcondoite form white to pale green compact sets of schistose, friable, soft material (Fig. 4h, i). These textures are easily distinguishable from those observed in the other garnierites described above (see Tauler et al., 2009 for a detailed study). Nisepiolite also occurs as fibrous aggregates embedded in a microcrystalline quartz matrix, which appear as silicified clasts in garnierite-cemented breccias in Figure 4d.

Structural formulae
Microprobe chemical analyses of the described garnierite types are summarized in  (Wells et al., 2009). The Fe 3+ and Al were allocated to the tetrahedral layer until fully occupied (Golightly and Arancibia, 1979). In addition, the fraction of serpentine (X serp ) and talc (X tlc ) were calculated for garnierite types II and III according to the formulae given by Brindley and Hang (1973).

Saprolite Ni-serpentine
Serpentine from the saprolite has the structural formula of an ideal serpentine, with 2 tetrahedral cations and about 3 octahedral cations. The serpentine is characterised by its

Type II garnierite
The structural formulae of type II garnierite differs significantly from ideal serpentine.
It has excess of tetrahedral occupancy (2.20 apfu) and an apparent deficit in the octahedral site (2.57 apfu). Similar values have been published for 7 Å-type garnierites by Brindley and Hang (1973) and Wells et al. (2009

X-ray element mapping
A detailed study of quantified X-ray element maps shows the relationships between textural position, sequence of crystallization and mineral composition of the various garnierite samples. In order to examine the distribution of major element in the different garnierites, the elements Si, Mg, Ni, Fe and Al were scanned in four selected areas containing saprolite serpentine (Fig. 7), serpentine-like, talc-like garnierites and their mixture (Fig. 8), talc-like garnierite (Fig. 9), and sepiolite-falcondoite (Fig. 10). The element maps show a complex and variable texture and mineral composition. Note that there is a close fit between spot analyses (section 5.1) and quantified pixel compositions (this section). The subtle divergences are the result of matrix effects (for these plots a single internal standard was used for all minerals of the scanned areas).
The textural and compositional features of saprolite serpentine are presented in Fig. 7.
The map shows a mesh texture in which Fe oxyhydroxide aggregates (brown areas in A C C E P T E D M A N U S C R I P T microcrystalline quartz, probably chrysoprase (Fig. 8b). In Figure 8c it is also evident the post-depositional diffusion transport of Fe from relictic (saprolite) serpentine within adjacent talc. To be noted also is the presence of a phase with a homogeneous intermediate composition between talc-and serpentine-like, comparable to that of karpinskite, which stands out clearly in the images associated with serpentine. This is confirmed by the quantified Si/(Mg+Fe+Ni) image (Fig 8c), that shows the correspondence between the theoretical values of this ratio in stoichiometric serpentine (0.67) and karpinskite (1.00) and the quantified pixels of both minerals. According to the triangular plot in Figure 8e, some spot and pixel analyses located in between major clusters represent mixtures of minerals. However, a small cluster located midway in between talc and serpentine corresponds to the area identified as karpinskite in the quantified XR maps. Note that this cluster plots in the line representing the solid solution of karpinskite.

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As seen in Figure 10, the textural and chemical variations suggest three stages of sepiolite crystallization, starting with homogeneous Ni-poor sepiolite aggregates, followed by Ni-rich sepiolite. In the last stage still Ni-richer sepiolite crystallized along fractures and/or channels. This progressive Ni enrichment in sepiolite was also observed by Tauler et al. (2009). Quartz crystallized in the wall of voids, within the matrix of

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sepiolite, and locally along fractures/channels. Ni-rich regions in quartz are noted at the rims of the quartz grains related to the third stage of Ni-rich sepiolite growth.

The Si-Mg-Ni system
All analyses performed on garnierite types I-IV, Ni-sepiolite-falcondoite (including results from Tauler  compositions between the kerolite-pimelite end members (Fig. 11b). Figure 11b also shows that the brown fragments in the microcrystalline quartz matrix have higher Ni contents than the brown botryoidal aggregates they coexist with; and that the talc-like replacements of saprolite fragments and spindle-shaped, talc-like, fine grained envelope in type II have similar compositions to those of type IV garnierite. The Ni-sepiolitefalcondoite analyses also plot continuously between sepiolite and falcondoite end members (Fig. 11c). The sepiolite analyses are, however, slightly enriched in Si probably due to microscopic intergrowths of quartz within sepiolite (Tauler et al., 2009). These results suggest the existence of a complete solid solution between Mg and Ni end members.
In general, Ni-Mg hydrous silicates from Falcondo Mine have similar structural and chemical characteristics to garnierite minerals examined in other worldwide Ni-lateritic deposits (Fig. 11d), except for the Ni-dominant serpentine-like phases, which have not been recorded in Falcondo, and falcondoite, which commonly occurs in this deposit.

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In general, unweathered serpentinised peridotites are massive and do not allow fluids to circulate except along fractures (Genna et al., 2005;Cluzel and Vigier, 2008 (Freyssinet et al., 2005). Ore thickness, grade and type of Nilaterite deposits are controlled by fractures, faults and shear zones in bedrock and regolith (Leguéré, 1976;Butt and Cluzel, 2013). Unfortunately, little attention has been placed on the effect of brittle tectonics on the formation of garnierites (Cluzel and Vigier, 2008).
In Also, the botryoidal habit of type IV garnierite (Fig. 4g), which represent precipitation from a colloidal solution in open spaces, are commonly fractured and brecciated (Fig.   4g, h), and the fragments are cemented by a second generation of garnierite and rounded quartz grains. These textures suggest multiple precipitation and deformation events, which may explain the origin of the bimodal composition of the botryoidal talc-like phase (Figs. 8 and 9). Similarly, garnierite clasts within breccias (Fig. 4d) indicate a

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mineralisation stage prior to a brittle deformation. The fracturing formed tectonic porosity that enhanced the circulation of solutions and the precipitation of a second generation of garnierite, namely type III.
In summary, our observations suggest a regional syn-tectonic environment for the generation of the Ni-ore garnierite deposits in Falcondo, even if the precipitates are pre-, syn-or post-kinematic with respect to the activity of a particular fault. This brittle syntectonic environment has played an important role in the formation of hydrous silicate Ni-laterites, since it triggered cyclic, recurrent processes of weathering and erosion.
These processes allowed the draining of fluids and reworking of previously formed ore, thus favouring remobilisation and reconcentration of ore minerals.

Garnierites and garnierites
Different greenish colours have been observed in the garnierite samples, which correspond to different mineral assemblages as demonstrated by XRD and EMP analyses. The various phases have been classified as type I to V garnierites. These garnierites show characteristic patterns of serpentine-like (type I), talc-like (type IV) and sepiolite-like minerals (type V). Also, mix phases with different proportion of serpentine-and talc-like phases (types II and III) are also common. The different colours of the studied garnierites (Fig. 3) are not an indication of the Ni content in the minerals as shown in Figure 11. This is contrary to the observation of Brindley and Hang (1973). However, in the sepiolite-falcondoite series, the greenness of the sample is directly correlated with the Ni concentration. The characteristic colour of type I to IV garnierites gives information on the dominant mineral phase present in the sample (serpentine-like, talc-like or mixtures), which is useful for a field mineral identification with the naked eye.
The following sections illustrate the complex mineralogical and compositional relations of garnierites. First, saprolite serpentine and type I serpentine-like garnierite compositions mainly display a lizardite [Mg 3 Si 2 O 5 (OH) 4 ] stoichiometry (Fig. 11a).
Deviations from the lizardite-népouite line toward the kerolite-pimelite line (types II and III) are due to mixing or interstratifications of 1:1 and 2:1 layers to form a mix phase (Faust, 1966;Berkhin, 1968, 1970). According to Brindley and Hang (1973), however, the same observations can be interpreted as deficiencies of octahedral cations in the 7 Å-type and of tetrahedral cations in the 10 Å-type garnierites. As pointed out by many authors and summarized by Galí et al. (2012), the results of this study suggest complete miscibility along the lizardite-népouite, kerolite-pimelite and the sepiolite-falcondoite lines under atmospheric conditions temperature. However, few data exist on the mechanism of Ni-Mg substitution in Ni-phyllosilicates (Manceau and Calas, 1985). The distribution and speciation of nickel has been studied in saprolite serpentine from New Caledonia (Dublet et al., 2012) and the Philippines (Fan and Gerson, 2011), in lizardite-népouite and kerolite-pimelite from New Caledonia (e.g. Manceau and Calas, 1985), and in Ni-sepiolite from the Dominican Republic (Roqué-Rosell et al., 2011). EXAFS (Extended X-ray Absorption Fine Structure) results suggest a heterogeneous distribution of Ni in the octahedral sheets of the Ni-phyllosilicates, forming discrete domains (clustering). In addition, Dublet et al. (2012) reported that Ni is randomly distributed in the Ni-serpentines from the bedrock (peridotite) and clustered in the Ni-serpentines from saprolite. Further XAS (X-ray Absorption Spectroscopy) is needed to define the mechanism of Ni-Mg substitution in garnierite minerals.

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The low oxide totals observed in talc-like garnierite can be explained by the presence of excess water, which may occupy interlayer positions in a 10 Å-type structure, as suggested by Brindley et al. (1979). Hence, we consider that for this type of phase, the terms kerolite and pimelite are more appropriate than talc and willemseite. Although kerolite and pimelite are not accepted as mineral species by the IMA, they have been described in other laterite deposits, for example in New Caledonia (e.g. Wells et al.,
Therefore, kerolite and pimelite are important components in not only hydrous silicate Ni-laterites but in other deposits worldwide.
The lack of iron is a chemical characteristic of garnierite minerals (Troly et al., 1978). Manceau and Calas (1985)  In the Falcondo deposit, the predominant garnierite is kerolite-pimelite (type IV), the 10 Å phase, having one of the highest Ni contents (up to 2.83 apfu). Also, Falcondo is the only locality in the world where the Ni-dominant sepiolite (falcondoite) occurs (Springer, 1976;Tauler et al., 2009). However, the Ni-dominant serpentine (népouite) has not been described yet. Possibly it only occurs as a mixture with Ni-kerolitepimelite at the nanoscale. Mixtures of lizardite-népouite and kerolite, and of lizarditenépouite and smectite are also described in New Caledonia (Fig. 20 in Pelletier, 1996;Wells et al., 2009). In contrast, in New Caledonia, the presence of a serpentine-like (népouite) garnierite is well documented (e.g. Glasser, 1907;Maksimović , 1973;Brindley and Wan, 1975). The difference among these two Ni-laterite localities may be explained by the lithology of the primary ultramafic rocks. In New Caledonia, the protolith is mainly harzburgite and dunite (e.g. Troly et al., 1979;Pelletier, 1983Pelletier, , 1996Perrier et al., 2006;Dublet et al., 2012) and lherzolite is rare (Cluzel et al., 2012), whereas in the Dominican Republic the protolith is mostly clinopyroxene-rich

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harzburgite and lherzolite (e.g. Marchesi et al., 2012). The higher content in pyroxenes suggests higher activity of silica in the Dominican than in the New Caledonian, leading to the preferential formation of talc rather than serpentine during weathering.

Origin of Falcondo garnierites
Similar to other hydrous silicate-type laterites, the Falcondo deposit in the Dominican Republic was formed by tropical weathering of ultramafic rocks undergoing continuous tectonic uplift. Coupled with a low water table, these conditions resulted in the development of a thick saprolite horizon, and a thin ferruginous oxide horizon (Golightly, 1981;Brand et al., 1998;Elias, 2002;Gleeson et al., 2003;Lewis et al., 2006;Butt and Cluzel, 2013).   Avias, 1978;Trescases, 1979;Golightly, 1981;Brandt et al., 1998;Freyssinet 2005). This interpretation is supported by the negative correlation of Mg with Ni and Fe in saprolite serpentines, where Ni increases more rapidly than Fe (Golightly and Arancibia, 1979). The Fe in saprolite serpentines is residual, and the Ni is imported from the upper lateritic levels (Trescases, 1973(Trescases, , 1979Avias, 1978). The Ni is incorporated first in the saprolite Ni-bearing serpentines (Pelletier, 1996). Once the Niserpentines are saturated, the excess Ni is precipitated as hydrous Mg silicates (garnierites) in open spaces including cracks and faults. The precipitation is caused by a sudden change in Eh/pH of the solution. The high stability of the octahedrally coordinated Ni 2+ ion (Burns, 1970) favours the formation of nickeliferous trioctahedral phyllosilicates (e.g. Trescases, 1975).
In general, the stability of Ni-bearing minerals in lateritic environments is determined by the Eh, pH, and chemical composition of permeating groundwater (e.g. Trescases, 1973). In an Al-free system, such as in the Falcondo profile, the stability of lizarditenépouite, kerolite-pimelite or sepiolite-falcondoite is controlled by the silica activity (Galí et al., 2012). The ideal formation of the Ni ore occurs as a succession of precipitation of mineral phases progressively enriched in Ni and with higher Si, because silica activity increases with time and through the profile. Thus, the first garnierite to precipitate is lizardite-népouite, followed by kerolite-pimelite, sepiolite-falcondoite and Ni-free sepiolite with quartz (Galí et al., 2012). This mineral sequence is coherent with field observations and textural relationships between types I-II and type IV. In this mineral sequence the silica activity increases progressively, as well as pH decreases and Eh becomes more oxidising (Golightly, 1981;Golightly, 2010;Gleeson et al., 2004). In alkaline conditions (pH > 8) garnierite minerals are the least-soluble Mg-Ni phyllosilicates, whereas at lower pH quartz may precipitate. The occurrence of later quartz indicates subsequent acidification conditions as a result of rapid access of meteoric surface waters (acidic) to deeper levels (e.g. Golightly, 1981).
According to our results, the first supergene Ni-phyllosilicate phase that was formed in the Falcondo deposit was an Fe-bearing, nickel-enriched, serpentine-like garnierite (type I). Its Fe content is similar to that of saprolite serpentine and suggests that type I garnierite precipitated in a Ni-saturated, rock-dominated system under more alkaline and reducing conditions. On the other hand, the other types of garnierite (type II-V) were precipitated under conditions in which Fe was insoluble as suggested by their low Fe content. Furthermore, the mix serpentine-and talc-like phases (types II and III) may have precipitated under an intermediate stage between the stability fields of Niserpentine and kerolite-pimelite (see Fig. 6 in Galí et al., 2012). These interpretations are coherent with the textures observed in the Falcondo garnierites, where talc-like phases frequently envelop type I and II garnierites (Fig. 4a, b), and the remaining porosity is finally filled by quartz and/or silica. In the particular case of Falcondo, the mineral sequence is more complex. For example, the polyphase infilings containing serpentine-and talc-like garnierite shown in Figure 4a, b suggest that the conditions of the system allowed the formation of serpentine-followed by talc-like phases in the same vein, when the fracture was open. Also, textural-chemical features revealed by the Xray maps indicate several stages of growth with strong oscillatory changes in Ni content

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in type IV (Figs. 8 and 9), and progressive Ni enrichment in type V (Fig. 10). These observations point to recurrent variations in the physical-chemical conditions during garnierite precipitation-dissolution in an aqueous medium.

Metallurgical implications
Variations in the chemical composition of garnierites imply changes in the behaviour of the laterite ore during mineral processing. The laterite ore is a mixture composed mainly In the case of Falcondo, in order to produce ferronickel containing ca. 40% nickel and 60% iron, operation in a conventional electric furnace requires high SiO 2 /MgO ratios (~2; wt.% units) and low FeO (~20 wt.% FeO) (Dalvi et al., 2004). This ratio acts as a flux in the smelting process. Thus a low SiO 2 /MgO leads to an increase of the temperature in the furnace, and to higher energy consumption. The garnierite typologies reported in Falcondo display different SiO 2 /MgO ratios: the highest average silica/magnesia is found in types III (6.5), IV (4.6) and V (3.2), followed by type II (2.3) and type I (1.5). The ratio of type I is similar to that of saprolite Ni-serpentine (1.5). In addition, in all cases, the Ni content and the SiO 2 /MgO ratio correlate fairly well because Ni substitutes for Mg in the octahedral position. As a result, processing ore containing remarkable amounts of type III or IV garnierites (with relatively high Si and Ni, thus high SiO 2 /MgO ratios) requires a lower temperature than processing saprolite

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serpentine with or without serpentine-like garnierites (with less Si than type III and IV and lower Ni, thus low SiO 2 /MgO ratios).
In summary, these variations in water content, Ni and SiO 2 /MgO ratio should be taken into account for the preparation of the laterite ore mixture prior to calcination and smelting. Therefore, an accurate mineralogical characterisation of garnierite-forming minerals is important from a metallurgical point of view.

Conclusions
This article synthesises previous information and provides new data on the mode of occurrence, mineralogy and mineral chemistry of the garnierites from the Falcondo Nilaterite deposit in the Dominican Republic. The following are some important conclusions from this study: 1. The garnierites in the Falcondo weathering profile were precipitated in a tectonically active regime in which Ni was reconcentrated through recurrent weathering-uplifterosion cycles. In some cases, the precipitation was syn-kinematic. 3. The formation of the ore occurred in successive stages becoming progressively enriched in Ni and Si. The precipitation sequence was as follows: iron-bearing Nienriched serpentine (Type I), iron-free mixtures between serpentine and kerolitepimelite (types II and III), kerolite-pimelite (type IV), and sepiolite-falcondoite (type V), being quartz the final product.

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4. The talc-like phase (10 Å-type), which shows one of the highest Ni contents, is the dominant garnierite phase in the profile. This contrasts with the New Caledonian Nilaterites, where the serpentine-like garnierite is the most common phase. This difference is probably due to the lower silica activity in the profile imposed by a harzburgite/dunite protolith, as opposed to a clinopyroxene-rich harzburgite/lherzolite protolith in the Loma Caribe peridotite.