Applied Clay Science 7778 (2013) 6878

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Applied Clay Science

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Chemistry, morphology and structural characteristics of syntheticAlNi and AlCo-lizardites

Mara Bentabol , Mara Dolores Ruiz CruzDepartamento de Qumica Inorgnica, Cristalografa y Mineraloga, Facultad de Ciencias, Universidad de Mlaga, Spain Corresponding author at: Departamento de QumicMineraloga, Facultad de Ciencias, Universidad de MlagMlaga, Spain. Tel.: +34 952131605; fax: +34 95213200

E-mail address: bentabol@uma.es (M. Bentabol).

0169-1317/$ see front matter 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.clay.2013.04.001a b s t r a c ta r t i c l e i n f oArticle history:Received 21 February 2011Received in revised form 2 April 2013Accepted 4 April 2013Available online 4 May 2013

Keywords:AmesiteBrindleyiteKaoliniteHydrothermal synthesisSerpentineAlNi and AlCo lizardites have been synthesized at hydrothermal conditions. The two phases display differentmorphologies: thin, dominantly curved platelets (AlNi); and stacks of very thin platy particles (AlCo). TheX-ray diffraction and transmission/analytical electron microscopy studies have been used to make accurate dis-tinctions among different structural types and chemical populations. AlNi bearing lizardite includes a 1Mpolytype and probably very subordinate amounts of chlorite. AlCo bearing lizardite includes two 1M polytypeswith different cell-parameters, one of these consisting of an apparently modulated structure. Chemically, AlNi-and specially AlCo lizardite consist of a mixture of two populations with tetrahedral compositions ~Si1.8Al0.2and Si2.0. In contrast with previously described Al-rich serpentines, our serpentines are characterized by anasymmetrical Al distribution, with VIAl on the order of 1.0 atom per formula unit.

2013 Elsevier B.V. All rights reserved.1. Introduction

In typical Mg-rich serpentine the substitutions of Al for Mg in theoctahedral sheet and of Al for Si in the tetrahedral sheet have beenclassically considered as the main factor controlling the structureand the morphology. The main idea was that with very scarce ornull substitution, the lateral dimensions of the tetrahedral sheet aresmaller than those of the octahedral sheet, leading to cylindrical orwavy structures characterizing chrysotile and antigorite, respectively.Tetrahedral substitution is generally higher in lizardite, favoring thefitting between the tetrahedral and the octahedral sheets, and leadingto planar structures (Wicks and O'Hanley, 1988).

Nevertheless, this topic is largely debated actually. Indeed,Mellini et al. (2010) have recently described the Mg-end term inlizardite (with Si = 2.0 apfu) and Evans et al. (2012) reportantigorite analyses with Si = 1.86 apfu. Etherovalent substitutionsaffect the structure not only because of the different size of the cat-ions involved, but also because of the different electrostatic interac-tions (e.g. Mellini, 1982).

Al-rich serpentines have been reported and synthesized (Bailey,1980, 1988, and references therein). Maxima tetrahedral and octahe-dral substitutions have been described in amesite (VIAl = IVAl = 1.0atoms per formula unitapfu). The Fe2+-analogous (berthierine)shows generally low octahedral occupancies (Newman and Brown,1987) and is known in two structural forms: 1T and 1M/3T (thea Inorgnica, Cristalografa ya, Campus de Teatinos. 290710.

rights reserved.ideal 1M and 3T layer sequences are similar for trioctahedral compo-sitions according to Bailey, 1969, 1980). In addition, the Ni-analogueof Al-lizardite was found by Weisse (1967) and termed brindleyiteby Maksimovic and Bish (1978). Brindleyite also has octahedral occu-pancy lower than 3 apfu. Both the natural and the synthetic AlNi ser-pentine include a mixture of several polytypes, 1M/3T and 1T beingdominant.

As appropriate substitutions of tetrahedral and/or octahedral cat-ions (mainly Al for Si and Mg) are considered as important factors de-termining the structure and the morphology of serpentines, we havestudied the influence of the octahedral cation size and of the paralleltetrahedral substitutions on the structure and morphology of Al-richserpentines. In order to test this influence, we have synthesized a se-ries of Al-rich serpentines (with Mg, Ni and Co in the octahedral site)all at identical hydrothermal conditions (200 C). The results for AlMg lizardite were reported in a previous paper (Bentabol et al.,2010). Morphologically Al-lizardite consists of thin platy particleson the order of 400 across and 150 thick. The X-ray diffractionpatterns indicate that the 2H2 polytype is predominant, with cell pa-rameters: a = 5.311 (0.006) ; c = 14.333 (0.013) and spacegroup P63. The high-resolution images revealed, however, the pres-ence of other polytypes, and abundant stacking disorder. Chemical-ly, Al-lizardite consists of a single population with averagetetrahedral composition Si1.74Al0.26. In contrast with previously de-scribed Al-rich serpentines, our Al-lizardite was characterized byan asymmetrical Al distribution, with VIAl on the order of 0.70 andIVAl of 0.25 apfu with the charge imbalance compensated for by va-cancies in the octahedral sheet.

The influence of the octahedral cation size and of the paralleltetrahedral substitutions on the structure and morphology of Al-rich

http://dx.doi.org/10.1016/j.clay.2013.04.001mailto:bentabol@uma.eshttp://dx.doi.org/10.1016/j.clay.2013.04.001http://www.sciencedirect.com/science/journal/01691317http://crossmark.crossref.org/dialog/?doi=10.1016/j.clay.2013.04.001&domain=pdf

Table 2Solid products of the reactions.

Systems Products of the reactions Mean particle size ()

Long () Thick ()

Ni-system AlNi-lizardite + Ni-kaolinite +boehmite Ni(OH)2

1000 (241) 180 (62)

Co-system AlCo-lizardite + Co-kaolinite +boehmite + hydralsite Co2AlO4

2500 (864) 850 (350)

69M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878serpentines is tested in the present work using synthetic Ni- and CoAl-rich serpentines. After characterization of the solid products, theinfluence of the chemical composition of the systems on the serpen-tine composition is analyzed. Our results can contribute to shedlight on the influence of both the octahedral substitutions and thecomposition of the weathering or hydrothermal solutions on the ser-pentine structure andmorphology, aswell as on the composition of ser-pentines and on the formation of the rarely described serpentine +kaolinite assemblage.

2. Methodology

Synthesis of serpentine generally has been done by starting from gelsof appropriate composition (Bailey, 1988; Perbost et al, 2003, and refer-ences therein). In contrast, we have followed the samemethodology weused in previous experiments for the formation of Mg-, Ni- and Co-richkaolinite (Bentabol et al., 2006, 2007, 2009), and AlMg lizardite(Bentabol et al., 2010), in order to favor the comparison among the prod-ucts formed in the several systems. Poorly crystalline kaolinite fromGeorgia (standard KGa-2, from the ClayMineral Society Source Clays Re-pository), which has been submitted to an intense grinding was used asstarting material and treated with hydroxide solutions. Details of thegrinding method have been previously described (Bentabol et al., 2006;Gonzlez Jess et al., 2000). Grinding produced rounded, partiallyamorphized particles with an average size of b0.5 m. The impuritiesdetected by XRD and TEM/AEM consist of abundant anatase and minorFe oxide, which were present in the unground kaolinite standard.

In our current study, synthesis was carried out in the systems NiOAl2O3SiO2H2O and CoOAl2O3SiO2H2O. The reactions studiedwere 1 kaolinite + 1.8 Ni(OH)2 and 1 kaolinite + 1.8 Co(OH)2 at tem-peratures of 200 C. A solid/solution ratio = 1:15was used in all the re-actions (Table 1). The hydrothermal treatments were conducted in50 cm3 Teflon-lined reactors (Parr 4744), which were maintained at aconstant temperature of 200 C (3 C), with reaction times from 1to 90 days. The products were immediately centrifuged and a smallfraction of the solutions was used for final pH measurement. Two sub-samples of the solutions were used for Al and Si analysis. The Si and Alcontents were determined using UVVis spectrometry by means ofthe molybdate blue and red S-alizarin complexes, respectively. Ni andCo analyses were carried out by atomic absorption.

The solid products of the reactions were characterized by XRD,FTIR, 27Al MAS-NMR, and TEMAEM. A detailed description of thesynthesis methodology and of the characterization methods is givenin Bentabol et al. (2010), and summarized in Table 1.

3. Results

The products formed through the various reactions include: lizardite,other possible serpentines, kaolinite, minor boehmite and hydralsite,and transitional Ni(OH)2 and Co2AlO4 (Table 2). The lizardite:kaoliniteTable 1Summary of methodology.

Systems composition (mol) Time(days)

XRD FTIR

Mlaga University Mlaga Un

1 kaolinite + 1.8 Ni (OH)2 130 Samples: random and oriented KBr pellets1 kaolinite + 1.8 Co(OH)2 190Reactor: Parr 4744 Philips X'Pert PRO MPD Nicolet spe

Solid:solution ratio = 1:15Temperature: 200 C(3 C)

CuK radiation (35 mA and45 kV) Step size = 0.01 2Counting time: 50800 s

Scanning raresolution:ratio has been estimated using the Rietveld-based X'Pert High ScorePlus software package (Panalytical) and the solutions composition. Thelizardite:kaolinite ratio increases at increasing run time and ranges from~75:25 in the Ni-bearing system to ~95:5 in the Co-bearing systemafter 90 days.

3.1. XRD-results

Lizardite, kaolinite and boehmite were present even after long reac-tion times in both systems (Fig. 1). Nevertheless, the mineral assemblageis more complex in the Co-bearing system, and the products obtainedafter 90 day reaction time also contain minor Co2AlO4 and hydralsite.In this system, increasing reaction times causes the decrease of theCo2AlO4 and kaolinite contents parallel to the increase of lizardite,boehmite and hydralsite (Fig. 2). In addition, splitting of the basal re-flections of serpentine is observed in the products obtained after thelongest reaction times (Fig. 3), suggesting the presence of two lizarditephases with different basal spacings.

Using Xpowder (Martn, 2004), the XRD pattern of AlNi lizardite hasbeen indexed as 1M polytype, with cell parameter: a = 5.313 (0.007) ;b = 9.162 (0.009) ; c = 7.316 (0.008) ; = 103.91 (0.09) (\dix 1,Table 3). TheXRDpattern ofAlCo lizardite has been indexed as amixtureof two 1M polytypeswith different cell parameters: a = 5.337 (0.004) ;b = 9.155 (0.008) ; c = 7.394 (0.006) ; = 104.88 (0.07); anda = 5.350 (0.004) ; b = 9.234 (0.006) ; c = 7.512 (0.006) ; =103.97 (0.06) (Appendix 1, Table 3).

3.2. TEM/AEM study

Although natural serpentines generally provide excellent TEM im-ages, in the case of our synthetic samples the TEM study was hinderedby the small particle size, which generally prevented obtaining SAEDpatterns with intense spots, and by the presence of abundant struc-tural defects. Associated kaolinite is characterized by the presenceof spherical morphologies and high Ni, and Co contents (Bentabol etal., 2007, 2009).

When the samples are observed at lowmagnification, it is evident thatsome segregation of phases occurred during precipitation, which pro-duced clusters of serpentine, kaolinite and oxide- or hydroxide. Fig. 427Al MAS-RMN TEM/AEM

iversity ICMM-CSIC (Madrid) 1. CIC (Granada University)2. Complutense University

(2% wt. sample) Powder samples Solid products, encased inepoxy resin and sliced

ctrometer (20XB) Bruker AVANCE-400 spec-trometer with Fouriertransform

1. Philips CM-20 microscope(200 kV)2. Jeol 3000 F (300 kV)

nge: 4000-400 cm1

1 cm1B0 = 9.4 T Magic-angle: 5444l = 5/2 pulse: /8 (2 s)

Fig. 1. XRD patterns of the solid products of (a) the Ni-bearing reaction (after 30 day reaction time), (b) of the Co-bearing reaction (after 90 day reaction time) and (c) the startingkaolinite. The more significant reflections of serpentine have been indexed in (a) and (b). One star indicates kaolinite reflections (or kaolinite contribution). Two stars indicateboehmite reflections. Three stars indicate anatase reflections. Crosses in (b), indicate AlCo oxide reflections and squares hydralsite reflections. In (c), only the more intensekaolinite reflections have been indexed. Kln: kaolinite. Srp: serpentine. Ant: anatase. Bhm: boehmite.

70 M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878shows representative low-magnification TEM images of serpentine-richareas formed in the two systems studied. Important differences canbe seen in particle-size and morphology between Ni- and Co-bearinglizardite. AlNi lizardite is characterized by the presence of curved plate-letswithmean size of 1000 180 , similar to those shownby Perbost etal. (2003). AlCo lizardite shows notably thicker particles with mean sizeof 2500 850 (Table 2).

Representative AEM data for the AlNi and AlCo lizardite and forthe associated kaolinite are reported in Table 4. The synthetic Ni- andCo-bearing lizardites contain two compositionally different popula-tions, with average Si contents of ~1.80 and 2.00 apfu (calculated for 7oxygens), both characterized by low octahedral occupancies. Thus, thecomposition of our serpentines clearly deviates from those describedin earlier studies (Bailey, 1980, 1988; Golightly and Arancibia, 1979;Maksimovic and Bish, 1978; Perbost et al., 2003), especially in the asym-metrical distribution of Al between the tetrahedral and the octahedralsheets. Indeed, asymmetrical distribution of Al has been only describedin brindleyite (Maksimovic and Bish, 1978). Deficiency in octahedral cat-ions, increasing in parallel to the Ni and Fe for Mg substitution was alsoobservedbyGolightly andArancibia (1979) inNi- and Fe-rich serpentinesfrom a lateritic profile. In this case, lateritic alteration could have entailedoxidation and Fe can be as Fe3+ into the structure, thus explaining the oc-tahedral vacancy. Although the presence of Fe3+ is also possible in ourSi-richer populations, the formulae calculated assuming that Fe is Fe3+

show negligible differences with these shown in Table 4, given the lowFe content.

Fig. 2. XRD patterns of the solids products obtained from the Co-bearing system at increasing run times. Symbols as in Fig. 1. Hy: hydralsite.

71M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878The lattice-fringe images also show differences among the synthe-sized serpentines. Ni-bearing lizardite shows a variety of packet sizesand structural defects (Fig. 5). Packets consisting of curved 7- layersare frequent (Fig. 5a). Although the curvature radii is small in somepackets (~500 ), it is notably higher than in chrysotile fibers (Yada,1967, 1971) and in tubular Ni-serpentine (Perbost et al., 2003). In addi-tion, some particles consist of subparallel packets with different orienta-tion, also suggestive of different structures. Thus, the platelet shown inFig. 5b consists of a lower packet with a wavy 7- serpentine structureand an upper curved packet suggestive of a 14- chlorite-like structure(Kameda, per. com.). True basal spacings can be easily observed in theFourier transfoms (FT) from both packets (Fig. 5b, inset). The boundarybetween both packets occurs through a thin areawith abundant disorder.

Variability in particle- and packet-size also characterizes Co-bearinglizardite (Fig. 6). Thick particles observed at low magnification (Fig. 4b)generally consist of stacks of thin (~50 ) subparallel platelets, whichshow regular shift along the X-axis characterizing the 1M polytype in im-ages obtained along [010] (Fig. 6a). Thicker planar particles with curvedlayers are also frequent (Fig. 6b, arrows). The FT (Fig. 6b, inset) revealsstreaking along c*. Images taken along the [010] zone axis also revealthat a wavy structure, which strongly resembles antigorite (Auzendeet al., 2002; Capitani and Mellini, 2005, 2007; Ddony et al., 2002;Grobty, 2003; Mellini and Zussman, 1986; Mellini et al., 1987; Wicksand O'Hanley, 1988), is frequent in some of these particles (Fig. 7).Although a wavelength of ~70 is dominant, the wavelength varies fre-quently within a single packet.

3.3. FTIR results

The FTIR spectra of our synthetic serpentines are strongly influencedby the presence of kaolinite, which is especially evident at the OH-stretching zone (Fig. 8a and b). Nevertheless, when these spectra arecompared with those of the Co- or Ni-bearing kaolinites (Bentabol et al.,2007, 2009) (Fig. 8c), it is evident that the spectramainly reflect the pres-ence of serpentine, and that the kaolinite influence can be minimized bysubtracting the sharp kaolinite bands present at high frequency, asshown in Fig. 9. The spectra of Ni- and Co-bearing lizardites are character-ized by the presence of a broad and intense band ~3458 cm1 (Fig. 8).Deconvolution of this band permits the observation of four distinctbands at ~3620, ~3460, ~3310, and ~3100 cm1 (Fig. 9). These bandsmust represent, according to Farmer (1974), Serna et al. (1977, 1979,1982), Velde (1980), Fuchs et al. (1998) and Balan et al. (2002), splittingof the OH-stretching band due to the presence of Al and vacancies in theoctahedral sheet, as also observed in AlMg lizardite (Bentabol et al.,2010).

At the low-frequency zone (1200400 cm1), the spectra showsome differences with those of typical Mg-serpentine and the pres-ence of kaolinite is also evident at this zone. The three bands

image of Fig.2

Fig. 3. XRD patters of the 001 (a and b) and 002 (c and d) zones of AlCo lizardite, showing the presence of two distinct basal reflections in the products obtained at longer run time(90 days). Ant: anatase.

72 M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878corresponding to perpendicular Si-O vibrations in kaolinite (Farmer,1974; Frost, 1995) are replaced by a doublet (1033 and 1010 cm1)with a weak shoulder at the high-frequency side. Differences with thespectra of typical Mg-serpentines can be tentatively related to the pres-ence of substitutions of Al for both Si and Co or Ni (Prencipe et al., 2009;Serna et al., 1979).

3.4. 27Al MAS-NMR spectroscopy

27Al MAS-NMR spectra were obtained with the aim of detectingthe presence of IVAl in the structure of the synthetic serpentines. Incontrast to synthetic AlMg lizardite, which provided spectra withthe IVAl and VIAl bands are clearly resolved, in the case of the Ni-and Co lizardites the central MAS-NMR transitions are stronglyinfluenced by quadrupolar interactions (Morris et al., 1990; Oldfieldet al., 1983). Moreover, whereas AlMg lizardite shows a single pop-ulation with mean tetrahedral composition of Si1.74Al0.26, the Ni- andCo lizardite show two coexisting populations with mean composi-tions of Si1.80Al0.20 and Si2.00Al0.00, leading to a lower intensity of theAl resonance at ~68 ppm (corresponding to Al in tetrahedral coordi-nation) (Fig. 10).

3.5. Composition of the solutions

The evolution of the pH and composition of the solutions atincreasing run times are shown in Fig. 11 for the two systems studied.In both cases a strong decrease of the pH is observed after short reac-tion time (1 day), as a consequence of the dissolution of kaolinite,which is followed by a later stabilization. Similarly, the increase ofTable 3Cell-parameters of serpentines calculated with Xpowder (Martn, 2004).

a () b () c () ()

AlNi-lizardite 5.313 (0.007) 9.162 (0.009) 7.316 (0.008) 103.91 (0.09)AlCo-lizardite 5.337 (0.004) 9.155 (0.008) 7.394 (0.006) 104.88 (0.07)AlCo-lizardite 5.350 (0.004) 9.234 (0.006) 7.512 (0.006) 103.97 (0.06)the Si and Al contents and the inverse behavior of Ni (Fig. 11a), andCo (Fig. 11b) also reflect the dissolutionprecipitation stages. Stabili-zation of the chemical parameters from 1 to 15 days of reaction timestrongly suggests an approach to the equilibrium.Fig. 4. Low magnification TEM images showing the size and morphology of (a) typicalAlNi lizardite and (b) AlCo lizardite.

image of Fig.3image of Fig.4

Table 4Representative AEM data for serpentine and average composition of associated kaolinite.

Ni-bearing system

Ni- lizardite Av. Ni-Kln

Planar particles Curved layers

1 2 3 4 Av. 1 2 3 4 Av.

Si 1.49 1.74 1.82 1.91 1.78 1.98 2.03 2.06 2.04 2.03 2.03IVAl 0.51 0.26 0.18 0.09 0.22 0.02 0.00 0.00 0.00 0.00 0.00VIAl 1.05 0.89 1.15 1.04 0.96 1.11 1.08 0.98 0.95 1.01 1.60Fe 0.08 0.09 0.05 0.06 0.05 0.04 0.02 0.02 0.02 0.03 0.04Ni 1.61 1.71 1.31 1.42 1.62 1.31 1.38 1.39 1.47 1.40 0.50oct. 2.73 2.69 2.51 2.52 2.63 2.46 2.49 2.39 2.44 2.44 2.14

Co-bearing system

Co- lizardite Av. Co-Kln

Planar particles Wavy structure

1 2 3 4 Av. 1 2 3 4 Av.

Si 1.62 1.76 1.85 1.94 1.81 2.01 2.02 2.03 2.03 2.02 2.00IVAl 0.38 0.24 0.15 0.06 0.19 0.00 0.00 0.00 0.00 0.00 0.00VIAl 0.84 0.89 1.03 0.98 0.88 0.95 0.87 0.82 0.91 0.89 1.74Fe 0.05 0.06 0.02 0.08 0.05 0.06 0.07 0.00 0.05 0.04 0.04Co 1.89 1.72 1.51 1.48 1.73 1.51 1.59 1.70 1.53 1.58 0.35oct. 2.77 2.67 2.56 2.54 2.65 2.51 2.53 2.52 2.49 2.51 2.13

73M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 68784. Discussion

Rapid precipitation of serpentine from kaolinite dissolution leads tonucleation of numerous fine-grained serpentine platelets, associated tominor amounts of kaolinite and subordinated amounts of boehmite,hydralsite, and Ni- and Co-hydroxide or oxide. These later phases aregenerally transitional and decrease at increasing reaction times.Fig. 5. Lattice-fringe images of AlNi lizardite. (a) Platy and curved serpentine platelets.Fringes with 14 spacing are common in some platelets. (b) HRTEM image showing twosub-parallel packets with different structure and orientation. The image of the upper packet,obtained along the [100] zone axis, consists of possible 14- chlorite layers curved along theX axis, whereas the lower packet, which is oriented perpendicularly to [010], shows a wavystructure with variable wavelength marked. Inset: Fourier transform from both packets.4.1. The assemblage kaoliniteserpentine

The association kaoliniteserpentine has been rarely described in nat-ural environments: in bauxitic deposits, weathered ultrabasic rocks andhydrothermally altered claystones (e.g. Ruiz Cruz and Barcel, 1984;Singh and Cornelius, 2006; Weisse, 1967). Given the wide distributionFig. 6. Representative lattice-fringe images of AlCo lizardite. (a) Thin packets showing acharacteristic offset of a 1M sequencemarked by arrows. (b) Thick packet where a planarstructure dominates. Curved layers and curvature inversions are marked by arrows. In b,the Fourier transform (inset) shows splitting of the basal reflections, suggesting coexistenceof the two types of AlCo lizardite.

image of Fig.5image of Fig.6

Fig. 7. Lattice-fringe image of AlCo lizardite particle obtained along [010], showing awavy structure with a wavelength ~70 .

Fig. 9. Fitting of the OH-stretching zone of (a) AlNi and (b) AlCo lizardite, showing

74 M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878of Fe in most geological environments, it can be expected that the as-semblage berthierine + kaolinite be widely developed, especially inweathering profiles and hydrothermal environments. This is the case,for example, of the hydrothermal berthierine + halloysite assemblagedescribed by Slack et al. (1992) and of the berthierine-bearing flintclays described by Moore and Hughes (2000), where the mineral as-semblage also includes kaolinite and boehmite. Following these reports,it seems evident that formation of Al-rich serpentines would requireeither a parent phase rich in Al or the availability of Al-rich solutions.

In our experimental systems (Bentabol et al., 2010; this study), theassociation kaoliniteserpentine systematically occurs at very vari-able hydroxide concentration and appears to be a stable assemblage,Fig. 8. FTIR spectra of the solid products of the two reactions after long reaction times(a) AlNi lizardite and (b) AlCo lizardite and (c) Co-kaolinite.

the presence of four overlapped bands and small kaolinite bands (Kln).as demonstrated by the evolution of the solution compositions (Fig. 11).Variations of the initial hydroxide concentration mainly influence theserpentine:kaolinite ratio of the reaction products although the serpen-tine composition also evolves with increasing hydroxide contents.

Table 5 summarizes the composition of the several serpentinemembers: 1) obtained in the present study; 2) those obtained in pre-vious experiments (Bentabol et al., 2006, 2007, 2009) using higherkaolinite:hydroxide ratio; and 3) those obtained in the Mg-bearingsystem using the same ratio used in this work (Bentabol et al.,2010). As a whole, serpentine formed in richer Al-systems (low hy-droxide concentration) was also richer in Al. The Al decrease is espe-cially significant in the Ni- and Co-bearing systems (high hydroxideconcentration). In addition, a notable decrease of the tetrahedral sub-stitution is also observed in these two systems. The average formulaof our AlMg lizardite could be similar to that of natural Al-lizardite(Wicks and O'Hanley, 1988), but with VIAl > IVAl, thus leading to an in-complete octahedral occupancy. Similarly, the formulae of the Si-poorpopulations of AlNi- and AlCo- compare well with a brindleyite-likeserpentine, alsowith stronger asymmetrical distribution of Al and lowoc-tahedral occupancies. Nevertheless, the Si-rich populations of AlNi- andAlCo lizardites clearly deviate from the typical serpentine formulae, es-pecially in the low octahedral occupancy. A possible explanation (notproved in our case) is that pure Ni (or Co) trioctahedral sheets alternatewith pure Al dioctahedral sheets (kaolinite-like sheets), as suggested byGolightly and Arancibia (1979) for Fe and Ni-rich serpentines. Brindley(1980) also noted that the nepouite (Ni-bearing serpentine with planar

image of Fig.7image of Fig.8image of Fig.9

Fig. 10. Comparison of the IVAl contents in (a) AlNi and (b) AlMg lizardites as observedafter deconvolution of the central components of the 27Al MAS-NMR spectra.

Fig. 11. Evolution of pH and composition of the solutions at increasing run times. (a) AlNilizardite; (b) AlCo lizardite.

75M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878lizardite structure) composition does not conform to a serpentine formulaalthough he suggested that the excess of Si and the low octahedral occu-pancy could be due to the presence of amorphous SiO2 or to the preferen-tial leaching of the octahedral R2+. This second possibility is, however,unlike in our synthetic samples.

The fact that AlMg lizardite shows a uniform chemistry and struc-ture, whereas AlNi- andAlCo lizardites consist of amixture of compo-sitionally and structurally different populations cannot be related withthe cation size but to other factors. It seems possible that, in the systemsstudied, one of the lizardite populations be metastable and evolves, atincreasing run times, toward the other structural type. In the case ofthe AlNi serpentine, the Si-rich term, which shows a basal spacing of7.111 (Appendix 1) predominates and splitting of the 00l reflectionsis not observed in the XRD patterns. This permits the adscription ofthe lower basal spacing in AlCo serpentine (7.179 ) to lizardite withthe Si-richer composition and the higher one (7.290 ) to the Si-poorterm. If this adscription is true, data in Fig. 3, which reveal the notableincrease of the polytype with higher basal spacing at increasing runtimes, would confirm the metastable nature of one of the polytypes.4.2. Serpentine composition vs. structure and morphology

In a first approximation, the deduced lattice parameters of our ser-pentine phases could be related to the size of the dominant octahe-dral cation. Indeed, plot of the b-parameter against the iconic radiiof our serpentines and some related terms (Fig. 12a) reveals a goodpositive correlation among the ionic radii and the b-cell parameter,including the two populations of Co-lizardite, brindleyite, amesite,and berthierine. Nevertheless, the Fe2+-richer term (greenalite),plots clearly out the line defined by the our Al-rich serpentines,suggesting that, in addition to the ionic radii of the dominant octahe-dral cation, the b-parameter is also controlled by the Al content in theoctahedral sheet.

Plot of the csin vs. the ionic radii (Fig. 12b) reveals a positive cor-relation defined by our samples (empty diamonds) and brindleyite,but excludes the Si-rich population of Co-serpentine (full diamond),suggesting that the csin values are also influenced by the size ofthe dominant octahedral cation. Amesite, berthierine and greenaliteplot, in contrast, far off the regression line, showing csin valueslower than expected on the basis of the ionic radii. This fact agreeswith observations in chlorite (Bailey, 1972), where the OHO dis-tances, and thus the basal spacings, decreased with the increase ofheavy cations content in the octahedral sheets.

Although the XRD patterns reveal some significant differencesamong the several serpentine phases, the HRTEM data provide moredetailed information. HRTEM images of AlNi lizardite show thatplaty packets with abundant stacking disorder show locally wavystructures with high wavelength, whereas the large curved particlesobtained after long reaction times (30 days) could include chloritedomains (Fig. 5). HRTEM images of AlCo lizardite indicate, on theother hand, that it consists of very thin particles with a dominant1M structure and thicker particles with an irregularly distributedwavy, possibly antigorite-like structure (Figs. 6 and 7).

image of Fig.10image of Fig.11

Table 5Average composition of synthetic serpentines.

Systems with low hydroxide concentration: 1 kaolinite + 0.6 (Mg2+, Ni2+, Co2+)

Mean composition oct. (apfu)

AlMg-serpentine (Al1.06Fe0.03Ni0.18Mg1.28)(Si1.83Al0.17)O5(OH)4 2.55 Bentabol et al. (2006)AlNi-serpentine (Al1.24Ti0.01Fe0.02Ni1.31)(Si1.58Al0.42)O5(OH)4 2.58 Bentabol et al. (2007)AlCo-serpentine (Al1.20Fe0.11Co1.27)(Si1.64Al0.36)O5(OH)4 2.58 Bentabol et al. (2009)

Systems with high hydroxide concentration: 1 kaolinite + 1.8 (Mg2+, Ni2+, Co2+)

Mean composition oct. (apfu)

AlMg-lizardite (Al0.73Fe0.02Mg2.00)(Si1.74Al0.26)O5(OH)4 2.76 Bentabol et al. (2010)AlNi-lizardite (Al0.96Fe0.05Ni1.62) (Si1.78Al0.22)O5(OH)4 2.63 This study

(Al1.01Fe0.03Ni1.40) (Si2.03)O5(OH)4 2.44 This studyAlCo-lizardite (Al0.88Fe0.05Co1.73)(Si1.81Al0.19)O5(OH)4 2.65 This study

(Al0.89Fe0.04Co1.58) (Si2.02)O5(OH)4 2.51 This study

76 M. Bentabol, M.D. Ruiz Cruz / Applied Clay Science 7778 (2013) 6878The different structures probably reflect, as in the case of typicalMg-serpentines, the different mechanisms leading to fitting be-tween the sheets of tetrahedra and octahedra in the compositionallydifferent members. In the case of AlMg lizardite and the Si-poorpopulations of AlNi- and AlCo lizardites, the tetrahedral substitu-tions (0.190.26 apfu) seem to be high enough to produce a good fitbetween both sheets, leading to planar structures. The Si-richer AlNi- and AlCo populations resolve the fitting between the twosheets in different ways. Curvature of the layers characterizes AlNi lizardite (Fig. 5a). In contrast, the presence of a wavy structure,which suggests the presence of inversions of the tetrahedral sheet,characterizes the Si-rich population of AlCo lizardite (Fig. 7). Thehigh Si content of this population could be a consequence of themodulated structure, as this would contain talc-like domains located atthe layer polarity reversal (Wunder et al., 1997). For testing this hypoth-esis, we have used the formula proposed by Capitani and Mellini (2004)Fig. 12. Relationships among (a) b-cell parameter and (b) csin versa the ionic radii forthe synthetic samples and some selected natural serpentine-group minerals. Ionic radiifrom Shannon and Prewitt (1969). Cell-parameters from Maksimovic and Bish (1978)and Bailey (1980, 1988). The two Co-lizardite populations are represented by emptyand full diamonds.for antigorite, Mg3m-3Si2mO5m(OH)4m-6, in which m = observed wave-length/lizardite subcell (2.54 ). According to this calculation, m = 27,and the calculated formula for an Al-free serpentine would be Mg79Si55O136(OH)103, which becomes Mg2.95Si2.05O5.08(OH)3.85 as calculated for 7oxygen, and M2+1.62Al0.88Si2.05O5.08(OH)3.85 when Mg is replaced byAl + Co (M2+). This formula is almost exactly coincident with the aver-age formulae deduced for the Si-rich population of AlCo serpentine inTable 5.

These results also suggest that, in synthetic Al-rich serpentines,the fit between the tetrahedral and the octahedral sheets requiresthe presence of abundant vacant sites in the octahedral sheet. This as-sumption is also supported by the low octahedral occupancies char-acterizing some natural Al-rich serpentines such as brindleyite orberthierine.

5. Conclusions

Synthesis of AlNi- and AlCo lizardite is rapid at hydrothermal con-ditions, using kaolinite as starting material, and Ni- and Co-bearing so-lutions. In these Al-rich systems, serpentine coexists with kaolinite andis characterized by high VIAl contents. Differences inmorphology, struc-ture and lattice-parameters between the Ni- and Co-terms appear to becontrolled by the size of the dominant octahedral cation and by the ex-tent of the octahedral substitutions. In both cases, platy particles showcompositions with average Si content = 1.8 apfu. In addition, particleswith Si content = 2.0 apfu show either curvedmorphologies (Ni-term)or a wavy structure (Co-term). Interpretation of the true nature of thislater structurewould require, however, the obtaining of larger syntheticcrystals, appropriate to provide informative SAED patterns.

The structural differences among the several serpentine types canbe interpreted as due to the different mechanisms leading to fittingamong the tetrahedral and the octahedral sheets, in the compositionallydifferent members. Our results suggest that a high number of vacanciesin the octahedral sheet are oneway of stabilizing the structure of Al-richserpentines with low tetrahedral substitution.

Acknowledgments

The authors are grateful to the Editor G. Christidis, to F.J. Wicksand to an unknown reviewer, whose comments and suggestions havenotably improved the manuscript. Thanks to M.M. Abad (Universidadde Granada) and to J.L. Baldonedo and A. Gmez (UniversidadComplutense) for their contribution to obtaining of the TEM/AEM dataand Isabel Sobrados (Instituto de Ciencias de Materiales de Madrid,CSIC) for her contribution of the NMR-MAS study. The authors aregrateful to Dr. Kameda for filtering and interpretation of some HRTEMimages. Our study has received financial support from the Project CGL2006-02481 (Ministerio de Educacin y Ciencia) and from the ResearchGroup RNM-199.

image of Fig.12

Ni-liz 1M Co-liz 1M Co-liz 1M

d(o) d(c) h k l I d(o) d(c) h k l I d(o) d(c) h k l I

7.1113 7.0990 0 0 1 100 7.1797 7.1587 0 0 1 100 7.2900 7.2854 0 0 1 1004.5702 4.5834 0 2 0 19 4.5701 4.5823 0 2 0 15 4.6263 4.6167 0 2 0 143.5661 3.5495 0 0 2 46 3.5781 3.5794 0 0 2 62 3.6418 3.6427 0 2 1 11

2.6558 2.6520 2 0 1 21 2.6480 2.6541 2 0 1 232.6174 2.6299 1 3 0 29 2.6326 2.6285 1 3 0 21 2.6480 2.6474 1 3 02.6161 2.5829 2 0 0 29 2.5900 2.5954 1 3 1 22

2.5440 2.5382 1 1 2 382.4871 2.4861 2 1 0 29 2.4405 2.4285 0 0 3 322.3780 2.3788 2 0 2 29 2.3801 2.4039 2 0 2 24 2.3957 2.4032 2 0 2 192.3780 2.3732 1 3 1 2.3801 2.3862 0 0 32.3780 2.3663 0 0 3 2.3759 2.3696 1 3 1 21 2.3957 2.3961 1 3 12.3033 2.3030 1 1 3 21 2.3435 2.3338 1 1 3 32 2.3445 2.3543 1 1 3 32

2.2947 2.2953 2 2 1 13 2.3062 2.3064 0 4 0 132.3033 2.2917 0 4 0 2.2947 2.2912 0 4 0 2.2966 2.3010 2 2 1 142.2645 2.2587 2 0 1 15 2.2534 2.2639 1 3 2 92.2516 2.2502 2 2 0 15 2.2534 2.2499 2 0 1 2.2534 2.2624 2 2 0 92.2516 2.2492 1 3 2 2.2534 2.2475 2 2 0

2.1858 2.1821 0 4 1 10 2.1958 2.2006 0 4 1 102.1248 2.1114 2 2 2 122.1248 2.1023 0 2 31.9820 2.0014 2 0 3 13 2.0219 2.0289 2 0 3 9 2.0425 2.0432 2 2 1 7

2.0219 2.0196 2 2 1 2.0425 2.0327 2 0 31.9820 1.9991 1 3 2 1.9918 1.9991 1 3 2 10 2.0219 2.0271 1 3 2 10

1.9478 1.9498 0 4 1 81.8907 1.8837 2 0 2 9 1.8963 1.8937 1 3 3 9 1.9188 1.9095 1 3 3 71.8907 1.8773 1 3 3 1.8622 1.8773 2 0 2 9 1.9188 1.9087 2 0 21.7713 1.7837 1 1 4 91.7713 1.7748 0 0 4 1.7862 1.7897 0 0 4 5 1.6996 1.7022 3 1 2 81.6898 1.6923 3 1 0 10 1.6886 1.6898 3 1 0 7 1.6996 1.7007 3 1 01.6898 1.6910 3 1 2 1.6883 1.6859 2 0 4 1.6937 1.6943 0 2 4 71.6600 1.6614 2 0 4 11 1.6695 1.6671 0 2 4 1.6937 1.6937 2 0 41.6600 1.6607 1 3 3 1.6619 1.6623 1 3 3 11 1.6883 1.6897 1 3 3 71.6452 1.6485 1 5 1 12 1.6521 1.6527 0 4 3 8 1.6762 1.6731 0 4 3 81.6452 1.6462 0 4 3 1.6521 1.6471 1 5 1 1.6619 1.6624 1 5 1 11

1.5759 1.5786 3 1 3 8 1.6203 1.6216 2 4 1 71.5688 1.5636 2 0 3 131.5574 1.5594 3 1 1 17 1.5759 1.5793 1 1 4 8

1.5358 1.5441 1 1 4 20 1.5509 1.5485 1 1 4 18 1.5759 1.5785 3 1 31.5759 1.5761 3 1 1

1.5358 1.5343 3 3 1 1.5358 1.5368 3 3 1 24 1.5401 1.5423 3 3 1 241.5299 1.5299 0 6 0 22 1.5226 1.5274 0 6 0 17 1.5401 1.5389 0 6 0

1.5183 1.5189 2 4 3 14 1.5226 1.5255 2 4 3 171.4917 1.5001 3 3 0 18 1.5062 1.5068 3 3 2 14 1.5092 1.5094 3 3 2 141.4917 1.4992 3 3 2 1.5006 1.4983 3 3 0 12 1.5077 1.5083 3 3 0 141.4917 1.4936 0 6 1 1.4895 1.4938 0 6 1 14 1.5077 1.5060 2 2 3

1.4823 1.4799 2 2 3 10 1.5077 1.5057 0 6 11.4830 1.4800 1 1 5 10

1.3996 1.4039 3 1 2 8 1.4021 1.4104 0 4 4 7 1.4303 1.4299 0 0 4 111.3996 1.4033 0 6 2 1.4021 1.4051 3 3 1 1.4303 1.4216 1 3 41.3996 1.4032 0 4 4 1.4021 1.4049 0 6 2 1.4303 1.4211 3 3 31.3996 1.4016 3 1 4 1.4021 1.3985 3 1 2 1.4303 1.4212 3 1 41.3996 1.3940 2 0 5 1.3946 1.3971 1 3 4 6 1.3393 1.3469 2 0 4 81.3996 1.3940 1 3 4 1.3393 1.3455 1 5 3 9 1.3354 1.3339 4 0 1 91.3180 1.3188 1 3 5 9 1.3303 1.3323 1 3 5 9 1.3303 1.3313 2 2 1 91.3128 1.3150 2 6 0 11 1.3120 1.3142 2 6 0 12 1.3265 1.3270 4 0 2 91.2838 1.2865 3 3 4 9 1.2859 1.2992 2 6 2 8 1.3245 1.3237 2 6 0 81.2838 1.2855 2 6 2 1.2859 1.2865 0 6 3 1.3001 1.3009 3 3 2 7

1.2833 1.2840 3 3 2 9 1.3001 1.2999 0 6 3

Appendix 1

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http://www.xpowder.com

Chemistry, morphology and structural characteristics of synthetic AlNi and AlCo-lizardites1. Introduction2. Methodology3. Results3.1. XRD-results3.2. TEM/AEM study3.3. FTIR results3.4. 27Al MAS-NMR spectroscopy3.5. Composition of the solutions

4. Discussion4.1. The assemblage kaoliniteserpentine4.2. Serpentine composition vs. structure and morphology

5. ConclusionsAcknowledgmentsReferences