chemical and structural variability of illitic phases ... · chemical and structural variability of...

(2006) 111–124www.elsevier.com/locate/clay

Applied Clay Science 32

Chemical and structural variability of illitic phases formed fromkaolinite in hydrothermal conditions

María Bentabol a, María Dolores Ruiz Cruz a,⁎, Francisco Javier Huertas b, José Linares b

a Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Málaga,Campus de Teatinos, 29071 Málaga, Spain

b Estación Experimental del Zaidín, CSIC, Prof. Albareda 1, 18008 Granada, Spain

Received 25 May 2005; received in revised form 13 December 2005; accepted 19 December 2005Available online 17 February 2006

Abstract

The transformations of kaolinite in the system Na2O–K2O–MgO–Al2O3–SiO2–H2O–HCl at 200 °C, with different MgCl2contents, have been investigated by X-ray diffraction and transmission/analytical electron microscopy. Two contrastingmechanisms of illite formation were identified: 1) direct precipitation of illite from dissolution of zeolites, which occurs at high pHand low Mg contents; and 2) formation of illitic phases from recrystallized kaolinite, either through dissolution–precipitationprocesses or by topotactic replacement. The second mechanism occurs at higher Mg contents and almost neutral conditions. Underthese later conditions, kaolinite dissolution–precipitation processes occur at the earliest stages of reaction, which lead to theformation of Mg-rich kaolinite.

The illitic phases formed through both mechanisms have different structural and chemical characteristics. Direct precipitationfrom solution produces well-ordered illites with scarce phengitic substitution and high Na contents. Illitic phases formed fromkaolinite show an evolution from illite/smectite mixed-layers to illite, at increasing run times. These phases are characterized byhigh octahedral Mg content (in the order of 0.8 apfu, for O10(OH)2), and lack of Na.

The activity diagrams constructed from the data of solutions confirm the data obtained from the study of the solid products.© 2006 Elsevier B.V. All rights reserved.

Keywords: Hydrothermal synthesis; Kaolinite; Illite; Mixed-layer minerals; Zeolite; TEM/AEM; XRD

1. Introduction

Experimental illitization of kaolinite in aqueoussolutions containing K has been a subject of study fora number of years. Velde (1965) synthesised muscovitefrom kaolinite and KOH solutions from 300 °C. Cher-mak and Rimstidt (1990) determined experimentally the

⁎ Corresponding author. Fax: +34 952132000.E-mail address: [email protected] (M.D. Ruiz Cruz).

0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.clay.2005.12.003

rate of kaolinite to illite transformation in KCl solutionsat temperatures between 210 and 307 °C and Huang(1993) reported results on the kinetics of kaolinite tomica conversion in alkaline solutions at temperatures of150, 175 and 200 °C. Bauer et al. (1998) studied thetransformation of kaolinite in high molar KOH solu-tions at low temperature (35 and 80 °C). According tothe results of these studies, the rate of illitization ismainly influenced by both temperature and pH. In ad-dition, the initial rate of illitization at basic pH is two orthree orders of magnitude faster than that of similar

112 M. Bentabol et al. / Applied Clay Science 32 (2006) 111–124

reactions at near neutral conditions (Huang, 1993). Inmost of these experiments, illite formation is accompa-nied by precipitation of K-zeolites, which behave astransitional phases that evolve toward illite or K-feld-spar at increasing reaction time. Nevertheless, the roleof zeolites in the illitization process has not been inves-tigated in detail.

Other factors can, moreover, influence the illitiza-tion reaction, such as the solution / solid ratio (Yatesand Rosemberg, 1996, 1997) and the chemical compo-sition of the system. Thus, Güven and Huang (1991),starting from illitic gels, found that the lack of Mg inthe system gives rise to poorly crystalline illite andrandomly interstratified illite–smectite. On the con-trary, illitic gels containing Mg produced no illite butpure smectite. Bentabol (2003) and Bentabol et al.(2003) described the results of the kaolinite illitizationin solutions containing NaOH, KOH, and MgCl2. Thepresence of Na+ ions in the system caused the forma-tion of analcime, whereas in most of the analyses ofillite Na was below the detection limit. On the otherhand, the presence of Mg2+ ions in the system origi-nated thin illite packets covered by 14-Å clinochlore-like layers. This study also suggested that both the illiteformation mechanisms and the illite composition wereinfluenced by the amount of Mg2+ in solution.

The present research was planed in order to investi-gate in detail the influence of the Mg content on thecomposition of the illite, and the role of zeolites on theillitization mechanism.

2. Methodology

The starting material consisted of poorly crystalline kao-linite from Georgia (standard KGa-2, from the Clay MineralSociety Source Clays Repository), after an intense grinding

Fig. 1. Unoriented X-ray diffraction pattern of the starting kaolinite (ground kof abundant amorphous material. Ant: anatase.

(HSM 100 vibration grinder). Transmission electron micros-copy (TEM) observations and granulometric analyses indicat-ed that the ground kaolinite is composed of rounded particleswith an average size of b0.05 μm (González Jesús et al.,2000). Grinding also led to amorphization of a high fractionof kaolinite, as deduced from the presence of a broad diffrac-tion band between 20 and 30 °2θ (Fig. 1). The impuritiesdetected by X-ray diffraction (XRD) and transmission/analyt-ical electron microscopy (TEM/AEM) consist of abundantgrains of anatase and minor ilmenite.

The following reactions were studied:

1kaolinite þ 0:6KOH þ 1:5NaOH ð1ÞSolution / solid ratio=15

1kaolinite þ 0:6KOH þ 1:5NaOH þ 0:3MgCl2 ð2ÞSolution / solid ratio=15

1kaolinite þ 0:6KOH þ 1:5NaOH þ 0:6MgCl2 ð3AÞSolution / solid ratio=4

1kaolinite þ 0:6KOH þ 1:5NaOH þ 0:6MgCl2 ð3BÞSolution / solid ratio=30.

Reactions 1 and 2 at a fixed time of 30 days, and Reaction3 with a solution / solid ratio=15 have been previously inves-tigated (Bentabol et al., 2003) and some of these results willbe used in this work for comparison.

The hydrothermal treatments were conducted in 50 cm3

Teflon-lined reactors (Parr 4744), which were maintained at aconstant temperature of 200 °C (±3 °C), with reaction timesfrom 1 to 90 days. At the end of the runs the reactors werequickly quenched in cold water. The solid products wererepeatedly cleaned with deionized water and centrifuged.

The solid products of the reactions were characterized byXRD and TEM/AEM. XRD patterns were obtained using aSiemens D-5000 powder diffractometer with CuKα radiationat 40 kV and 30 mA, with a step size of 0.02 °2θ and a

aolinite). The background between 20 and 30 °2θ reflects the presence

113M. Bentabol et al. / Applied Clay Science 32 (2006) 111–124

counting time of 1 s. Randomly oriented XRD patterns werescanned between 2 and 65 °2θ. Oriented samples werescanned between 2 and 30 °2θ, in the air-dried state, aftersolvation with ethylene glycol (EG), and after heating at 350°C.

The TEM study was carried out using a Philips CM-20microscope operated at 200 kV and a Jeol 3000 F, at 300 kV,both fitted with an ultrathin window, solid-state Si(Li) detectorfor energy dispersive X-ray analysis (EDAX and OXFORD,respectively). The solid products were encased within anepoxy resin and then sliced. The atomic percentage was cal-culated by the Cliff–Lorimer thin-film ratio criteria (Lorimerand Cliff, 1976).

3. Results

3.1. XRD results

The unoriented XRD patterns of Reactions 1, 2, 3Band 3A, after 5 days are shown in Fig. 2. The pattern ofthe solid products of Reaction 1 (Fig. 2A) reflects anintense dissolution of the starting kaolinite, accompa-nied by the precipitation of phillipsite (main reflections

Fig. 2. X-ray diffraction patterns (obtained from randomly oriented samples)Ant: anatase. Chl: chlorite. Kln: kaolinite. Bhm: boehmite. P: phillipsite. A:have been labelled. Mineral assemblages: Reaction 1: phillipsite+analcimeReaction 3B: kaolinite+chlorite+ illitic phases; Reaction 3A: kaolinite+phil

at 9.45, 3.19 and 2.93 Å), analcime (main reflections at3.42 and 2.93 Å) and nepheline (main reflections at8.09, 7.56 and 4.36 Å). Weak reflections of boehmitewere also observed in this pattern (6.11 and 3.16 Å).

The XRD pattern of the solid products of Reaction 2(Fig. 2B) shows reflections of kaolinite, phillipsite andanalcime. The disappearance of the amorphous bandpresent in the XRD pattern of the starting kaolinite(see Fig. 1) reflects a process of dissolution–precipita-tion of kaolinite, together with precipitation of zeolites.

The XRD pattern of the solid products of Reaction3B (Fig. 2C) shows reflections corresponding to kao-linite, a trioctahedral phase and a mica-like phase. Thekaolinite reflections are more intense, as compared withthe anatase reflections, than those in the pattern of thestarting material, suggesting a kaolinite recrystalliza-tion and a better orientation of kaolinite after the amor-phous fraction has been dissolved. The micaceousphase produced a broad and asymmetrical reflectionat ∼11.6 Å, which expands slightly after solvationwith ethylene glycol and contracts at ∼10 Å afterheating, showing the characteristic behaviour of illite/

of the products of Reactions 1, 2, 3B and 3A after 5 days reaction time.analcime. The main reflections of analcime, phillipsite and nepheline+nepheline+boehmite; Reaction 2: kaolinite+phillipsite+analcime;lipsite+bohemite.


smectite interstratification (Reynolds, 1980). A weakreflection at ∼14 Å suggests the presence of a chlo-rite-like phase. Heating causes a slight contraction ofthe basal spacing, suggesting the presence of mixed-layers chlorite/smectite in addition to chlorite. Thepresence of a trioctahedral phase is also confirmedby the reflections present in the 06 region of thediagrams. The position of the 060 reflection of illite(d060=1.506 Å) suggests a phengitic composition. TheXRD pattern of the solid products of Reaction 3A(Fig. 2D) shows an increase in intensity of the kaolin-ite reflections, that indicates recrystallization, togetherwith another phillipsite polytype (main reflections at7.19, 5.06, 4.13, and 3.19 Å) and minor boehmite.

Increasing reaction times to 10 and 15 days causescarce modifications in the solid products of Reactions1 and 2, with the exception of the relative ratio of theseveral phases. Nepheline reflections increase in inten-sity, whereas the intensity of the phillipsite reflectionsslightly decreases. A slight decrease of the basal spac-ing of the illitic phases is observed in the solid productsof Reaction 3B. In addition, the 14-Å reflection is lack-ing in the patterns obtained from 15 days. Nevertheless,

Fig. 3. X-ray diffraction patterns (obtained from randomly oriented samplestime. Ant: anatase. Kln: kaolinite. Il: illite. P: phillipsite; N: nepheline. A: anabeen labelled. Mineral assemblages: Reaction 1: illite+phillipsite+analcnepheline; Reaction 3B: kaolinite+ illitic phases; Reaction 3A: kaolinite+ ill

weak shoulders on the low-angle side of the 060 reflec-tion of illite in the patterns obtained from 15 days (seeFigs. 3 and 4) suggests the presence of minor amountsof a trioctahedral phase, probably serpentine.

Increasing the reaction time to 30 days causes in bothReactions 1 and 2 two main effects: i) notable decreaseor disappearance of the phillipsite reflections; ii) de-crease (Reaction 2) or disappearance (Reaction 1) ofthe kaolinite reflections; and iii) presence of a new 10-Åreflection (Fig. 3A, B). This reflection does not varyappreciably after solvation with ethylene-glycol or heat-ing and can be ascribed to illite. The presence of intensezeolite reflections prevents the precise determination ofthe d060 spacing of illite. The XRD pattern of the solidproducts of Reaction 3B (Fig. 3C) shows again adecrease of the basal spacing of the illitic phase, at∼10.53 Å, revealing the decrease of the smectite-layers interstratified in the illite structure. The patternof the solid products of Reaction 3A (Fig. 3D) issimilar to that of the products of Reaction 3B butcontaining small phillipsite reflections.

The XRD patterns of the solid products of the reac-tions after 60 and 90 days are very similar. The phases

) of the products of Reactions 1, 2, 3B and 3A after 30 days reactionlcime. The main reflections of analcime, phillipsite and nepheline haveime+nepheline; Reaction 2: kaolinite+ illite+phillipsite+analcime+itic phases+phillipsite.

Fig. 4. X-ray diffraction patterns (obtained from randomly oriented samples) of the products of Reactions 1, 2, 3B and 3A after 90 days reactiontime. Kln: kaolinite. Il: illite. Ant: anatase. Bhm: boehmite. N: nepheline. A: analcime. The main reflections of analcime, phillipsite and nephelinehave been labelled. Mineral assemblages: Reaction 1: illite+analcime+nepheline; Reaction 2: illite+analcime+nepheline; Reaction 3B: kaolinite+ illite+(phillipsite)+ (analcime); Reaction 3A: kaolinite+ illite+bohemite+(phillipsite)+ (analcime).


identified in the products from Reactions 1 and 2 areillite, analcime and nepheline (Fig. 4A, B). The pro-ducts of Reaction 3B (Fig. 4C) consist of illite, kaolin-ite and minor zeolites, reflecting an increase in theillite /kaolinite ratio at increasing reaction times. Thepattern of the solid products of Reaction 3A shows, inaddition, the analcime and boehmite reflections (Fig.4D). The illitic phases formed in Reactions 1 and 2show basal reflections narrower than those observed atshorter reaction times. The behaviour after ethylene-glycol solvation and after heating indicates that thisphase is true illite. The illitic phases formed throughReactions 3A and 3B show asymmetrical reflections,which do not change in shape after the several treat-ments. This behaviour would correspond to poorlycrystalline illite.

3.2. TEM/AEM results

TEM observation of the solid products of Reactions1 and 2 after 90 days show the presence of large crystals

of zeolites and analcime, and clusters of packets of theillitic phases with sizes ranging from ∼50 to N500 Å(Fig. 5). The illite formed from Reaction 1 produceswell-defined SAED patterns and high-resolution imageswith a regular 10-Å periodicity (Fig. 6A). Defects con-sisting of layer terminations and presence of whitefringes, as observed in the products obtained after 30days reaction time (Bentabol et al., 2003) are absent inthe products obtained after 90 days. The illite packetsformed from Reaction 2 show, at their surfaces, layerswith basal spacing of 14 Å (Fig. 6B), similar to thosedescribed by Bentabol et al. (2003).

Some selected AEM data for the illitic phasesformed through Reactions 1 and 2 are shown in Table1. The analyses of the illites obtained from Reaction 1are homogeneous, with a Si content in the order of 3.3apfu (atoms per formula unit, calculated for O10OH)2).These analyses show some Fe, coming probably fromilmenite dissolution. The rapid loss of some Na and Kduring the obtaining of the analyses originates formulaewith relatively low interlayer charge (0.50–0.87). The

Fig. 5. Low magnification TEM images of illite packets formed fromReactions 1 (A) and 2 (B), after 90 days.

Fig. 6. Lattice-fringe images of illites formed from Reactions 1 (A)and 2 (B) after 90 days. In A, the illite packet shows regular 10-Åperiodicity. The Fourier transform (inset) indicates the presence of aone-layer polytype. The filtered image is also shown. In B, the illitepackets with regular 10-Å periodicity, appear covered by 14-Å lightlayers (arrows).


compositions range from typical K-rich illite (analysis4) to intermediate Na–K–illite (analysis 3). The analy-ses of the illitic phases obtained from Reaction 2 show ahigher variability. The Si content ranges from 3.68 to3.00 apfu. Simultaneously, the interlayer charge variesbetween 0.30 and 0.74, which suggests the presence oftrue illite and illite/smectite mixed-layers with a vari-able composition. The Mg content ranges from 0.08 to0.32 apfu. Na has been detected in most of the analyses,reaching up to 0.23 apfu.

The TEM observation of the solid products formedfrom Reaction 3B at short reaction times (5 days)reveals the presence of several chemically differentphyllosilicates: kaolinite, a Mg-rich phase (chlorite)and illitic phases. Kaolinite forms thick packets (inthe order of 500 Å in thickness), whose SAED pat-terns reveal the presence of a polytype with abundantstacking disorder (Fig. 7A). On the contrary, both theillitic packets and those with Mg-rich composition arevery thin and do not allow us to obtain SAED patterns(Fig. 7B).

Some representative AEM data for these phases areshown in Table 2 (analyses 1–9). Kaolinite containsimportant amounts of Mg (in the order of 0.30 apfu,

calculated for O5(OH)4). Since the Si content is close to2 apfu, the presence of Mg in the kaolinite analysescannot be related to a sort of contamination but to eitherMg for Al replacement or presence mixed-layering ka-olinite/serpentine. Only some of the analyses obtainedfrom the Mg-rich phases lead to structural formulaetypical of chlorite (Table 2, analysis 3). Generally, theanalyses show a high Si content and the presence ofappreciable amounts of K, suggesting that they corre-spond to either mixtures of illite and chlorite or mixed-layered structures. According to this later assumption,the analyses of these phases have been recalculated forO20(OH)10 (Table 2, analyses 4–6). The illitic phasesalso show systematically a high Si content (3.50–7 3.70apfu), characteristic of smectite and illite/smectitemixed-layers. The low interlayer charge appears to con-firm this assumption. These phases show appreciableMg contents, which lead in cases (Table 2, analysis 9) toa di/trioctahedral composition. Na has not been detectedin the analyses.

Table 1Selected AEM data for the solid products (illitic phases) of Reactions 1 and 2

Reaction 1 Reaction 2

1 2 3 4 5 6 7 8 9 10 11 12 13

Si 3.37 3.33 3.32 3.29 3.68 3.54 3.43 3.43 3.36 3.26 3.20 3.17 3.00AlIV 0.63 0.67 0.68 0.71 0.32 0.46 0.57 0.57 0.64 0.74 0.80 0.83 1.00AlVI 1.95 1.93 2.00 1.87 1.93 1.96 2.00 1.80 1.98 1.99 2.02 1.90 2.00Mg – – – – 0.13 0.11 0.12 0.32 0.12 0.08 0.08 0.19 0.17Fe 0.10 0.12 – 0.11 – – – – – – – – –Σoct. 2.05 2.05 2.00 1.98 2.06 2.07 2.12 2.12 2.10 2.07 2.10 2.09 2.17Na 0.11 0.10 0.31 0.12 – 0.06 0.18 0.23 0.21 0.06 – 0.18 0.18K 0.47 0.50 0.38 0.75 0.30 0.30 0.17 0.39 0.24 0.22 0.57 0.56 0.48O 11 11 11 11 11 11 11 11 11 11 11 11 11


Increasing reaction times causes two main modifica-tions in the products of Reaction 3B i) disappearance ofthe chloritic phases and ii) increase in size of the illiticpackets. The SAED patterns show a uniform 10-Åperiodicity but the lattice fringe images show an evolu-tion from areas with wavy fringes and periodicities upto 15 Å, to areas with well-defined 10-Å fringes, indi-cating the presence of smectite-bearing mixed-layers

Fig. 7. A. Low magnification TEM image of Mg-rich kaolinite. TheSAED pattern (inset) shows the 7-Å periodicity and almost continuous0kl reflection rows, indicating stacking disorder. B. Thin and curvedpackets of Illite and chlorite. (Reactions 3A and 3B, 5 days).

and true illite (Fig. 8A and B). Indeed, the AEM dataobtained from several types of packets reveal a highvariability in Si content (Table 2, analyses 12–16).Nevertheless, the small size of most of the illitic packetsanalysed causes a rapid loss of K, leading to low inter-layer occupancies. Kaolinite does not show any appre-ciable chemical difference relative to the reaction atlower times (Table 2, analyses 10–11).

3.3. Chemical data of the solutions

The initial pH of the reactions ranged from 13.63(Reaction 1) to 11.97 (Reaction 3A). The pH values(Table 3) decreased strongly after short reaction timesand slightly increased later. The strongest decrease ofthe pH occurred in the solutions of Reactions 3A and3B. The pH values after 90 days varied between 12.92(Reaction 1) and 7.15 (Reaction 3B).

Fig. 9 shows the variation with time of the Si, Al,Mg, Na and K concentration in solution, measured atroom temperature. A sudden increase of both Si and Alis observed in all the reactions at short reaction times,reflecting a first stage of kaolinite dissolution. Theseconcentrations decrease after 15–30 days of reactiontime, reflecting the precipitation stage. Na, K and Mgcontents show a parallel behaviour in the three reac-tions: a strong decrease in concentration at short reac-tion times (from 1 day) that stabilize for longer times.Nevertheless, whereas Mg and K concentration is verylow in the solutions of the three reactions, the Nacontent decreases from Reaction 3A to Reaction 1 andreflects the variable content in zeolites in the threecases.

Equilibrium among clay minerals at 200 °C has beenportrayed on the equilibrium activity diagrams shown inFigs. 10 and 11. Activities of the species in solution(Table 3) were calculated using the SOLMINEQ.88computer program for aqueous speciation calculations

Table 2Selected AEM data for the solid products of Reactions 3A and 3B

Reaction time: 5 days Reaction time: 60 days

Kaolinite Chloritic phases Illitic phases Kaolinite Illitic phases

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Si 2.08 1.98 3.03 6.16 6.17 5.63 3.64 3.62 3.49 2.00 1.91 3.85 3.50 3.16 3.13 3.05AlIV 0.00 0.02 0.97 1.84 1.83 2.37 0.36 0.38 0.51 0.00 0.08 0.15 0.50 0.84 0.87 0.95AlVI 1.70 1.79 1.36 2.79 2.11 2.36 1.47 1.53 1.22 1.77 1.75 1.52 1.56 1.72 1.62 1.59Mg 0.29 0.31 4.43 5.65 6.65 6.44 0.86 0.78 1.34 0.32 0.42 0.62 0.73 0.68 0.79 0.86Σoct. 1.99 2.10 5.79 8.44 8.76 8.80 2.33 2.31 2.56 2.09 2.17 2.14 2.29 2.40 2.41 2.45Na – – – – – – – – – – – – – – – –K 0.03 – 0.02 0.02 0.15 0.24 0.26 0.30 0.14 0.05 0.04 0.38 0.31 0.30 0.43 0.45O 7 7 14 25 25 25 11 11 11 7 7 11 11 11 11 11


(Kharaka et al., 1988). Activity plots were built usingthermodynamic data from the EQ3/6 software database(Wolery, 1992), implemented with specific equilibriumconstants calculated in previous studies (Table 4). Thesolubility constants for particular chemical composi-tions of smectites and illites were computed assuminga solid solution among the end members. The clino-chlore solubility constant was derived from the resultsof Reaction 2. In this set of experiments, the assemblagechlorite–illite in small packages observed by TEM wasinterpreted as equilibrium between both minerals, aspreviously assumed by Bentabol et al. (2003). Thefree energy and enthalpy of formation at 25 °C of theNa- and K-phillipsite end members (Bowers and Burns,1990) were used to calculate the solubility constants at200 °C using SUPCRT92 software (Johnson et al.,1992). Mineral equilibrium reactions were written con-

Fig. 8. Lattice fringe images of illite/smectite mixed-layers (A) and illite (Bshows regular 10-Å periodicity.

sidering Al as an immobile element. For these diagramswe have used the chemical data obtained from Reaction3A, since the low solution / solid ratio should favour theequilibrium between solids and solutions.

The activity plots were drawn using combinations ofa(H4SiO4), a(Al

3+) /a(H+)3, a(Mg2+) /a(H+)2 a(K+) /a(H+) and a(Na+) /a(H+). On the a(Al3+) /a(H+)3 vs a(H4SiO4) diagram, the points corresponding to the threereactions appear aligned, spanning from the kaolinitefield to the boehmite field (Fig. 10A), in agreement withthe XRD data. On the a(Na+) /a(H+) vs a(H4SiO4)diagram (Fig. 10B), the solutions, also aligned, spanfrom the albite (2, 3A and 3B) to the analcime fields(Reactions 1 and 2). Whereas analcime is an abundantproduct of Reactions 1 and 2, albite has not been clearlyidentified in the products of Reactions 2, 3A and 3B.XRD data also shows that nepheline and phillipsite

) formed from reaction 3 after 60 days. The Fourier transform (inset)

Table 3Logarithm of the activity of the species in solutions at the experimental temperature (200 °C), calculated with SOLMINEQ.88 (Kharaka et al., 1988)

Reaction 1 (pH initial at 25 °C=13.63)

Time (days) pH H4SiO4 Al3+ Na+ K+ H+

5 12.62 −2.855 −28.97 −1.374 −2.713 −10.06210 12.11 −2.741 −27.45 −1.542 −2.517 −9.64815 12.27 −2.728 −28.04 −1.510 −2.890 −9.81330 12.50 −3.144 −28.38 −1.748 −2.376 −9.83960 12.70 −3.100 −29.19 −1.286 −7.298 −10.04390 12.92 −3.412 −29.70 −1.255 −3.189 −10.203

Reaction 2 (pH initial at 25 °C=13.20)

Time (days) pH H4SiO4 Al3+ Mg2+ Na+ K+ H+

5 11.68 −2.659 −25.73 −5.370 −1.069 −0.116 −9.27310 11.38 −2.543 −25.18 −5.214 −1.072 −2.072 −9.14115 10.89 −2.259 −24.96 −5.091 −1.068 −2.050 −9.05430 10.10 −2.302 −20.14 −3.152 −1.125 −1.909 −7.79760 11.27 −2.938 −23.44 −4.526 −0.942 −2.282 −8.68490 10.65 −2.462 −22.76 −4.121 −1.021 −2.168 −8.503

Reaction 3A (pH initial at 25 °C=11.97)

Time (days) pH H4SiO4 Al3+ Mg2+ Na+ K+ H+

1 8.56 −1.621 −18.55 −3.949 −0.474 −1.550 −7.5353 6.91 −1.201 −15.39 −2.336 −0.494 −1.266 −6.6355 9.31 −1.849 −21.08 −4.431 −0.590 −1.647 −8.15310 10.41 −2.415 −22.59 −4.780 −0.602 −1.657 −8.52115 7.83 −2.240 −17.91 −4.315 −0.457 −1.610 −7.26830 8.39 −2.007 −22.40 −3.907 −0.559 −1.685 −8.48760 8.72 −1.649 −18.68 −3.524 −0.510 −1.859 −7.573


precipitate at very short reaction times and later theyprogressively dissolved. Such behaviour as metastablephases is also consistent with the activity plots, sincephillipsite stability field does not appear in the activitydiagrams, which indicate that phillipsite is not a stablephase under our experimental conditions of temperatureand pressure. Two independent diagrams a(Mg2+) /a(H+)2 vs a(H4SiO4) and a(Mg2+) /a(H+)2 vs a(K+) /a(H+) have been drawn for Reactions 2, 3A and 3B. Thefirst set (Fig. 11A, C) was constructed using illite withlow phengitic component (for Reaction 2) and the sec-ond set (Fig. 11B, D), using illite with 0.8 Mg (for thesolutions of Reactions 3A and 3B). In the first set, theexperimental points distribute along the clinochlore–illite equilibrium boundary. Equilibrium was assumedin order to calculate the clinochlore solubility constant,according to the observation by TEM of the thin illitepackets coated by clinochlore-like layers. The solutionsof Reactions 3A and 3B show in Fig. 11B and D anevolution from clinochlore towards the illite (or mont-morillonite) field, probably within the field (not drawn)of the illite/smectite mixed-layers, in accordance withthe evolution observed by XRD.

4. Discussion

The different concentration of MgCl2 in the systemsstudied influences slightly the initial pH of the reactionsand especially determines the lowering of the pH atincreasing reaction times (from 12.92 in Reaction 1 to7.15 in Reaction 3B). The high pH values characteriz-ing Reaction 1 cause the almost immediate dissolutionof kaolinite and the precipitation of zeolites and neph-eline. The coupled kaolinite dissolution/zeolite–nephe-line precipitation produces the drop of the solution pHduring the first reaction stage. On the contrary, the firstprocess observed in Reaction 3B was the dissolution–precipitation of kaolinite, accompanied by formation ofchlorite-like and illite-like phases. As a consequence ofthe dissolution–precipitation process, the rounded par-ticles of low-crystalline starting kaolinite are replaced atshort reaction times by relatively large packets of Mg-rich kaolinite (Fig. 7A). Reaction 2 shows an interme-diate behaviour.

Natural kaolinites frequently show slight deviationsfrom ideal stoichiometry due to small quantities of Fe,Ti, K and Mg (Deer et al., 1972; Giese, 1991). The Mg

Fig. 9. Room temperature concentrations of Si, Al, Mg, Na and K in solutions as a function of the reaction time.


content is generally b0.02 apfu in kaolinite whereas theFe content can reach up to 0.1 apfu (Newman andBrown, 1987). Nevertheless, the synthesis of Fe3+-kao-linites up to 0.6 Fe apfu in octahedral positions demon-strates that kaolinite permits a considerable degree ofisomorphic substitution as revealed by the linear corre-lation between the b-parameter and the Fe for Al sub-stitution (Iriarte et al., 2005). A detailed study of theseanomalous Mg-rich kaolinites is in preparation.

The early precipitation of zeolites in Reactions 1 and2 withdraws Na and K from solution and prevents theformation of illitic phases in the first stages of thereaction. This control of the illitization process is alsoknown in natural systems, where the presence of K-rich

zeolites (K-clinoptilolite) can retard or even prevent theillite formation (e.g. Altaner and Grim, 1990). Theformation of illitic phases is also retarded in the caseof Reaction 3A, relative to Reaction 3B, due to its lowersolution / solid ratio. On the contrary, in Reaction 3B,the illitic phases formed from 5 days.

Increasing reaction times cause the progressive dis-solution of phillipsite and nepheline in Reactions 1 and2, and the parallel precipitation of illitic phases, whereasincreasing run times is reflected, in Reaction 3B, in thedisappearance of the chlorite, the increase of the illite /kaolinite ratio and the evolution of the chemical andstructural characteristics of the illitic phases. These fea-tures reveal two contrasting mechanisms of illitization:

Fig. 10. Plots of the solutions of Reactions 1, 2 and 3A on the activity diagrams a(Al3+) /a(H+)3 vs a(H4SiO4) (A) and a(Na+) /a(H+) vs a(H4SiO4) (B).

Fig. 11. Plots of the solutions of Reactions 2, 3A and 3B on the activity diagrams a(Mg2+) /a(H+)2 vs a(H4SiO4) (A and B) and a(Mg2+) /a(H+)2 vs a(K+) /a(H+) (C and D).


Table 4Solubility constants (at 200 °C) used to construct the activity diagrams

Mineral log K

Quartz SiO2 −2.438 a

Amorphoussilica

SiO2 −1.819 a

Boehmite AlOOH −0.052 a

Kaolinite Al2Si2O5(OH)4 −5.597 b

Na−montmorillonite

Na0.33(Mg0.33Al1.67)(Si4)O10(OH)2 −6.083 a

K−montmorillonite

K0.33(Mg0.8Al1.434)(Si4)O10(OH)2 −3.102 c

Illite 1 K0.6(Mg0.25Al1.8)(Al0.5Si3.5)O10(OH)2 −4.612 a

Illite 2 K0.6(Mg0.25Al1.8)(Al0.5Si3.5)O10(OH)2 −0.526 c

Clinochlore Mg3(Mg2Al)(AlSi3)O10(OH)8 54.187 d

Albite NaAlSi3O8 −2.389 a

Microcline KAlSi3O8 −3.774 a

Analcime NaAlSi2O6 H2O −0.108 a

Nepheline NaAlSiO4 4.369 a

Na-phillipsite Na2Al2Si5O14·5 H2O 0.813 e

K-phillipsite K2Al2Si5O14·5 H2O −5.347 e

a EQ3/6 database.b Estimated from kaolinite recrystallisation in water (Bentabol,

2003).c Calculated from the end-members data (Bentabol, 2003).d This study, assuming clinochlore–illite equilibrium in Reaction 2.e Estimated with SUPCRT92 (Johnson et al., 1992) using the ΔG°f

and ΔH°f values reported by Bowers and Burns (1990).


i) direct precipitation from solution (Reactions 1 and2); and ii) formation of illitic phases directly fromkaolinite, either by a dissolution–precipitation processor by topotactic replacement (Reactions 3A and 3B,and possibly 2).

The first mechanism leads to formation of large illitepackets from 30 days of reaction whereas the secondmechanism originates illite/smectite mixed-layers thatgradually evolve to illite. These structural differencesare also accompanied by notable chemical differences.

The illites formed in Reaction 2 by direct precipita-tion are characterized by low Mg content in the octahe-dral sheet. The illites formed in Reaction 1 (Mg-freesystem) incorporate, however, minor amounts of Fe,coming from impurities dissolution. In both cases, theillites are also characterized by the presence of appre-ciable amounts of Na in the interlayer. The entry of Naappears to be controlled by the Na availability in solu-tion. Thus, the illites formed at 30 days, when thezeolite content is high, do not contain appreciableamounts of Na (Bentabol et al., 2003), whereas the Nacontent increases notably at longer reaction times, whenthe dissolution of the Na-rich phases increases. The Nacontent is higher than that generally observed in naturalillite, and clearly within the solvus muscovite-parago-nite (Guidotti, 1987). Nevertheless, intermediate Na–K

dioctahedral micas have been described in diagenetic tolow-grade terrains (e.g. Frey, 1987, and referencestherein). These intermediate micas were firstly inter-preted as mixed-layers muscovite/paragonite (Frey,1969, 1970; Kisch, 1983; Merriman and Roberts,1985), but the most recent studies indicate that theyare metastable phases, with intermediate compositionbetween paragonite and muscovite, which evolve atincreasing grades toward discrete muscovite and para-gonite (Jiang and Peacor, 1993; Li et al., 1994; Livi etal., 1997).

The illitic phases formed from Reactions 3A and 3Bare, however, characterized by their high Mg contentand lack of Na as interlayer cation. Both chemicalcharacteristics are related, since it is known that thestructural modifications induced by the entry of Mg orFe2+ in the octahedral sheet modifies the size of theinterlayer cavities. As a consequence, the phengite con-tent on mica increases and the Na / (Na+K) ratiodecreases, in such a way that only minor amounts ofFe2+Mg are possible in the structure of paragonite(Guidotti, 1987). The octahedral Mg content is ratheruniform in the products obtained at different reactiontimes, suggesting that this Mg content represents equi-librium conditions among solid products and solutions.Illites with such Mg-contents have not been observed innatural environments. Divalent cations can reach up to aquarter of the octahedral sites in natural micas (e.g.Hower and Mowatt, 1966; Weaver and Pollard, 1973)and it is generally a consequence of a phengitic substi-tution (Guidotti, 1987). Since the total octahedral occu-pancy is clearly N2 in the illitic phases formed fromReactions 3A and 3B, we assume that the deviationfrom the ideal dioctahedral mica toward trioctahedralmica occurs through a substitution 2R3+ =3R2+, as dis-cussed by Brown (1968) and Guidotti (1987). Previousresults reported by Bentabol et al. (2003) indicate thatthe illites obtained at 300 °C in the presence of Mg arealso Mg-rich, the Mg content being higher in the illitesformed at 300 °C than in those originated at 200 °C.These features suggest that it is possible to obtain ex-perimentally micas with a wide range of Mg contents.Indeed we have recently synthesised illites (probablymetastable) with Mg-contents higher than those de-scribed here (A manuscript on this subject is in prepa-ration by the authors of the present paper).

Comparison of the results obtained in Reaction 3Band those obtained by Bentabol et al. (2003) using alower solution / solid ratio indicates that the solid pro-ducts re-equilibrate more rapidly at higher solution /solid ratios. These authors describe the formation ofthin illitic packets, coated by clinochlore-like layers,


similar to those described here in the products of Reac-tion 2, whereas discrete packets of chlorite were notobserved. The results presented here indicate that, ef-fectively, the assemblage chlorite+ illitic phases form atshort reaction times and high solution / solid ratio. Nev-ertheless, this assemblage is, in its turn, metastable,since the chloritic phases disappear at longer run timesand lead to Mg-rich illites.

The study of the solutions basically confirms themineral evolution identified by XRD and TEM/AEMat increasing run times, which indicate that illites arethe prevalent phases in long time experiments. At veryshort time, the dissolution of kaolinite at highly alka-line pH produces high concentrations of silica andaluminium, which induces the precipitation of nephe-line, phillipsite and analcime in Reactions 1 and 2.Phillipsite is thermodynamically unstable under theseconditions of temperature and pressure and its stabilityfield is not portrayed in the activity plots (Fig. 10B).XRD data indicate the progressive dissolution ofnepheline and phillipsite; analcime remains as a stablephase in Reaction 1, and albite or microcline are notdetected despite they appear as stable minerals inReactions 2 and 3A.

The stability diagrams indicate that the illite for-mation is controlled by the availability of Mg and Kin solution. Initial precipitation of the metastable phil-lipsite affects the activity of K and inhibits precipita-tion of illite. In Reaction 1, where there is no Mg insolution, some Fe released by dissolution of impuri-ties allows the formation of illite. This is consistentwith the observations of Fiore et al. (2001), whoreported that Fe or Mg was necessary for the smectitecrystallisation in the initial stage of alteration of rhy-olitic obsidian. In Reaction 2, the assemblage illite–chlorite controls the Mg activity (Fig. 11A and C).Concerning Reaction 3B, Fig. 11B and D are consis-tent with XRD data: initial formation of chlorite-likeand illite-like phase close to the equilibrium boundaryand later progressive dissolution of chlorite and for-mations of additional illite crystals.

5. Concluding remarks

In the system studied, the amount of Mg in solu-tion strongly controls the pH and, consequently, theproducts formed from kaolinite. Low Mg contentsfavour high pH values, causing dissolution of kaolin-ite and early precipitation of zeolites±nepheline.Under these conditions, increasing run times causethe partial dissolution of zeolites and simultaneouslythe precipitation of illite. Higher Mg contents produce

the lowering of the initial pH of solutions up toalmost neutral conditions, and the formation of illiticphases occurs at short reaction time. The phasesformed are essentially illite/smectite mixed-layersthat evolve toward illite at increasing run times.

The composition of the illites formed from these twocontrasting mechanisms is also different. The amount ofMg in the octahedral sheet is controlled by the Mgconcentration in solution and the composition of themicas formed ranges from typical illites to Mg-richterms. This suggests that it is possible to obtain exper-imentally micas with a wide range of Mg contents. TheMg content in the octahedral sheet of the illitic phasescontrols, in its turn, the entry of Na in the interlayer, insuch a way that Mg-rich illites only contain K whereasthe Na content is high in the typical illites.

According to the results shown here, it is apparentthat: 1) illitization of kaolinite can be inhibited in natu-ral systems either by formation of K-consuming phases(such as zeolites) or in the presence of low fluid/rockratios, and 2) Mg concentration in solution can controlthe formation of either typical K-illites of intermediateNa–K white micas.

Acknowledgements

The authors are grateful to two unknown reviewers,whose corrections and suggestions have notably im-proved the manuscripts. We are also grateful to A.Gómez and J.L. Baldonedo for their assistance withthe TEM/AEM. This study has received financial sup-port from the Project BTE-2003-01382 (Ministerio deEducación y Cultura) and from the Research GroupRNM-199 (Junta de Andalucía).

References

Altaner, S.P., Grim, R.E., 1990. Mineralogy, chemistry, and diagenesisof tuffs in the Sucher Creek Formation (Miocene), eastern Oregon.Clays Clay Miner. 38, 561–572.

Bauer, A., Velde, B., Berger, G., 1998. Kaolinite transformation inhigh molar KOH solutions. Appl. Geochem. 5, 619–629.

Bentabol, M. 2003. Transformaciones hidrotermales de la caolinita.Ph.D. Thesis. Universidad de Málaga. 395 pp.

Bentabol, M., Ruiz Cruz, M.D., Huertas, F.J., Linares, J., 2003.Hydrothermal transformation of kaolinite to illite at 200 °C. ClayMiner. 38, 161–172.

Bowers, T.S., Burns, R.G., 1990. Activity diagrams for clinoptilolite:susceptibility of the zeolite to further diagenetic reactions. Am.Mineral. 75, 601–619.

Brown, E.H., 1968. The Si4+ content of natural phengites: a discussion.Contrib. Mineral. Petrol. 17, 78–81.

Chermak, J.A., Rimstidt, J.D., 1990. The hydrothermal transformationrate of kaolinite to muscovite/illite. Geochim. Cosmochim. Acta54, 2979–2990.


Deer, W.A., Howie, R.A., Zussman, J., 1972. Rock-forming minerals.Sheet Silicates, vol. 3. Longman, London. 270 pp.

Fiore, S., Huertas, F.J., Huertas, F., Linares, J., 2001. Smectiteformation from rhyolitic obsidian as inferred by microscopic(SEM–TEM–AEM) investigation. Clay Miner. 36, 489–500.

Frey, M., 1969. A mixed-layer paragonite/phengite of low-grademetamorphic origin. Contrib. Mineral. Petrol. 24, 63–65.

Frey, M., 1970. The steps from diagenesis to metamorphism in peliticrocks during Alpine orogenesis. Sedimentology 15, 261–279.

Frey, M., 1987. Very low-grade metamorphism of clastic sedimentaryrocks. In: Frey, M. (Ed.), Low Temperature Metamorphism.Blackie and Son Ltd, Glasgow, pp. 9–58.

Giese, R.F., 1991. Kaolin minerals: structures and stabilities. In:Bailey, S.W. (Ed.), Hydrous phyllosilicates (exclusive of micas)Rev. Mineral. Min. Soc. Am., vol. 19, pp. 29–66.

González Jesús, J., Huertas, F.J., Linares, J., Ruiz Cruz, M.D., 2000.Textural and structural transformations of kaolinite in aqueoussolutions. Appl. Clay Sci. 17, 245–263.

Guidotti, C.V., 1987. Micas in metamorphic rocks. In: Bailey, S.W.(Ed.),Micas. Rev. InMineral.Min. Soc. Am., vol. 13, pp. 357–467.

Güven, N., Huang, W.L., 1991. Effect of Mg2+ and Fe2+ on thehydrothermal illitization reaction. Clays Clay Miner. 39, 387–399.

Hower, J., Mowatt, T.C., 1966. The mineralogy of illites and mixed-layers illite/montmorillonite. Am. Mineral. 51, 825–854.

Huang, W.-L., 1993. The formation of illitic clays from kaolinite inKOH solution from 225 °C to 350 °C. Clays Clay Miner. 41,645–654.

Iriarte, I., Petit, S., Huertas, F.J., Fiore, S., Grauby, O., Decarreau, A.,Linares, J., 2005. Synthesis of kaolinite with a high level of Fe3+

for Al substitution. Clays Clay Miner. 53, 1–10.Jiang, W.-T., Peacor, D.R., 1993. Formation and modification of

metastable sodium potassium mica, paragonite and muscovite inhydrothermally altered metabasites from northern Wales. Am.Mineral. 78, 782–793.

Johnson, J.W., Oelkers, E.H., Helgeson, H.C., 1992. SUPCRT92: asoftware package for calculating the standard molal thermody-namic properties of mineral, gases, aqueous species and reactionsfrom 1 to 5000 bar and 0 to 1000 °C. Comput. Geosci. 18,899–947.

Kharaka, Y.K., Gunter, W.D., Aggarwal, P.K., Perkin, E.H., De Braal,J.D., 1988. SOLMINEQ.88: a computer program for geochemicalmodelling of water–rock interaction. U.S. Geological Survey

Water-Resources Investigation Report 88-4227. Washington.420 pp.

Kisch, H.J., 1983. Mineralogy and petrology of burial diagenesis (bu-rial metamorphism) and incipient metamorphism in clastic rocks.In: Larsen, G., Chilinger, G.V. (Eds.), Diagenesis in Sedimentsand Sedimentary Rocks. Elsevier, Amsterdam, pp. 289–493.

Li, G., Peacor, D.R., Merriman, R.J., Roberts, B., 1994. The diageneticto low-grade metamorphic evolution of matrix white micas in thesystem muscovite–paragonite in a mudrock from central Wales,United Kingdom. Clays Clay Miner. 42, 369–381.

Livi, K.J.T., Veblen, D.R., Ferry, J.M., Frey, M., 1997. Evolution of2 :1 layered silicates in low-grade metamorphosed Liassic shalesof Central Switzerland. J. Metamorph. Geol. 15, 323–344.

Lorimer, G.W., Cliff, G., 1976. Analytical electron microscopy ofminerals. In: Wenk, H.R. (Ed.), Electron Microscopy in Mineral-ogy. Springer-Verlag, New York, pp. 506–519.

Merriman, R.J., Roberts, B., 1985. A survey of white mica crystallinityand polytypes in pelitic rocks of Snowdonia and Llyn, NorthWales. Mineral. Mag. 49, 305–319.

Newman, A.C.D., Brown, G., 1987. The chemical constitution ofclays. In: Newman, A.C.D. (Ed.), Chemistry of Clays and ClayMinerals. Mineralogical Society, London, pp. 1–128.

Reynolds, R.C., 1980. Interstratified clay minerals. In: Brindley, G.W.,Brown, G. (Eds.), Crystal Structures of Clay Minerals and their X-ray identification. Mineralogical Society, London, pp. 249–303.

Velde, B., 1965. Experimental determination of muscovite polytypesstabilities. Am. Mineral. 50, 436–449.

Weaver, C.E., Pollard, L.D., 1973. The Chemistry of Clay Minerals.Elsevier. 213 pp.

Wolery, T.J., 1992. EQ3/6, a Software Package for GeochemicalModelling of Aqueous Systems. (Version 7) Laurence LivermoreNational Laboratory, CA. UCRL-MA-110662.

Yates, D.M., Rosemberg, P.E., 1996. Formation and stability ofend-member illite: I. Solution equilibration experiments at100 to 250 °C and PV,sol. Geochim. Cosmochim. Acta 60,1873–1883.

Yates, D.M., Rosemberg, P.E., 1997. Formation and stability of end-member illite: II. Solid equilibration experiments at 100 to 250 °Cand PV,sol. Geochim. Cosmochim. Acta 61, 3135–3144.

chemical and structural variability of illitic phases ... · chemical and structural variability of...

Documents