Study of Natural Halloysite from the Dragon Mine, Utah (USA)

ARTICLE

DOI: 10.1002/zaac.200900076

Study of Natural Halloysite from the Dragon Mine, Utah (USA)

Francisco J. Garcıa Garcıa,[a] Sergio Garcıa Rodrıguez,[b] Andreas Kalytta,[a] andArmin Reller*[a]

Dedicated to Professor Reinhard Nesper on the Occasion of His 60th Birthday

Keywords: Halloysite nanotubes; X-ray diffraction; Electron microscopy; Structural characterisation; Thermal stability

Abstract. A natural sample from the Dragon Mine, Tintic District,Silver City, Utah (USA), has been characterised. It contains a mix-ture of halloysite and alunite. The former shows the characteristictubularly shaped particles while the latter is found to crystallise inplaty particles. The crystal structure of halloysite is stable up to400�450 °C and before decomposition some crystallisation is

IntroductionHalloysite is a clay mineral belonging to the kaolinite

group [1]. It was first studied as early as 1826 by Berthier[2]. Since then, the related literature is extensive and a vastnumber of publications concerning this mineral have beenpublished. We have carried out a non-official literaturesearch in the subject (Through the “ISI web of knowledge”by selecting halloysite as topic) and the result is quite re-markable: 296 published articles only in this century. Thereason is easily understandable after a short review of themain subject matters of those reports. This mineral is exten-sively used in industry for ceramics, cements and fertilisers,among others. As a curiosity, we mention that in the pastit has been used to produce the highest-quality porcelain inChina. Of special interest is the number of potential appli-cations derived from the peculiar tubularly shaped particlesnormally found in this mineral. Such a peculiarity opens upthe interest [3] as a template in nanotechnology, i.e. [4, 5]. Inthis context, it is physically impossible to review the wholeliterature concerning this mineral and we just refer to arecent publication [6], which constitutes a compact andextensive review on this mineral.

The general stoichiometry is Al2Si2O5(OH)4 ·nH20 withn � 0 or 2 [7] for the anhydrous and the fully hydrated

* Prof. Dr. A. RellerFax: �49-8215983002E-Mail: [email protected]

[a] Lehrstuhl für Festkörperchemie, Institut für PhysikUniversität AugsburgUniversiätstrasse 186159 Augsburg, Germany

[b] Instituto de Catalisis y Petroleoquımica, CSICC/Marie Curie, 2, Cantoblanco28049 Madrid, Spain

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observed. Amorphisation is observed with further increasing theannealing temperature. However, the halloysite particles conservethe original morphology (tubes) up to 900 °C. The size distributionof the tubes is found to be quite broad. This is argued to be respon-sible for the poor measured surface area.

halloysite, respectively. The symmetry is either monoclinic(C2/m, C21/c or Cc have been reported) or triclinic, i.e [8].Unit cell parameters are a � 5.14(4) A, b � 8.90(4) A,c � 14.9(1) A, β � 101.9° with � � γ � 90° for the mono-clinic case and both different but close to 90° in the tricliniccase. These values correspond to the anhydrous compoundand the insertion of water molecules strongly influences thec parameter, which is increased to � 20.4 A for n � 2 (fullyhydrated). In principle, any value between these two ex-tremes is possible and, hence, c is a direct indication of thewater content. Thus, in the literature this is referred to ashalloysite-(7 A) and halloysite-(10 A) emphasising on theposition of the 002 reflection in the X-ray powder diffrac-tion pattern. It is noteworthy that this is the reflection withthe highest 2θ value to appear.

Halloysite is a layer silicate and the crystal structure isdescribed as formed by two building blocks [9]: (i) sheets ofcorner sharing [SiO4] tetrahedra and (ii) sheets of edge-sharing [AlO6] octahedra, see Figure 1 (b) and (c). In the[SiO4] sheets the tetrahedra share three corners and the un-bounded tetrahedral apices are all pointing along the samedirection. These are shared with the neighbouring octa-hedral sheets as shown in Figure 1. Within the octahedralsheet, only 2/3 of the existing octahedra sites are filled byaluminium. It is referred to be a dioctahedral layer (termnormally used in mineralogy). The layers building up thefinal structure are composed by one of each of these sheetsso that the crystal structure of halloysite is described as 1:1dioctahedral layer silicate. The final structure is obtainedby piling up these layers in the manner as seen in Figure 1.The water molecules are sitting between two consecutivelayers (indicated in the Figure). Slight differences in therelative orientation between neighbouring layers might giverise to the differences in the symmetry of the final structure.


Figure 1. In (a) the crystal structure of halloysite is shown down adiection slightly away from [100]. Si, Al, O and H are representedby black, dark grey, light grey and white spheres. Coordinationpolyhedra for Si and Al are drawn. The positions of the water mole-cules in the structure are indicated. The sheets of edges-sharing[AlO6] octahedra and corner sharing [SiO4] tetrahedra are shownin (b) and (c), respectively, in a projection parallel to [001] of thestructure.

This description of the structure of halloysite im-mediately elucidates the close similarity to the structure ofkaolinite and explains the subsequent difficulties to exper-imentally distinguish between them. This is a rather contro-versial point which has been extensively treated in the litera-ture. As a matter of fact, the structure of halloysite-(7 A) iscompletely analogous to a disordered kaolinite structure.Tubular or cylindrical particles of different sizes are nor-mally found in halloysite even though some other morpho-logies have also been reported (fibres, platy, etc.). Thus, thepowder X-ray diffraction patterns are rather complicatedwith extensive overlapping of reflections accompanied bya high background. As a result, X-ray diffraction powderpatterns are virtually identical for both structures. As statedin [2], the most noticeable difference is probably the particleshape. Thus, we adopt the convention that the presence ofcylindrical or tubular particles is indicative of halloysitewhile samples with platy particles might be named askaolinite [10, 11].

Here we present the results of our investigations on anatural sample of halloysite from the Dragon Mine, Tintic

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District, Silver City, Utah (USA). We have characterisedthe as-received sample along with specimens resulting fromannealing the sample at different temperatures. Two pointsconstitute the major target to be presented in this article:(i) a careful characterisation of this mineral sample and (ii)establishment of the stability ranges of the tubes of halloy-site in function of the annealing temperature. Results pre-sented here constitute the base we are following in our on-going research using halloysite as a template to synthesisefunctional materials.

Results and Discussion

X-ray Powder Diffraction

X-ray powder diffraction patterns are presented in Figure2. Indexing of the observed reflections have been donebased on the presence of two different crystalline phases.Indicated with arrows pointing up reflections from the hal-loysite structure are marked. The index of some of them,the most intense ones, are given in the Figure. Reflectionsof the second phase, in the Figure indicated with arrowspointing down, could be indexed according to the crystalstructure of alunite [KAl3(SO4)2(OH)6, a � 6.970(1) A, c �17.27(1) A, space group R3m, n° 166] [12]. Additionallythere exist two more weak reflections at � 27° and � 29°respectively, that could not be indexed. Interestingly, thesetwo reflections do not disappear with annealing of thesamples and they are also present in the patterns of samplesannealed up to 900 °C (not shown). Furthermore, an extra

Figure 2. X-ray powder diffraction patterns of the samples studied.Annealing temperatures are given. Reflections indexed with arrowspointing down and up correspond to the alunite and halloysitestructures, respectively. The index fort he most intense reflectionsfor the latter are given. Unindexed reflections are indicated withasterisk: two in the range 25° < 2θ < 30° and one at � 44° thatshows up with the annealing of the samples. Note the weak sharpe-ning of the 001 reflections for halloysite and more importantly thefact that these reflections do not change the position when exposedto the annealing treatment.

F. J. Garcıa Garcıa, S. Garcıa Rodrıguez, A. Kalytta, A. RellerARTICLEreflection appears in the sample annealed at 250 °C thatincreases the intensity in the pattern of the sample annealedat 450 °C, to finally disappear at 500 °C. All these threereflections are indicated with asterisk in the Figure 2.

As mentioned above, the close structural relationship be-tween halloysite and kaolinite gives rise to similar powderdiffraction patterns. Over the years this subject has been abig issue in the literature and nowadays it is generally ac-cepted that in the case of halloysite-(7 A) the intensity ratiobetween the 002 and 020/110 reflections should be � � 2.In the case of 10 A halloysite, doubts fade away as long asthe 002 reflection is pushed to lower diffracting angles dueto the increase of the c parameter as a consequence of in-serting water molecules between the 1:1 dioctahedral layers.

The 002 reflection is observed at an angle of � 12.4° sothat the corresponding c unit cell parameter is � 14.3 A,which is formally the one for anhydrous halloysite. In fact,higher values than this one have been reported in the litera-ture for kaolinite (which is anhydrous). As examples, wemention here two careful refinements of the structure, [12,13], where the c parameters were found to be close to14.8 A. Besides, the reflection stays at the very same posi-tion after annealing at different temperatures. These two ex-perimental findings strongly suggest that the water contentis minima, if not zero, in the as-received sample. In agree-ment with this result, a conclusion to be drawn from thepowder X-ray diffraction patterns is simply that some re-crystallisation process in function of annealing temperatureoccurs. This is observed in the sharpening of reflectionswhen the sample is annealed at 400 °C. Note that in thissample the 111 reflection is already present while it is partof the continuous decreasing background right after the020/110 relflection in the as-received sample. A peculiaritythat could catch one’s eye is the difference in the intensityratio between 001 and 020/110 in function of temperature.According to this, we could assume that the annealing pro-cedure renders a soft transformation of halloysite into kao-linite as the ratio becomes roughly 3 for the sample an-nealed at 400 °C. Nonetheless, and according to the trans-mission electron microscopy studies, the particle shape ismaintained as tubular independently of the annealing tem-perature (see below). Hence, and in an internally self con-sistent manner with the criteria adopted here to distinguishbetween halloysite and kaolinite, we should refer to a pro-cess of crystallisation of halloysite itself and not to a trans-formation into kaolinite. Concerning the thermal stabilityof the sample, all reflections from the halloysite have disap-peared at some point between 400 °C and 450 °C, in agree-ment with the literature, i.e. [14]. The alunite is still presentat this temperature along with the three unindexed peaks.Further temperature increasing leads to the decompositionof alunite so that only the two former unindexed peaks re-main at 500 °C and higher temperatures.

Thermal Analysis

TG and DSC curves are presented in Figure 3. Theseexperiments show that the decomposition process takes

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place in two main steps: (i) at the first endothermic peakaround 100 °C water is released, surely corresponding towater absorbed on the surface of the particles. The con-comitant FTIR experiment clearly shows that only watervapour is released. (ii) Structural decomposition of halloy-site and alunite take place in the second endothermic peakat 400�550 °C. This signal is composed by decompositionof halloysite and alunite all together as seen in the X-raypowder diffraction study. Thus, no absolute value for thedecomposition of halloysite can be given according tothermal analysis and we stick to the observed value frompowder diffraction (400�450 °C). The FTIR analysis showsat this step a more complex situation where H2O, CO2 andSO2 are obtained.

Figure 3. TG, DSC (upper part) and FTIR (bottom) curves areshown. For the latter the wave-numbers that have been used for thespectroscopic analysis are given in the inset. The signal of SO2 hasbeen multiplied by 50 in order to make it more clear.

Figure 4. Nitrogen physical absorption experiments of the as recei-ved halloysite sample as shown. The surface area was calculatedaccording to the BET method (Brunauer, Emmett and Teller) andthe pore size distribution according to the BJH method (Barrett,Joyner and Halenda).

In Figure 4 the nitrogen adsorption-desorption isothermis presented. The surface area was calculated according to


BET method (Brunauer, Emmett and Teller) and the poresize distribution according to the BJH method (Barrett,Joyner and Halenda). The isotherms are the typical onesfor halloysite and the calculated value for the pore size is29 m2/g. This value is significantly smaller than the one re-ported in some other samples of halloysite, i.e. [5]. Thismight be explained as a consequence of the mixture withalunite along with the presence of tubes of halloysite with-out inner pores (see below).

Transmission Electron Microscopy

Low magnification images of the as-received sample arepresented in Figure 5 (a) and (b). The Figure is completedwith higher magnification images, (c) (d) (e), to show inmore detail the main characteristic of the tubes of halloy-site. Two types of particles are found in the sample: tubularand platy particles. Electron diffraction experiments, alongwith compositional EDX analysis, confirm that the formerbelong to halloysite and the latter to alunite. In Figure 5(a) a particle of alunite is indicated in order to stress themorphological difference.

Figure 5. Transmission electron microscopy images of the halloysitetubes recorded in the as received sample. (a) and (b) were recordedat lower magnification than (c), (d) and (e) to give a general over-view of the sample. Different details are indicated by: a platy par-ticle of alunite in (a), a couple of small diameter tubes that containpores along the whole tube in (c) and (d). Note the presence of atube oriented along the axis in (d) and how the inner part of par-ticle in (e) is filled.


The tubular particles are of rather different sizes both indiameter and in length. Tubes of � 50 nm diameter, Figure5 (c), up to 2 μm, in (a) and (b), are present. Tightly connec-ted with this result we have observed that tubes of smalldiameter, and independently of the length, are normallyopen on both sides so that they contain a tubular pore inthe inner part. A couple of these are indicated with arrowsin (c) and (d). In (e) a filled tube is shown, and thereforethe cylindrical pore is closed. Note the larger diameter inthis case in comparison with those tubes indicated in (c)and (d). In some cases we could obtain images of the tubesalong the axis and one such particle is indicated in (d). Inall cases the structure of the walls is quite disordered andit seems the contact between the layers is lost. A possibleinterpretation has been given in terms of layer separationas a result of the dehydration process [15].

Several tubular particles were analysed by EDX in themicroscope and no other elements than silicon, aluminium,and oxygen were observed. This is somehow surprising asit is well known that halloysite contains normally smallamounts of Fe2O3, Cr2O3, Mg, Fe, Ni, Cu or Zn [2]. Thepresence of these impurities influencing shape and size ofthe tubes is obviously not relevant in the present study.

High resolution imaging of halloysite is much more com-plicated due to decomposition of the crystalline structureunder the beam of electrons. To partly overcome this prob-lem we have used imaging plates instead of traditionalphotographic negatives or CCD camera owing to the highdynamical range and sensibility. Hence, we were able to re-cord images at much shorter exposure time that, eventhough they did not show fine details of the crystallinestructure, allowed us to draw some conclusions. In Figure6 a high resolution image of a tube in the as-receivedsample is presented. The axis of the tube is indicated andthe parallel fringes observed correspond to the 002 reflec-tion. Hence, the axis of the tube is perpendicular to the[001] of the halloysite tube [1, 6].

In the same manner we could also record electron diffrac-tion patterns as the ones presented in Figure 7. Two typicalelectron diffraction patterns of the tubular halloysite par-ticles in the as-received sample, (a), and in the sampleannealed at 400 °C, (b), are shown. These patterns were al-ways recorded without tilting the crystals. They, therefore,correspond to the direction of the crystals exactly as theyare laying on the grid. The major peculiarity of the diffrac-tion patterns of cylindrical crystals is the coexistence of sev-eral reciprocal directions in one reciprocal plane [16, 17].Furthermore, we carryied out tilting experiments in theelectron microscope with no changes in the diffraction pat-terns as might be expected from tubes. According to theunit cell parameters of halloysite, as observed in the X-raypowder diffraction patterns, the direction of the axis of thetubes could be determined to corresponding to [010]. In-dexation of reflections in (b) has been done according tothat unit cell and the two major present zone axis are indi-cated with numbers in different colours. Furthermore, wealso give the indexing of a group of reflections under 11l.

F. J. Garcıa Garcıa, S. Garcıa Rodrıguez, A. Kalytta, A. RellerARTICLE

Figure 6. In (a) a high resolution image is presented where some fringes can be seen parallel to the axis of the tube (indicated). Thecorresponding Fast Fourier Transform, FFT, in inset and in (b) the inverse FFT is presented. Note the presence of some stacking disorderin the area encircled.

Figure 7. Typical electron diffraction patterns of the tubular halloysite particles of the as-received sample, (a), and of the sample annealedat 400 °C, (b). The particle in (b) is clearly better crystallised as a consequence of annealing. Indexing of the present reflections is donein this pattern. All peculiarities indexed in (b) are present in (a) but in the latter much more apparent streaking of reflections is observed.Indexing of the two coexisting reciprocal directions are presented in two different colours to make it more clear (see text). Additionallya group of reflections (parallel to [001]*) where all 11l are included is indicated.

The difference between these two patterns as a result of thecrystallisation with annealing of the sample is apparent.Note that in (b) the 060 reflection is clearly visible and welldefined while almost completely absent in (a) and that allpeculiarities indexed in (b) are present in (a) but in the lattermuch more apparent streaking of reflections is observed.

Indexation of all reflections give the following extinctionconditions: F(hkl) � 0, unless h � k � 2n, F(h0l) � 0,unless h, l � 2n and F(0k0) � 0 unless k � 2n. Accordingto these two possible space groups are possible: C2/c andCc. The only difference between them is the presence of aninversion center. In principle, it is reasonable to think thatthere exists no inversion center and therefore the correctspace group should be Cc.

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Conclusions

A natural sample from the Dragon Mine, Tintic District,Silver City, Utah (USA) is found to be composed by halloy-site and alunite. The halloysite mineral particles adopttubular shapes while the alunite prefers to crystallise for-ming platy shapes. Halloysite decomposes at 400�450 °Cand alunite at 450�500 °C. Surprisingly the characteristictubular morphology of halloysite is maintained up to900 °C, even though the crystalline structure is destroyedgiving rise to an amorphous structure. There exist no im-purities in the halloysite tubes as only Al, Si and O couldbe obtained in the compositional analysis. Powder X-rayand electron diffraction show that the halloysite is basically


anhydrous in agreement with the observed unit cell param-eters. Thus, it must be described as halloysite-(7 A) accord-ing to the terminology normally used in the related litera-ture. The cross section images of the tubes always show arather disordered stacking of the silicate layers. A connec-tion between tube diameter and porosity has been found.Long diameter tubes use to be closed. In contrast, shortdiameter tubes present internal pores along the whole tube.This experimental observation may explain the poor surfacearea of 29 m2 ·g�1.

Experimental SectionSamples were supplied by Nanoclays and Technology Inc.. Differ-ent portions of the as-received specimen were annealed in air at250 °C, 400 °C, 450 °C, 500 °C and 900 °C to study the thermalstability of the sample. Two facts push us to carry out the annealingtreatments in air and not in hydrothermal conditions: (i) the pow-der patterns clearly show that the original sample belongs to theso-called halloysite (7 A) and it is well known that the dehydrationof halloysite is a non-reservible process (ii) the experiments withthis samples are being carried out, or at least at some intermediatestep, in high temperature conditions.

Powder X-ray diffraction patterns were recorded with a Seifert3003-TT diffractometer, operating with Cu-Kα radiation. Thermalinvestigations, TG and DSC, were carried out with a Netzsch STA409 PC thermobalance from room temperature up to 920 °C insynthetic air. The gases released by the samples while heating upin the thermobalance were analysed by Fourier Transformation In-frared Spectroscopy, FTIR, with a Bruker Equinox 55 apparatusconnected by a heated transfer line (130 °C) to the thermo balance.The obtained signals were integrated versus temperature and theused wave numbers were as follows: CO2 2440�2175 cm�1, H2O3766�3501 cm�1 and SO2 1407�1278 cm�1. Pore size distributionand surface area were determined by nitrogen adsorption with aQuantachrome Nova 2000 apparatus. The surface area was calcu-lated according to the BET method (Brunauer, Emmett and Teller)and the pore size distribution according to the BJH method (Bar-rett, Joyner and Halenda). These calculations were carried out ap-plying the NovaWin2 software, version 2.2.


Transmission electron microscopy studies were performed with aJEOL JEM 2100F operated at 200 KV. An EDAX detector for X-ray energy dispersive analysis is attached to this microscope. Dueto the susceptibility of the samples to radiation damage under thebeam of electrons, the electron diffraction patterns were recordedand digitalised with a DITABIS imaging plate system. Images wererecorded with a CCD camera Gatan Ultrascan 1000.

References

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114, 2445; Angew. Chem. Int. Ed. 2002, 41, 2446.[4] A. F. Gualtieri, Phys. Chem. Minerals 2001, 28, 719.[5] S. Barrientos-Ramırez, E. V. Ramos-Fernandez, J. Silvestre-

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[12] R. Wang, W. F. Bradley, H. Steinfink, Acta Crystallogr. 1965,18, 249.

[13] R. B. Neder, M. Burghammer, T. Grasl, H. Schulz, A. Bram,S. Fiedler, S. Clays and Clay Minerals 1999, 47(4), 487.

[14] G. W. Brindley, K. Robinson, J. Goodyear, Am. Mineral. 1948,34 (3�4), 423.

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[16] G. Honjo, K. Mihama, Acta Crystallogr. 1954, 7, 511.[17] G. Honjo, N. Kitarumra, K. Mihama, Clay Miner. Bull. 1954,

2, 133.Received: January 30, 2009

Published Online: April 3, 2009

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Study of Natural Halloysite from the Dragon Mine, Utah (USA)