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Behavior of halloysite clay under formamide treatment
Applied Clay Science 35 (2007) 17 – 24 www.elsevier.com/locate/clay
Behavior of halloysite clay under formamide treatment Emmanuel Joussein a,⁎, Sabine Petit a , Bruno Delvaux b b
a CNRS UMR 6532-HydrASA Université de Poitiers. 40, av. du Recteur Pineau. 86022 Poitiers cedex, France Université Catholique de Louvain, Unité des Sciences du Sol, 2/10, Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium
Received 27 July 2004; received in revised form 3 November 2005; accepted 2 July 2006 Available online 8 August 2006
Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. When both halloysite and kaolinite are present in a given sample, differentiation between the two minerals is problematic particularly when the halloysite constituent is dehydrated. Formamide intercalation test is widely used to differentiate halloysite-(7 Å) from kaolinite. The soil used in this study was taken from an Udalf B horizon. It was sampled in 1986, immediately studied, and then stored. This soil was selected due to its high halloysite-(10 Å) content. The formamide intercalation test was performed on 3 clay fractions (b 0.1, 0.1–1, and 1–2 μm) re-extracted from the stored soil. X-ray diffraction shows that the hydrated halloysite, initially present (in the fresh sample), is totally dehydrated after storage. Since the interlayer water in halloysite is weakly held, halloysite-(10 Å) can readily and irreversibly dehydrate to give the corresponding halloysite-(7 Å) form. Halloysite-rich samples should therefore be kept under controlled humidity conditions or in a water-saturated atmosphere if this clay mineral is to remain fully hydrated. The abundance of tubes and spheroids, estimated from transmission electron microscopy (TEM) of the re-extracted sample, is proportional to the amount of halloysite derived from the formamide test on the fresh sample. On the other hand, after storage and natural dehydration, the formamide test markedly underestimates the concentration of halloysite (30–40% in the halloysite-rich fractions). For the reference halloysites, however, the state of hydration does not affect formamide intercalation. After progressive formamide treatment, almost all the kaolinite is intercalated whereas only 50% of the halloysite is expanded in the re-extracted soil sample. Thus, the actual amount of halloysite in the fresh soil sample, estimated by formamide intercalation and TEM, cannot be reached even after progressive treatment with formamide. Formamide treatment indicates that the behaviour and physico-chemical properties of halloysite can be modified by dehydration. © 2006 Elsevier B.V. All rights reserved. Keywords: Halloysite; Intercalation; Formamide; Morphology; Dehydration
1. Introduction Halloysite is a dioctahedral 1:1 clay mineral that occurs widely in soils of wet tropical and subtropical regions (Parfitt and Wilson, 1985) and weathered rocks, being formed by weathering of many types of igneous and non-igneous rocks (Churchman, 2000; Joussein
⁎ Corresponding author. Tel.: +33 5 49 45 49 03; fax: +33 5 49 45 42 41. E-mail address: [email protected] (E. Joussein). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.07.002
et al., 2005). The structure and chemical composition of halloysite are similar to those of kaolinite but halloysite can intercalate a monolayer of water molecules (Hofmann et al., 1934; Churchman and Carr, 1972, 1975) giving a basal spacing near 10 Å. Because the interlayer water is weakly held, halloysite-(10 Å) can readily and irreversibly dehydrate to give the corresponding halloysite-(7 Å) form (Alexander et al., 1943; Joussein et al., 2006). It is therefore very difficult, if impossible, to handle halloysite-(10 Å) without altering its hydration status.
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The particles of halloysite can adopt a variety of morphologies. The most common is the elongated tube although short tubular, spheroidal, and platy shapes have been widely reported (Kunze and Bradley, 1964; de Souza Santos et al., 1966; Dixon and McKee, 1974; Tazaki, 1982; Wada and Mizota, 1982; Churchman and Theng, 1984; Noro, 1986; Bailey, 1990). Transmission electron microscopy (TEM) could provide an independent estimate of halloysite concentrations by assuming that the particles occur only as tubes or spherules (Brindley et al., 1963; Beutelspacher and van der Marel, 1968; Churchman and Theng, 1984). In mixtures with kaolinite, halloysite-(10 Å) is easily identified by its basal (d001) reflection at 10 Å in the Xray diffraction (XRD) pattern. Although many attempts have been made to differentiate between kaolinite and halloysite-(7 Å) (Alexander et al., 1943; Brindley and Robinson, 1946; Brindley et al., 1963; Chukhrov and Zvyagin, 1966; Churchman and Carr, 1975; Churchman et al., 1984; Churchman and Theng, 1984; Churchman, 1990) this point remains problematic. However, halloysite (either hydrated or dehydrated) has a greater propensity for intercalating organic molecules than does kaolinite (Theng, 1974). For this reason, intercalation methods have been developed to identify and quantify halloysite-(7 Å) and kaolinite in mixtures (Wada, 1961; Churchman et al., 1984; Theng et al., 1984). The most widely used test, developed by Churchman et al. (1984) is based on differences in the rate and extent of formamide intercalation between halloysite and kaolinite. The formamide intercalation test is conclusive for naturally dehydrated halloysites differing in occurrence, mineralogy, and crystal chemistry. The test, however, is inconclusive for halloysites that have been oven-dried at 110 °C. This also applies to some halloysites that have been dried at 40 °C (Churchman, 1990), although some water would still be present in the interlayer space. Several authors (e.g., Takahashi et al., 1993; de Oliveira et al., 1997; Takahashi et al., 2001) have even reported that some naturally dehydrated halloysites do not expand with formamide. Here we investigate the intercalation of formamide into soil and reference halloysites in order to gain insight into the behaviour and reactivity of these minerals toward organic compounds. 2. Materials and methods 2.1. Clay samples The halloysites used for this study derived from a basaltic ash soil formed under humid tropical conditions in Cameroon.
The soil (b 2 mm) was sampled in 1986 and characterized by Delvaux et al. (1989, 1990a,b). The sample was stored under controlled conditions: 20 °C, 30% RH. For this study, we “reextracted” the b2 μm fraction from the major Udalf B horizons (SN2 in Delvaux et al., 1989, 1992) because of the high content in halloysite-(10 Å) and its high Cs+ affinity (Joussein et al., 2004). After treatment with H2O2 (6%, 40 °C) to oxidize organic matter, the soil was dispersed by passing through a Na+-resin (Rouiller et al., 1972; Bartoli et al., 1991). The fine clay (b 0.1 μm) subfraction was separated from the whole clay (b 2 μm) fraction by continuous flow ultracentrifugation. Two other subfractions (0.1–1 and 1–2 μm) were obtained by sedimentation according to Stokes' law. Three reference halloysites from New Zealand: Te Puke, Opotiki, and Matauri Bay (Theng et al., 1982; Churchman and Theng, 1984) were also used. The clay (b2 μm) fractions from the reference specimens were obtained by sedimentation after treatment with 1 M NaCl. The samples were analysed by XRD after saturation with Ca2+ using 0.5 M CaCl2. XRD of oriented samples was carried out at room temperature using a Siemens D500 diffractometer (CuKα, 40 KV, 30 mA) coupled with a DACO-MP multichannel data analyser. Samples were step-scanned at 0.4°(2θ)/min. Particles morphology was observed by transmission electron microscopy (TEM) using a JEOL 1010 instrument. The samples were prepared by evaporating a small droplet of clay suspension on to a carbon TEM grid. 2.2. Formamide intercalation Halloysite is known to have a capacity for interlayer adsorption of certain organic compounds (MacEwan, 1948). In the 1980s, the interactions of halloysite with amides, dimethylsulphoxide, and hydrazine were investigated to distinguish halloysite-(7 Å) from kaolinite in mixtures (Olejnik et al., 1968; Range et al., 1969; Lipsicas et al., 1985; Costanzo and Giese, 1986; Churchman, 1990; Frost et al., 1999; Joussein et al., 2005). Compounds that form complexes with halloysite are polar, either acids or bases, and contain two functional groups (preferably OH or NH2 groups). Churchman and Theng (1984) have shown that complex formation with formamide (HCONH2), a polar organic molecule with a high dipole moment (3.71 Debye), was rapid and complete, irrespective of differences in crystallinity, morphology, and iron content among the halloysites. The interlayer orientation of formamide in halloysite or kaolinite is still debated (Olejnik et al., 1970; Xie and Hayashi, 1999a,b; Michalkova et al., 2002). Churchman et al. (1984) developed a rapid and simple test for differentiating halloysite from kaolinite in mixtures, based on differences in the rate and extent of formamide intercalation. With halloysite, interlayer complex formation is both rapid and complete whereas no significant intercalation occurs with kaolinite until 4 h of contact. Thus, when the mixture is sprayed with formamide, and X-rayed after 1 h of spraying, halloysite gives a basal peak at 10.3–10.4 Å whereas kaolinite
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Fig. 1. XRD patterns of oriented deposits before (bottom) and after (top) immediate formamide treatment for three New Zealand reference halloysites.
does not expand beyond 7.2 Å. The relative proportions of halloysite and kaolinite in the sample are related to their respective peak intensities. The test is inconclusive for halloysites that have been oven-dried at 110 °C, and even at 40 °C for some samples (Churchman, 1990). Halloysite content with respect to kaolinite may be estimated by the “immediate” formamide intercalation test; that is, the samples are X-rayed immediately after spraying with formamide (Churchman et al., 1984). The halloysite content is then calculated from the I10 / (I10 + I7) ratio where I10 and I7 denote the intensity of the basal (d001) peaks at 10 and 7 Å, respectively. The effect of “progressive” formamide treatment, involving 4 successive additions of formamide, is also studied. After the immediate formamide treatment, the samples are placed in a dessicator for 4 h, then sprayed 4 times at 1.5 h intervals. XRD analysis is carried out after each spraying.
3. Results 3.1. Reference halloysite samples The XRD patterns for the 3 reference halloysites are shown in Fig. 1. The basal (d001) reflections are typical of fully hydrated (Opotiki, 10 Å), partially hydrated (Te Puke, 7–10 Å), and dehydrated (Matauri Bay, 7 Å) halloysites. After immediate formamide treatment, all 3
Fig. 2. XRD patterns of oriented deposits for (a) the fine (b0.1 μm), (b) the intermediate (0.1–1 μm), and (c) the coarse (1–2 μm) clays fractions from Cameroon soil after (i) immediate formamide treatment (i.F.T.) (Churchman et al., 1984), and progressive formamide treatment (p.F.T.).
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halloysites give a basal spacing at 10.2 Å, consistent with the presence of a single layer of amide molecules in the interlayer space. However, Matauri Bay halloysite shows a residual reflection near 7 Å indicating the presence of some unexpanded layers. 3.2. Soil samples The XRD patterns of the b 2 μm clay fraction, separated from the fresh sample (Delvaux et al., 1990b) show basal reflections at 10 Å and 7 Å indicative of halloysite-(10 Å) and kaolinite, respectively. The presence of gibbsite and quartz is also noted. Using immediate formamide treatment on the fresh sample, Delvaux et al. (1992) calculated 63% and 79% of halloysite (with respect to kaolinite) in the b 2 μm and b0.1 μm fraction, respectively. The XRD patterns of the clay fractions of the re-extracted sample (i.e. after storage) show a prominent, rather broad peak near 7 Å whatever the clay fraction. The fine clay fraction (Fig. 2a) is dominated by halloysite-(7 Å) typified by (i) a broad peak at 7.40 Å, and (ii) a broad hk band at 4.4 Å due to tubular or spheroidal morphology and high degree of disorder (Brindley, 1980). A small shoulder at 10 Å indicates the occurrence of some hydrated halloysite. The XRD pattern of the 0.1–1 μm fraction (Fig. 2b) also shows halloysite (broad 7.40 Å component, hk band, 10 Å shoulder) and kaolinite with typical narrower 00 ℓ reflections at 7.20 and 3.58 Å. Gibbsite and quartz are also present. In the XRD pattern of the coarse fraction (1–2 μm), the 7.15 and 3.58 Å reflections are due to kaolinite, representing the main component, together with quartz, gibbsite, and a small amount of mica. The hk band of halloysite is absent (Fig. 2c). These results from the re-extracted sample accord with the finding by Delvaux et al. (1992) that halloysite decreases relative to kaolinite as particle size increases. After immediate formamide treatment, only small changes in XRD intensity are observed as compared with the natural sample (Fig. 2). The relative intensity of the broad peak at 10 Å increases for the fine and intermediate fractions. Using the test of Churchman et al. (1984), the estimated halloysite content is about 30% for the fine clay fraction, 28% for the intermediate fraction, and less than 10% in the coarse clay fraction (Table 1). After progressive formamide treatment, the 10 Å component clearly increases for all particle-size fractions. Almost the entire coarse fraction is expanded (Fig. 2c), whereas about 50% of the fine and intermediate clay fractions are expanded (Table 1). Note that the 10 Å peak
Table 1 Estimation (in %) of halloysite content in different fractions of halloysitic soil Soil fractions
b0.1 μm (in %)
0.1–1 μm (in %)
1–2 μm (in %)
TEM investigation (H/(H + K)1 Immediate formamide intercalation on fresh sample2 Immediate formamide intercalation after storage2 Progressive formamide intercalation after storage3
80 (210) 79
60 (180) 55–604
22 (130) b254
30
28
10
50
–
–
Values in brackets denote the number of particles counted. Estimated using the number of tubular and spheroidal particles. 2 Estimated following Churchman et al. (1984)'s procedure. 3 Estimated after progressive formamide treatment (see text). 4 Estimated from Delvaux et al. (1992). – Not given due to kaolinite intercalation. 1
that appears after progressive treatment for the intermediate and coarse clay fractions, is narrow and corresponds to intercalated kaolinite. 3.3. Transmission electron microscopy investigations Fig. 3 shows representative TEM micrographs of the reference halloysites and the clay fractions from soil. The reference Matauri Bay halloysite consists dominantly of thick tubes of about 0.7 μm in length (Fig. 3a), while Te Puke halloysite is charaterized by short, narrow tubes (mean size about 0.3 μm) (Fig. 3b). Opotiki halloysite is mostly made up of spheroidal particles (Fig. 3c) with a mean diameter of 0.4 μm. These observations accord with those reported by Churchman and Theng (1984). The dominant particle shape in the fine clay fraction (Fig. 3f) is tubular with few spheroidal particles, whereas platy particles are dominant in the coarse clay fraction (Fig. 3d). The intermediate clay fraction shows a mixture of tubular and platy particles (Fig. 3e). The mean size of particles is about 0.05 μm, 0.3 μm and 1.4 μm for the fine, the intermediate and the coarse fractions, respectively. The size of the particles measured by TEM is in agreement with that collected by sedimentation and centrifugation. It is well known that tubular or spheroidal shapes are characteristic of halloysite, while a platy morphology is more typical of kaolinite (Dixon, 1989). In accordance with XRD data, the coarse fraction is kaolinite-rich, while the fine one is halloysite-rich. 4. Discussion When Delvaux et al. (1990b, 1992) investigated the halloysite-rich soils from Cameroon they found fully
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Fig. 3. TEM micrographs of (i) reference halloysites (a) Te Puke, (b) Matauri Bay and (c) Opotiki, and (ii) of (d) coarse (1–2 μm), (e) intermediate (0.1– 1 μm), and (f ) fine (b0.1 μm) clays fractions from the soil after storage.
hydrated halloysite in the B horizon of these soils. For this study, we separated (“re-extracted”) the clay fractions from the same sample (horizon) after longterm storage in controlled room temperature (20 °C) and humidity (30% RH). XRD shows that the halloysite-
(10 Å), initially present in the (fresh) sample, has since been completely dehydrated. That halloysite-(10 Å) could partially dehydrate during transportation and storage has also been reported by Rattigan (1967). Halloysite can be kept fully hydrated only by storing in a
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sealed container in contact with water or in a watersaturated atmosphere. The XRD patterns of the different clay fractions clearly indicate an increase in halloysite content relative to kaolinite with decreasing particle size. Using TEM, the halloysite content may be estimated from the abundance of tubular or spheroidal particles, assuming that kaolinite particles are platy or polygonal (Beutelspacher and van der Marel, 1968; Churchman and Carr, 1975; Churchman and Theng, 1984; Dixon, 1989). From representative TEM micrographs, about 80, 60, and 20% of tubular and spheroidal particles are determined for the fine, intermediate, and coarse fractions, respectively, in agreement with XRD data. The proportion of characteristic halloysite shapes in the re-extracted sample closely matches the amount of halloysite content measured by Delvaux et al. (1992) using the formamide test on the fresh sample (Table 1). Formamide intercalation indicates differences in reactivity between the fresh and re-extracted halloysite. After immediate formamide treatment, 30% of the reextracted fine clay (b0.1 μm) fraction intercalates formamide as compared with 79% for the corresponding fraction from the fresh sample (Table 1). About 28 and 10%, respectively, of the intermediate (0.1–1 μm) and coarse (1–2 μm) clay fractions from the stored soil can intercalate formamide, while the correponding values, measured by Delvaux et al. (1992) for the fresh sample, are 55–60% and b 15% (Table 1). Thus, the formamide test highly underestimates the halloysite content of the fine and intermediate clay fractions. The difference in halloysite content between the fresh and the re-extracted samples decreases as particle size increases. This is because the coarse clay fraction is kaolinite-rich, and little formamide is intercalated after immediate treatment. Similarly, the formamide test considerably underestimates the halloysite content of soil samples (30–40% in the halloysite-rich fractions) after storage and natural dehydration. By contrast, the state of hydration does not affect the reactivity of the reference halloysites toward formamide (Fig. 1). Both Te Puke halloysite (partially dehydrated) and Matauri Bay halloysite (fully dehydrated) readily intercalate a single layer of formamide after immediate treatment. After progressive formamide treatment, almost all the kaolinite is intercalated whereas only 50% of the halloysite is expanded (Table 1). This observation is consistent with the finding by Churchman et al. (1984) that kaolinite intercalates formamide after 4 h of contact. However, the amount of halloysite measured by progressive formamide treatment is less than the actual value (Table 1). Apparently, a fraction of
dehydrated halloysite intercalates less formamide than kaolinite. Churchman et al. (1984) have shown that intercalation is influenced by particle crystallinity and size. Highly reactive halloysites are characterized by large particles, and a high crystallinity. Conversely, small particles with low crystallinity are relatively nonreactive. Since the hydration state of halloysite can profoundly affect its behaviour toward formamide (e.g. Churchman and Theng, 1984; Joussein et al., 2005), due care should be exercised in applying the formamide test to halloysite-rich soil samples that are not kept in a wet state. The halloysite from a Cameroon soil, used in this study, has a cation exchange capacity (CEC) of about 26 cmolc/kg as well as a high K+ selectivity (Delvaux et al., 1990a, 1992), and Cs+ affinity (Joussein et al., 2004). Similarly, Takahashi et al. (1993, 2001) found that formamide did not intercalate into halloysite-rich clays with a high CEC and K+ selectivity from northern California. We propose that the presence of interlayer Kspecific sites (Delvaux et al., 1990a; Joussein et al., 2004) keep the halloysite layers collapsed after dehydration, preventing formamide from entering the interlayer space. That natural or induced dehydration would cause a decrease in the CEC of halloysite (Grim, 1968; Bailey, 1990) is in keeping with this hypothesis. 5. Conclusion The halloysite from a Cameroon soil is fully hydrated (halloysite-10 Å) in the fresh sample but appears to be completely dehydrated after storage and re-extraction. The proportion of characteristic halloysite shapes in the re-extracted sample, estimated by TEM, closely matches the amount of halloysite derived from the formamide test on the fresh sample. The formamide test greatly underestimates the halloysite content of the clay fractions separated from the stored soil. Even after repeated treatments with formamide, the estimated amount of halloysite in the soil clay fractions is less than the actual value. In the case of the reference halloysites, however, the state of hydration does not influence formamide intercalation. Thus, due care should be taken in using the formamide test to determine the halloysite content of samples that are not kept in a wet state. On the positive side, the formamide test clearly indicates that dehydration can modify the behaviour and reactivity of K-selective halloysites toward organic compounds. Because of the presence of K-specific sites, dehydration would promote strong interaction between halloysite layers within a particle, inhibiting formamide intercalation.
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volcanic materials from northern California. Soil Science Society of America Journal 65, 516–526. Theng, B.K.G., 1974. The chemistry of clay-organic reactions. Adam Hilger, London. Theng, B.K.G., Russell, M., Churchman, G.J., Parfitt, R.L., 1982. Surface properties of allophane, halloysite, and imogolite. Clays and Clay Minerals 30, 143–149. Theng, B.K.G., Churchman, G.J., Whitton, J.S., Claridge, G.G.C., 1984. Comparison of intercalation methods for differentiating halloysite from kaolinite. Clays and Clay Minerals 32, 249–258. Wada, K., 1961. Lattice expansion of kaolin minerals by treatment with potassium acetate. American Mineralogist 46, 78–91. Wada, S.I., Mizota, C., 1982. Iron-rich halloysite (10 Å) with crumpled lamellar morphology from Hokkaido, Japan. Clays and Clay Minerals 30, 315–317. Xie, X., Hayashi, S., 1999a. NMR study of kaolinite intercalation compounds with formamide and its derivatives. 1. Structure and orientation of guest molecules. Journal of Physical Chemistry B103, 5949–5955. Xie, X., Hayashi, S., 1999b. NMR study of kaolinite intercalation compounds with formamide and its derivatives. 2. Dynamics of guest molecules. Journal of Physical Chemistry B103, 5956–5962.
Behavior of halloysite clay under formamide treatment Emmanuel Joussein a,⁎, Sabine Petit a , Bruno Delvaux b b
a CNRS UMR 6532-HydrASA Université de Poitiers. 40, av. du Recteur Pineau. 86022 Poitiers cedex, France Université Catholique de Louvain, Unité des Sciences du Sol, 2/10, Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium
Received 27 July 2004; received in revised form 3 November 2005; accepted 2 July 2006 Available online 8 August 2006
Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. When both halloysite and kaolinite are present in a given sample, differentiation between the two minerals is problematic particularly when the halloysite constituent is dehydrated. Formamide intercalation test is widely used to differentiate halloysite-(7 Å) from kaolinite. The soil used in this study was taken from an Udalf B horizon. It was sampled in 1986, immediately studied, and then stored. This soil was selected due to its high halloysite-(10 Å) content. The formamide intercalation test was performed on 3 clay fractions (b 0.1, 0.1–1, and 1–2 μm) re-extracted from the stored soil. X-ray diffraction shows that the hydrated halloysite, initially present (in the fresh sample), is totally dehydrated after storage. Since the interlayer water in halloysite is weakly held, halloysite-(10 Å) can readily and irreversibly dehydrate to give the corresponding halloysite-(7 Å) form. Halloysite-rich samples should therefore be kept under controlled humidity conditions or in a water-saturated atmosphere if this clay mineral is to remain fully hydrated. The abundance of tubes and spheroids, estimated from transmission electron microscopy (TEM) of the re-extracted sample, is proportional to the amount of halloysite derived from the formamide test on the fresh sample. On the other hand, after storage and natural dehydration, the formamide test markedly underestimates the concentration of halloysite (30–40% in the halloysite-rich fractions). For the reference halloysites, however, the state of hydration does not affect formamide intercalation. After progressive formamide treatment, almost all the kaolinite is intercalated whereas only 50% of the halloysite is expanded in the re-extracted soil sample. Thus, the actual amount of halloysite in the fresh soil sample, estimated by formamide intercalation and TEM, cannot be reached even after progressive treatment with formamide. Formamide treatment indicates that the behaviour and physico-chemical properties of halloysite can be modified by dehydration. © 2006 Elsevier B.V. All rights reserved. Keywords: Halloysite; Intercalation; Formamide; Morphology; Dehydration
1. Introduction Halloysite is a dioctahedral 1:1 clay mineral that occurs widely in soils of wet tropical and subtropical regions (Parfitt and Wilson, 1985) and weathered rocks, being formed by weathering of many types of igneous and non-igneous rocks (Churchman, 2000; Joussein
⁎ Corresponding author. Tel.: +33 5 49 45 49 03; fax: +33 5 49 45 42 41. E-mail address: [email protected] (E. Joussein). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.07.002
et al., 2005). The structure and chemical composition of halloysite are similar to those of kaolinite but halloysite can intercalate a monolayer of water molecules (Hofmann et al., 1934; Churchman and Carr, 1972, 1975) giving a basal spacing near 10 Å. Because the interlayer water is weakly held, halloysite-(10 Å) can readily and irreversibly dehydrate to give the corresponding halloysite-(7 Å) form (Alexander et al., 1943; Joussein et al., 2006). It is therefore very difficult, if impossible, to handle halloysite-(10 Å) without altering its hydration status.
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The particles of halloysite can adopt a variety of morphologies. The most common is the elongated tube although short tubular, spheroidal, and platy shapes have been widely reported (Kunze and Bradley, 1964; de Souza Santos et al., 1966; Dixon and McKee, 1974; Tazaki, 1982; Wada and Mizota, 1982; Churchman and Theng, 1984; Noro, 1986; Bailey, 1990). Transmission electron microscopy (TEM) could provide an independent estimate of halloysite concentrations by assuming that the particles occur only as tubes or spherules (Brindley et al., 1963; Beutelspacher and van der Marel, 1968; Churchman and Theng, 1984). In mixtures with kaolinite, halloysite-(10 Å) is easily identified by its basal (d001) reflection at 10 Å in the Xray diffraction (XRD) pattern. Although many attempts have been made to differentiate between kaolinite and halloysite-(7 Å) (Alexander et al., 1943; Brindley and Robinson, 1946; Brindley et al., 1963; Chukhrov and Zvyagin, 1966; Churchman and Carr, 1975; Churchman et al., 1984; Churchman and Theng, 1984; Churchman, 1990) this point remains problematic. However, halloysite (either hydrated or dehydrated) has a greater propensity for intercalating organic molecules than does kaolinite (Theng, 1974). For this reason, intercalation methods have been developed to identify and quantify halloysite-(7 Å) and kaolinite in mixtures (Wada, 1961; Churchman et al., 1984; Theng et al., 1984). The most widely used test, developed by Churchman et al. (1984) is based on differences in the rate and extent of formamide intercalation between halloysite and kaolinite. The formamide intercalation test is conclusive for naturally dehydrated halloysites differing in occurrence, mineralogy, and crystal chemistry. The test, however, is inconclusive for halloysites that have been oven-dried at 110 °C. This also applies to some halloysites that have been dried at 40 °C (Churchman, 1990), although some water would still be present in the interlayer space. Several authors (e.g., Takahashi et al., 1993; de Oliveira et al., 1997; Takahashi et al., 2001) have even reported that some naturally dehydrated halloysites do not expand with formamide. Here we investigate the intercalation of formamide into soil and reference halloysites in order to gain insight into the behaviour and reactivity of these minerals toward organic compounds. 2. Materials and methods 2.1. Clay samples The halloysites used for this study derived from a basaltic ash soil formed under humid tropical conditions in Cameroon.
The soil (b 2 mm) was sampled in 1986 and characterized by Delvaux et al. (1989, 1990a,b). The sample was stored under controlled conditions: 20 °C, 30% RH. For this study, we “reextracted” the b2 μm fraction from the major Udalf B horizons (SN2 in Delvaux et al., 1989, 1992) because of the high content in halloysite-(10 Å) and its high Cs+ affinity (Joussein et al., 2004). After treatment with H2O2 (6%, 40 °C) to oxidize organic matter, the soil was dispersed by passing through a Na+-resin (Rouiller et al., 1972; Bartoli et al., 1991). The fine clay (b 0.1 μm) subfraction was separated from the whole clay (b 2 μm) fraction by continuous flow ultracentrifugation. Two other subfractions (0.1–1 and 1–2 μm) were obtained by sedimentation according to Stokes' law. Three reference halloysites from New Zealand: Te Puke, Opotiki, and Matauri Bay (Theng et al., 1982; Churchman and Theng, 1984) were also used. The clay (b2 μm) fractions from the reference specimens were obtained by sedimentation after treatment with 1 M NaCl. The samples were analysed by XRD after saturation with Ca2+ using 0.5 M CaCl2. XRD of oriented samples was carried out at room temperature using a Siemens D500 diffractometer (CuKα, 40 KV, 30 mA) coupled with a DACO-MP multichannel data analyser. Samples were step-scanned at 0.4°(2θ)/min. Particles morphology was observed by transmission electron microscopy (TEM) using a JEOL 1010 instrument. The samples were prepared by evaporating a small droplet of clay suspension on to a carbon TEM grid. 2.2. Formamide intercalation Halloysite is known to have a capacity for interlayer adsorption of certain organic compounds (MacEwan, 1948). In the 1980s, the interactions of halloysite with amides, dimethylsulphoxide, and hydrazine were investigated to distinguish halloysite-(7 Å) from kaolinite in mixtures (Olejnik et al., 1968; Range et al., 1969; Lipsicas et al., 1985; Costanzo and Giese, 1986; Churchman, 1990; Frost et al., 1999; Joussein et al., 2005). Compounds that form complexes with halloysite are polar, either acids or bases, and contain two functional groups (preferably OH or NH2 groups). Churchman and Theng (1984) have shown that complex formation with formamide (HCONH2), a polar organic molecule with a high dipole moment (3.71 Debye), was rapid and complete, irrespective of differences in crystallinity, morphology, and iron content among the halloysites. The interlayer orientation of formamide in halloysite or kaolinite is still debated (Olejnik et al., 1970; Xie and Hayashi, 1999a,b; Michalkova et al., 2002). Churchman et al. (1984) developed a rapid and simple test for differentiating halloysite from kaolinite in mixtures, based on differences in the rate and extent of formamide intercalation. With halloysite, interlayer complex formation is both rapid and complete whereas no significant intercalation occurs with kaolinite until 4 h of contact. Thus, when the mixture is sprayed with formamide, and X-rayed after 1 h of spraying, halloysite gives a basal peak at 10.3–10.4 Å whereas kaolinite
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Fig. 1. XRD patterns of oriented deposits before (bottom) and after (top) immediate formamide treatment for three New Zealand reference halloysites.
does not expand beyond 7.2 Å. The relative proportions of halloysite and kaolinite in the sample are related to their respective peak intensities. The test is inconclusive for halloysites that have been oven-dried at 110 °C, and even at 40 °C for some samples (Churchman, 1990). Halloysite content with respect to kaolinite may be estimated by the “immediate” formamide intercalation test; that is, the samples are X-rayed immediately after spraying with formamide (Churchman et al., 1984). The halloysite content is then calculated from the I10 / (I10 + I7) ratio where I10 and I7 denote the intensity of the basal (d001) peaks at 10 and 7 Å, respectively. The effect of “progressive” formamide treatment, involving 4 successive additions of formamide, is also studied. After the immediate formamide treatment, the samples are placed in a dessicator for 4 h, then sprayed 4 times at 1.5 h intervals. XRD analysis is carried out after each spraying.
3. Results 3.1. Reference halloysite samples The XRD patterns for the 3 reference halloysites are shown in Fig. 1. The basal (d001) reflections are typical of fully hydrated (Opotiki, 10 Å), partially hydrated (Te Puke, 7–10 Å), and dehydrated (Matauri Bay, 7 Å) halloysites. After immediate formamide treatment, all 3
Fig. 2. XRD patterns of oriented deposits for (a) the fine (b0.1 μm), (b) the intermediate (0.1–1 μm), and (c) the coarse (1–2 μm) clays fractions from Cameroon soil after (i) immediate formamide treatment (i.F.T.) (Churchman et al., 1984), and progressive formamide treatment (p.F.T.).
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halloysites give a basal spacing at 10.2 Å, consistent with the presence of a single layer of amide molecules in the interlayer space. However, Matauri Bay halloysite shows a residual reflection near 7 Å indicating the presence of some unexpanded layers. 3.2. Soil samples The XRD patterns of the b 2 μm clay fraction, separated from the fresh sample (Delvaux et al., 1990b) show basal reflections at 10 Å and 7 Å indicative of halloysite-(10 Å) and kaolinite, respectively. The presence of gibbsite and quartz is also noted. Using immediate formamide treatment on the fresh sample, Delvaux et al. (1992) calculated 63% and 79% of halloysite (with respect to kaolinite) in the b 2 μm and b0.1 μm fraction, respectively. The XRD patterns of the clay fractions of the re-extracted sample (i.e. after storage) show a prominent, rather broad peak near 7 Å whatever the clay fraction. The fine clay fraction (Fig. 2a) is dominated by halloysite-(7 Å) typified by (i) a broad peak at 7.40 Å, and (ii) a broad hk band at 4.4 Å due to tubular or spheroidal morphology and high degree of disorder (Brindley, 1980). A small shoulder at 10 Å indicates the occurrence of some hydrated halloysite. The XRD pattern of the 0.1–1 μm fraction (Fig. 2b) also shows halloysite (broad 7.40 Å component, hk band, 10 Å shoulder) and kaolinite with typical narrower 00 ℓ reflections at 7.20 and 3.58 Å. Gibbsite and quartz are also present. In the XRD pattern of the coarse fraction (1–2 μm), the 7.15 and 3.58 Å reflections are due to kaolinite, representing the main component, together with quartz, gibbsite, and a small amount of mica. The hk band of halloysite is absent (Fig. 2c). These results from the re-extracted sample accord with the finding by Delvaux et al. (1992) that halloysite decreases relative to kaolinite as particle size increases. After immediate formamide treatment, only small changes in XRD intensity are observed as compared with the natural sample (Fig. 2). The relative intensity of the broad peak at 10 Å increases for the fine and intermediate fractions. Using the test of Churchman et al. (1984), the estimated halloysite content is about 30% for the fine clay fraction, 28% for the intermediate fraction, and less than 10% in the coarse clay fraction (Table 1). After progressive formamide treatment, the 10 Å component clearly increases for all particle-size fractions. Almost the entire coarse fraction is expanded (Fig. 2c), whereas about 50% of the fine and intermediate clay fractions are expanded (Table 1). Note that the 10 Å peak
Table 1 Estimation (in %) of halloysite content in different fractions of halloysitic soil Soil fractions
b0.1 μm (in %)
0.1–1 μm (in %)
1–2 μm (in %)
TEM investigation (H/(H + K)1 Immediate formamide intercalation on fresh sample2 Immediate formamide intercalation after storage2 Progressive formamide intercalation after storage3
80 (210) 79
60 (180) 55–604
22 (130) b254
30
28
10
50
–
–
Values in brackets denote the number of particles counted. Estimated using the number of tubular and spheroidal particles. 2 Estimated following Churchman et al. (1984)'s procedure. 3 Estimated after progressive formamide treatment (see text). 4 Estimated from Delvaux et al. (1992). – Not given due to kaolinite intercalation. 1
that appears after progressive treatment for the intermediate and coarse clay fractions, is narrow and corresponds to intercalated kaolinite. 3.3. Transmission electron microscopy investigations Fig. 3 shows representative TEM micrographs of the reference halloysites and the clay fractions from soil. The reference Matauri Bay halloysite consists dominantly of thick tubes of about 0.7 μm in length (Fig. 3a), while Te Puke halloysite is charaterized by short, narrow tubes (mean size about 0.3 μm) (Fig. 3b). Opotiki halloysite is mostly made up of spheroidal particles (Fig. 3c) with a mean diameter of 0.4 μm. These observations accord with those reported by Churchman and Theng (1984). The dominant particle shape in the fine clay fraction (Fig. 3f) is tubular with few spheroidal particles, whereas platy particles are dominant in the coarse clay fraction (Fig. 3d). The intermediate clay fraction shows a mixture of tubular and platy particles (Fig. 3e). The mean size of particles is about 0.05 μm, 0.3 μm and 1.4 μm for the fine, the intermediate and the coarse fractions, respectively. The size of the particles measured by TEM is in agreement with that collected by sedimentation and centrifugation. It is well known that tubular or spheroidal shapes are characteristic of halloysite, while a platy morphology is more typical of kaolinite (Dixon, 1989). In accordance with XRD data, the coarse fraction is kaolinite-rich, while the fine one is halloysite-rich. 4. Discussion When Delvaux et al. (1990b, 1992) investigated the halloysite-rich soils from Cameroon they found fully
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Fig. 3. TEM micrographs of (i) reference halloysites (a) Te Puke, (b) Matauri Bay and (c) Opotiki, and (ii) of (d) coarse (1–2 μm), (e) intermediate (0.1– 1 μm), and (f ) fine (b0.1 μm) clays fractions from the soil after storage.
hydrated halloysite in the B horizon of these soils. For this study, we separated (“re-extracted”) the clay fractions from the same sample (horizon) after longterm storage in controlled room temperature (20 °C) and humidity (30% RH). XRD shows that the halloysite-
(10 Å), initially present in the (fresh) sample, has since been completely dehydrated. That halloysite-(10 Å) could partially dehydrate during transportation and storage has also been reported by Rattigan (1967). Halloysite can be kept fully hydrated only by storing in a
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sealed container in contact with water or in a watersaturated atmosphere. The XRD patterns of the different clay fractions clearly indicate an increase in halloysite content relative to kaolinite with decreasing particle size. Using TEM, the halloysite content may be estimated from the abundance of tubular or spheroidal particles, assuming that kaolinite particles are platy or polygonal (Beutelspacher and van der Marel, 1968; Churchman and Carr, 1975; Churchman and Theng, 1984; Dixon, 1989). From representative TEM micrographs, about 80, 60, and 20% of tubular and spheroidal particles are determined for the fine, intermediate, and coarse fractions, respectively, in agreement with XRD data. The proportion of characteristic halloysite shapes in the re-extracted sample closely matches the amount of halloysite content measured by Delvaux et al. (1992) using the formamide test on the fresh sample (Table 1). Formamide intercalation indicates differences in reactivity between the fresh and re-extracted halloysite. After immediate formamide treatment, 30% of the reextracted fine clay (b0.1 μm) fraction intercalates formamide as compared with 79% for the corresponding fraction from the fresh sample (Table 1). About 28 and 10%, respectively, of the intermediate (0.1–1 μm) and coarse (1–2 μm) clay fractions from the stored soil can intercalate formamide, while the correponding values, measured by Delvaux et al. (1992) for the fresh sample, are 55–60% and b 15% (Table 1). Thus, the formamide test highly underestimates the halloysite content of the fine and intermediate clay fractions. The difference in halloysite content between the fresh and the re-extracted samples decreases as particle size increases. This is because the coarse clay fraction is kaolinite-rich, and little formamide is intercalated after immediate treatment. Similarly, the formamide test considerably underestimates the halloysite content of soil samples (30–40% in the halloysite-rich fractions) after storage and natural dehydration. By contrast, the state of hydration does not affect the reactivity of the reference halloysites toward formamide (Fig. 1). Both Te Puke halloysite (partially dehydrated) and Matauri Bay halloysite (fully dehydrated) readily intercalate a single layer of formamide after immediate treatment. After progressive formamide treatment, almost all the kaolinite is intercalated whereas only 50% of the halloysite is expanded (Table 1). This observation is consistent with the finding by Churchman et al. (1984) that kaolinite intercalates formamide after 4 h of contact. However, the amount of halloysite measured by progressive formamide treatment is less than the actual value (Table 1). Apparently, a fraction of
dehydrated halloysite intercalates less formamide than kaolinite. Churchman et al. (1984) have shown that intercalation is influenced by particle crystallinity and size. Highly reactive halloysites are characterized by large particles, and a high crystallinity. Conversely, small particles with low crystallinity are relatively nonreactive. Since the hydration state of halloysite can profoundly affect its behaviour toward formamide (e.g. Churchman and Theng, 1984; Joussein et al., 2005), due care should be exercised in applying the formamide test to halloysite-rich soil samples that are not kept in a wet state. The halloysite from a Cameroon soil, used in this study, has a cation exchange capacity (CEC) of about 26 cmolc/kg as well as a high K+ selectivity (Delvaux et al., 1990a, 1992), and Cs+ affinity (Joussein et al., 2004). Similarly, Takahashi et al. (1993, 2001) found that formamide did not intercalate into halloysite-rich clays with a high CEC and K+ selectivity from northern California. We propose that the presence of interlayer Kspecific sites (Delvaux et al., 1990a; Joussein et al., 2004) keep the halloysite layers collapsed after dehydration, preventing formamide from entering the interlayer space. That natural or induced dehydration would cause a decrease in the CEC of halloysite (Grim, 1968; Bailey, 1990) is in keeping with this hypothesis. 5. Conclusion The halloysite from a Cameroon soil is fully hydrated (halloysite-10 Å) in the fresh sample but appears to be completely dehydrated after storage and re-extraction. The proportion of characteristic halloysite shapes in the re-extracted sample, estimated by TEM, closely matches the amount of halloysite derived from the formamide test on the fresh sample. The formamide test greatly underestimates the halloysite content of the clay fractions separated from the stored soil. Even after repeated treatments with formamide, the estimated amount of halloysite in the soil clay fractions is less than the actual value. In the case of the reference halloysites, however, the state of hydration does not influence formamide intercalation. Thus, due care should be taken in using the formamide test to determine the halloysite content of samples that are not kept in a wet state. On the positive side, the formamide test clearly indicates that dehydration can modify the behaviour and reactivity of K-selective halloysites toward organic compounds. Because of the presence of K-specific sites, dehydration would promote strong interaction between halloysite layers within a particle, inhibiting formamide intercalation.
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