surface properties and clay mineralogy …minersoc.org/pages/archive-cm/volume_25/25-2-141.pdf ·...

Clay Minerals (1990) 25, 141-160

S U R F A C E P R O P E R T I E S A N D C L A Y M I N E R A L O G Y OF H Y D R A T E D H A L L O Y S I T I C SOIL C L A Y S .

II: E V I D E N C E F O R T H E P R E S E N C E OF H A L L O Y S I T E / S M E C T I T E ( H / S m ) M I X E D - L A Y E R

C L A Y S

B. D E L V A U X * , A. J. H E R B I L L O N $ , L. V I E L V O Y E t AND M. M. M E S T D A G H *

*IRFA/C1RAD, Unitd de Chimie des Interfaces, ~'Section de Physico-chimie Mindrale du Musde Royal de l'Afrique Centrale, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium, and ~:Centre de Pddologie

Biologique, UP 6831 du CNRS, associ~e it l'Universitd de Nancy L BP 5, F-54501 Vandoeuvre-les-Nancy Cedex, France

Received 6 May 1989; revised 6 September 1989)

A B S TRACT: Six clays from volcanic ash soils at different stages of weathering differ in their relative halloysite content with respect to kaolinite and several surface properties, namely CEC, and exchange selectivity for K +. These three parameters are related to each other in that they all decrease with increasing soil weathering stage. XRD data show that the hydrated 1 : 1 layer-silicates in these clays combine with smectite to form interstratified H/Sm clay minerals. In these mixed-layers, the content and layer charge of smectitic units decrease as the relative halloysite content in the clay decreases. These clays thus depict a weathering sequence that is parallel to the weathering sequence of the soils from which they originate. It is also shown that the smectites in the H/Sm minerals have the distinctive composition and ESR spectrum of Fe-rich 2:1 clay minerals belonging to the beidellite-nontronite series. The information obtained explains why these clays have high CEC and distinct affinities for K + . It is hoped that this study will help to clarify the controversy concerning the CEC and related surface properties attributed to hydrated halloysite.

As noted by Newman & Brown (1987), the cation exchange capacity (CEC) of halloysite is still open to debate. The occurrence in soils, tephra and saprolites of halloysites exhibiting a high charge contrasts with that of halloysites exhibiting CEC values more usual for 1 : 1 layer-silicates (i.e. <10 mEq/100 g according to Lim et al., 1980). Since the paper of Garrett & Walker (1959), several reasons have been evoked for explaining such discrepancies. Some authors have suggested that the charge of halloysite (10 A) originates from isomorphous substitutions in the 1 :1 layers (Wada & Mizota, 1982; Okamura & Wada, 1984; Wada & Kakuto, 1985). According to others, the high charge of some hydrated halloysites is due either to contaminat ion by 2 : 1 phyllosilicates (Quant in et al., 1984; Delvaux et al., 1988) or to some external surface properties of the hydrated 1 : 1 clay which

were not precisely defined (Noro, 1986). It is also known that some halloysites have chemical compositions departing notably from

that of ideal kaolin (see e.g. Weaver & Pollard, 1973 and Wada & Mizota, 1982). On the other hand, Tazaki (1982) who noticed a relation between the chemical composition and the morphology of halloysites, found that iron was a characteristic cation in some halloysites.

In the first part of this study, Delvaux et al. (1990) reported, for the soil clays investigated

�9 1990 The Mineralogical Society

142 B. De lvaux et al.

in this work, a direct relationship between relative halloysite content, CEC, and exchange selectivity for K +. They concluded that such surface properties pointed to the presence of smectites closely associated with halloysite in these samples. As a second part of this study, the present paper is aimed at documenting this view by mineralogical characterization of the same clays, with special emphasis on the identification of their 2 : 1 contaminants. As a conclusion to this study, the mineralogical information will be combined with the surface properties already described to construct a model of the halloysite/smectite (H/SIn) mixed- layer clays found in these samples.

M A T E R I A L S A N D M E T H O D S

The origin and major characteristics of the selected clay samples (<2/~m) were presented in part I of this study (see Table 1 in Delvaux et al., 1990). Elemental analysis of the deferrated clays was achieved after a sulpho-fluorhydric acid attack and digestion of calcined clay materials. Aluminium, Fe, Ca, Mg, K, Na, and Mn were determined by atomic absorption spectrophotometry, Ti by chromotropic acid colorimetry, and Si by difference (Voinovitch et al., 1962).

The deferrated clays were saturated with K + and M f + ions. X-ray diffraction (XRD) patterns of the clays deposited on ceramic plates were obtained using a Philips diffractometer using Ni-filtered Cu-Ko~ radiation. The oriented clays were submitted to the following treatments: ethylene glycol (EG) vapour treatment under vacuum (10 -2 mbar) for 15 h, heating at 105~ (18 h), 300~ and 550~ (4 h). Similar treatments were also applied to three reference low-charge halloysite clay samples (<2 /~m): the Te Puke, Matauri Bay and Opotiki halloysites (Theng et al., 1982; Churchman & Theng, 1984).

Thermogravimetric analyses (TGA) were performed on both K +- and MgZ+-saturated clays equilibrated at 66% relative humidity, using a Setaram mtb 10.8 microbalance at a heating rate of 6~

Infrared (IR) spectra were scanned between 4000 and 500 cm-I on a Fourier Transform IR Brucker IFS88 spectrometer using 400 scans and a resolution of 2 cm-l : KBr pellets with 2% clay, i.e. 0.68 mg/cm 2, were examined after 9 h dehydration pretreatment at 180~ in vacuum-tight quartz cells with NaC1 windows; the residual pressures in the cells were of the order of 10 4 torr.

Electron spin resonance (ESR) measurements of powdered samples were performed at 25~ on an X-band Varian E-12 spectrometer as previously described by Mestdagh et al. (1980). Deferrated clay samples were examined by ESR spectroscopy (i) without any further pretreatment, and (ii) after heating at 550~ and subsequent deferrification treatments with 0.2 M oxalic acid-ammonium oxalate at pH 3 (Blakemore, 1983) and Na dithionite-citrate-bicarbonate (DCB) (Mehra & Jackson, 1960).

R E S U L T S A N D D I S C U S S I O N

Chemical analysis

Chemical analyses are presented in Table 1. As SN4 clay contains allophane and phyllosilicates (Delvaux et al., 1990), analysis was carried (a) before, and (b) after the extraction of allophanic materials with a buffer solution of 0.2 M oxalic acid-ammonium oxalate at pH 3 (Blakemore, 1983). The analyses of sample SN4 (b) and the other allophane-free materials show that these clays all have the characteristic composition of I : 1

Hydrated halloysitic soil clays: mixed-layer H/Sm 143

TABLE 1. Chemical analyses of the deferrated Na+-saturated clays (wt%, oven dried basis, 105~

SN4 (a) (b) SN5 SN2 1R13 MUI MK1

Ignition loss 18.00 14-52 14.26 13-33 13.08 13-81 13-18 SiO2 37-14 43-19 42.06 43-95 43-33 43.70 40.37 A1203 32-73 34.78 34.74 33.80 32.59 35.18 30.98 Fe203 6-05 3-95 4.90 4-36 5-37 4.04 7.25 TiO~ 2-95 1-07 1.77 2.89 4.12 2.01 7.07 Mn304 0.07 0.04 0-03 0.03 0.03 0.04 0.04 CaO 0.13 0.38 0.03 0.02 0.03 0.02 0.01 MgO 1.09 0.94 0.92 0.47 0.36 0-30 0.41 K20 0.20 0-23 0.17 0-52 0-39 0-20 0.17 Na20 1-64 0-90 1.12 0.63 0-70 0.70 0.52 SIO2/A1203" 1-92 2.10 2.05 2.20 2.25 2.10 2-21 SIO2/R203" 1"72 1"96 1"89 2"04 2"04 1"96 1.92

* Molar ratios.

dioctahedral phyllosilicates. Nevertheless, most of the clays studied exhibit SIO2/A1203 molar ratios slightly greater than 2.0 and have rather high Fe (4-7% Fe203) and Mg (0.3- 1-0% MgO) contents. The molar ratio SIO2/R203 is close to 2.0, as for the "ferrihalloysites" studied by Weaver & Pollard (1973), and also for Fe-rich "halloysites" (Askenasy et al., 1973; Wada & Mizota, 1982; Wada & Kakuto, 1985; Quantin et al., 1984; Quantin et al., 1988).

Fig. 1 shows Fe and Mg contents plotted against Si/A1 atomic ratios. The symbols in the shaded areas represent the clay materials studied while the other symbols refer to both low- charge (Si/A1 ~ 1.0) and "high-charge" halloysites reported in the literature. The low- charge halloysites (Theng et al., 1982; Churchman & Theng, 1984) exhibit Si/A1 atomic ratios close to 1-0; they have the lowest Fe contents (0.0-2-6% Fe) and virtually no magnesium. The "high-charge halloysites" can be distinguished by higher Fe contents (% Fe > 5), very low to moderate Mg contents (0.04).2% Mg) and higher Si/A1 atomic ratios. Relative to these two groups, the clays studied (shaded area in Fig. 1) have intermediate Fe contents (3-5% Fe) but they are richer in Mg (0.24).6% Mg). In addition, Fig. 1 suggests an inverse relationship between Fe and Mg in these clays.

The low measured Si/A1 atomic ratios provide little evidence of the contamination of I : 1 clays by 2 : 1 swelling layers. Nevertheless, they may indicate that they are not of montmorillonitic type but rather beidellitic or nontronitic. The rather high Fe, Mg contents might be related to such a contamination and suggest the presence of Fe-smectites in the clay samples under study.

X R D patterns

X R D patterns are illustrated in Figs. 2, 3 and 4, Fig. 2 for the halloysite-rich clay SN5 in the 2-30~ region, and Figs. 3 and 4 for selected 20 ranges of the three reference low- charge halloysites (Te Puke, Matauri Bay and Opotiki samples), and the soil clays investigated in this paper, respectively.

144 B. D e l v a u x et al.

6

f ........ ::::i::::i ~ 2 ~:: : : : : :G

0 .5

0.3

:: :::: ::: : ::i:3.

0.1

1.0 1.1 1.2 1.3 1.4

ATOMIC R A T I O S i / A I

Fx6.1. Relationship between Si/Al atomic ratios and Fe, Mg contents of both the deferrated clays and some halloysites described in the literature, ii~ii This study. �9 Churchman & Theng (1984). A Wada

& Mizota (1982). �9 Quantin et al. (1984). ~ Quantin et al. (1988).

S a m p l e SN5 . On Fig. 2a, the pattern shows the typical 00l basal reflections of hydrated halloysite (10 A, 3-33 A) as well as the broad h k band at 4.4 A typical of this mineral. The presence of kaolinite (7.15 ,~, 3.56 ~) , gibbsite (4.82 ~ ) and quartz (3.33 ~ on Fig. 2f) is also to be noted. After saturation of the clay by EG vapour, the position and the shape of the reflection close to 10 ~ varies with the nature of the exchangeable cation. For the Mg2+-saturated clay, the reflection is shifted to 10.5 A and exhibits a distinct broadening towards the low diffraction angles; simultaneously, a weak diffraction peak at 14.9 appears which cannot be assigned to halloysite. A similar shift is not detected on the pattern of the K+-saturated, EG-treated clay. The X-ray profile obtained after prolonged dehydration (18 h) at 105~ shows a 7 A peak assigned to dehydrated halloysite, characterized by an important tailing towards the low-angle region in which, in spite of the duration of the dehydration treatment, a 10 A peak is still present. After heating the clay at 300~ the 7-2 A peak of dehydrated 1 : 1 layer-silicates is associated with a shoulder at 7.37 A showing a tailing towards lower diffraction angles. A similar asymmetry also characterizes the 3-56 ,~ reflection. At 550~ the 1 : 1 clays are dehydroxylated and do not diffract.

Two peculiarities clearly distinguish the X-ray patterns in Fig. 2 from those performed under identical conditions on the three reference halloysites (Fig. 3). When compared to the latter, the X-ray patterns of sample SN5 show (i) a shift in the position of the 10 reflection after the EG treatment of the MgZ+-clay (d Mg 2+, EG > d K +, EG) and (ii) a collapse after dehydration at 300~ giving rise to a 7 A peak that is neither located at the


same position nor as symmetric as the 7 A reflection due to the dehydrated reference halloysites (Fig. 3).

The first observation (d Mg 2+, EG > d K +, EG), is a clear indication that, in the clay SN5, the 10 A reflection is not solely due to the 001 reflection of hydrated halloysite. In ideal halloysite (10 A), the interlayer space is, indeed, not accessible to exchangeable cations (see e.g., the organization of water in this hydrate as described by Cruz et al., 1978) and therefore, a change in the nature of these cations cannot induce any differential swelling. In contrast, such a differential swelling (d Mg 2+, EG > d K +, EG) in the presence of polar

Mg 550 ~

Mg 300 ~

Mg 105 ~

K EG

Mg EG

Mg

R.H. 66 %

3.56 7.13 7.37 j ~ e

3.56 7.13 91"9 ]

10.5 7.13

I I 1 4 . 9 /

3.56 4 41 9.9 �9 713 I ] 3.33 ~ I I ^ /

~J~ A 4"82 ~ , / ~

I I I I I I I I [ I I I I I I _ _

30 20 8 2

~ Fla. 2. X-ray (Cu-Ka') diffraction patterns of oriented SN5 clay. Spacings in ~.


, , , , i , , ,

K 550"C J

7.19 I

A / K 300"C j ~ - ~

10.2 I

K EG

Mg ~

, , , , , , , , i i , . . . . ,

__5I/ - - 7 . 13

7.13 I

- - 10 .1 - - 10.1

J

S

J Te Puke Opotiki Matauri Bay

I i i I I I I I I t I I I I I I I i ] I / L L t

8 2 8 2 8 2

~

FI6.3. X-ray (Cu-Ko 0 diffraction patterns of three reference halloysites (oriented clays <2 Hm) from New Zealand (Theng et al., 1982; Churchman & Theng, 1984). Spacings in A.

molecules is known to occur in high-charge 2 : 1 swelling clays (Harward & Brindley, 1965; Suquet et al., 1977; Suquet, 1978; Egashira et al. , 1982; Malla & Douglas, 1987). On the other hand, the 10-5 A reflection appearing with the Mg2+-EG clay (Fig. 2c) is in the same range of d values as the combined basal 001/001 reflection due to a mixed-layer clay composed of a 10 A mineral (a mica for instance) and a 2 : 1 swelling clay mineral (see e.g. the peak-migration curve given by Reynolds (1980) for a biotite/vermiculite mixed-layer system).

As for the difference z~ (A = (d Mg 2+, EG) - (d K +, EG)) between the basal spacing of the Mg 2+- and K+-saturated clay, an interstratified mica/2: l swelling system can also explain this observation. As any high-charge swelling clay saturated with K + and then treated with EG generally shows a lower d(001) spacing than after saturation with Mg 2+, it is expected that, in a 10/k/2 : 1 high-charge swelling clay mixed-layer system, the 001/001 combined basal reflection will have a lower d value for a K+-sample than for a MgZ+-one. Hence a differential swelling A similar to that observed. Fig. 2 therefore illustrates that, in

K 550~

Hydrated halloysitic soil clays: mixed-layer H/Sm

7.15 7.3

713 7.8 /

K 300~ - ~

K EG ~ ~ ~,~ 9.9 105 /

Mg EG ~ k ~ ~'~] 99

7.13 74 ]

7.15 I

25~ / U 715919 ' ~ 71\5 0 9 ~

SN4 (0.98) SN5 (0.82) SN2 (0.63) IR13 (0.50) MU1 (0.60) I I I L I I I I I ] I I I I P I i I I I k I I I I i I F I I I I I I I

8 2 8 2 8 2 8 2 8 2

147

Y /725 1/7.25

725

V J

7.115

MK1 (0.38)

8 2

~

FIG. 4. X-ray (Cu-K0~) diffraction patterns of the soil clays studied; spacings in A. Relative halloysite contents (H/H + K) in each sample are given in brackets.

sample SN5, the halloysite (10 A) is interstratified with a 2 : 1 swelling clay, the layer charge of which is high enough for swelling in the presence of EG to be affected by the nature of the exchangeable cation (d Mg 2+, EG > d K +, EG). Both vermiculites and high-charge smectites belong to such a category of 2 : 1 swelling clay minerals.

The X-ray pattern of the dehydrated SN5 clay (300~ Fig. 2e) also supports the above identification. On Fig. 2e, the 7.37 A shoulder of the 7 A peak can, as in a dehydrated kaolinite/smectite mixed-layer mineral, be assigned tO a 001 (7/~)-001 (10 A) combined basal reflection. The position (7.37 A) further indicates that the smectite content is --20% in the halloysite/smectite (H/Sm) mixed-layer mineral present in sample SN5 (Cradwick & Wilson, 1972). It is to be noted, however, that as this combined reflection is close to the 7 position, the X-ray estimation of the content of 2 : 1 units in the H/Sm cannot be very accurate. In sample SN5, accurate identification is further hampered by the presence of a segregated kaolinite phase which also shows a 7 A reflection.

Other samples (& ', SN2, IR13, MU1, MK1). XRD patterns in the range 2-14~ are

148 B. Delvaux et al.

shown on Fig. 4. Comparison of these patterns with those for sample SN5 (Fig. 4) and the reference halloysites (Fig. 3) leads to several observations:

(a) The 10 A reflection for these samples does not respond to the different treatments in the same way as for the 10 ,~ peak of the reference halloysites.

(b) One major difference concerns again the position and shape of the 10 ~ peaks after EG treatment. In this respect, the clays can be divided into two categories. In the first one (SN4, SN2), the clays behave similarly to sample SN5 and there is important differential swelling A. As discussed above, it can be inferred from such a differential swelling that these clays contain H/Sm mixed-layer minerals, in which the 2:1 swelling components belong to the high-charge category. The second category of clays (IR13, MU1, MK1) is characterized by a lower proportion of halloysite with respect to kaolinite. A shift in the 10 A peak after EG treatment is observed, not only when they are saturated with Mg 2+, but also when they are K+-saturated. As low-charge smectites also show lower differential swellings in response to the nature of exchangeable cation when treated with EG, this observation suggests the presence of smectites with lower layer charge in the H/Sm clay minerals of this second category of samples.

(c) For all samples, the position and the symmetry of the 7 A reflection shown by the dehydrated clays (300~ supports the above identification of H/Sm instead of pure halloysite. Such an identification is especially obvious for SN4. For this clay, which is devoid of segregated kaolinite, reference to the peak-migration curve computed by Cradwick & Wilson (1972) allows estimation of the smectite content of the H/Sm as 30%. For the other samples, a similar quantitative estimation is more difficult, and the evidence for interstratification is based only on the shoulders and/or the asymmetry of the 7 ,~ peak (Thiry & Weber, 1977).

(d) Another difference with respect to both reference halloysites and clay SN5 concerns the samples SN2, IR13, MU1 and MK1. The first three of these show X-ray evidence for the presence of segregated 2 : 1 clay minerals by exhibiting a 10 A reflection on the patterns obtained after heating at 300~ However, when the 1 : 1 layer-silicates of these clays are dehydroxylated (550~ the 10 .& peak broadens considerably (samples SN2, IR13, MU1) while in sample MK1 (in which no evidence for segregated 2 : 1 clays could be detected at 300~ there is a very broad diffraction band at ~12 A. In fact, the 10-14 A region of the X-ray patterns of these dehydroxylated clays can be taken as a further evidence for the presence of 1 : 1/2 : 1 mixed-layer clays in these samples. As noted by Herbillon etal. (1981), it has often been reported that the dehydroxylation of 1 : 1/2 : 1 mixed-layer clays gives rise to such XRD features that have been attributed to remnants of 2 : 1 layers intercalated with either metakaolinite (Wiewiora, 1973) or hydroxyaluminium polymers. With sample MK1, for which the X-ray features at 550~ are the most typical in this respect, a hot citrate treatment (Tamura, 1958) of the clay led to (i) an improvement of the symmetry of the 7 peak after heating the sample at 300~ and (ii) the resolution of a more defined 10/~ reflection on the X-ray pattern of the dehydroxylated sample (550~ (results not illustrated). As the hot citrate treatment also dissolved 3-1% A1203 and no SiO2, these observations suggest the presence of hydroxyaluminium vermiculite or smectite in the H/Sm mixed-layer clay in sample MK1.

In conclusion, the XRD patterns in Fig. 4 are useful for two different purposes. They document that the halloysitic soil clays investigated contain hydrated 1 : 1 layer-silicates that are not genuine halloysites, but interstratified H/Sm clay minerals. They show also that, as relative halloysite content with respect to kaolinite (H/H + K) decreases in the clays, the


propor t ion and the layer charge of smecti te associated with halloysite in the H/Sm minerals also decrease. Finally, with the MK1 clay, evidence has been obta ined to suggest that the reduction of the layer charge of the smectite units associated with halloysite might be due to their interlayering with hydroxyaluminium polymers. Therefore , this collection of soil clays, which originate from soils from different weather ing stages, depicts a paral lel weathering sequence of clays that can be schematically writ ten as follows: smectite-rich, high-charge smecti te/halloysite H/Sm (samples SN4, SN5) ~ smect i te-poor , lower-charge smecti te/halloysite H/Sm + kaolinite (sample MK1).

Thermogravimetric analysis

T G A curves were obta ined on clay samples sa turated with ei ther M f + or K + and equi l ibrated at a relative humidity of 66% for 15 days. Al l the curves showed a clear inflection at 250-300~ The computed values for the weight loss of the hydrated clays are listed in Table 2. The weight toss measured between 20~ and 300~ involves the loss of hydrat ion water (20-250~ and the possible loss of structural O H groups of gibbsite (250 ~ 300~ The weight loss measured at 300~ indicates higher hydrat ion-water contents for Mg a+- as opposed to K+-saturat ion, as occurs in smectites (Suquet, 1978).

The weight loss of the anhydrous clays (300--1000~ is related to the dehydroxylat ion of 1 : 1 and 2 : 1 layer-silicates in these samples. The structural water contents listed in Table 2 are almost identical after ei ther M f +- or K+-saturat ion. These values are lower than 13.9%, i.e. the theoret ical weight loss of anhydrous kaolini te and/or halloysite, and would indicate much higher 2 : 1 layer-sil icate contents than those est imated from X-ray data. Such discrepancies may be due to the presence of o ther anhydrous impurit ies (quartz, TiO2) and/or unreacted 2 : 1 layer-silicates (Brindley et al., 1983). It is, however, satisfactory to find that the percentage dehydroxyla t ion loss appears to be re la ted to both the CEC and the est imated internal surface area (SEGME - SBET, see Delvaux et al., 1990), as i l lustrated in Fig. 5. F rom this graph, ext rapola t ion of structural water content at CEC = 0 yields a value of 12.5% and this value was arbitrari ly chosen to compute smectite contents, on the basis of a weight loss of 4.9% for the anhydrous (300~ 2 : 1 mineral (Brindley et al., 1983). As listed in Table 2, the inferred smectite content decreases from 31 to 14% with increasing soil

TABLE 2. Weight loss measured from TGA curves performed on Mg 2+- and K+-saturated clays. Inferred smectite contents estimated from the structural water content measured on K+-saturated clays (300-1000~ weight loss %).

% Weight loss

20--300~ 300-1000~ Clay Mg K Mg K

Inferred smectite content

%

SN4 -- -- -- 10.24 30 SN5 14-48 13.55 10-64 10.17 31 SN2 10-45 9.57 10-83 10-50 27 IR13 9.96 8.40 10-85 10-60 25 MUI 9.76 8.24 11.51 11.50 14 MK1 9-75 8.53 11.00 10.92 21


weathering stage, in reasonable agreement with the values estimated from X-ray data (30% to -~10%).

1 2

% I

o

10

\ Q

D \

L O i I / i I 312 i ~ f rO i i i i i i 2 26 100 120 140

CEC pH 7 me/lO0 g (SEGME - SBE T) rn2/g

F16.5. Relationship between structural water content computed from TGA and (left) CEC measured at pH 7, and (right) estimation of the internal surface area (SEGME--SBET) for the clays studied (see

Delvaux et al., 1990).

F T I R spectra

The main variations in the FTIR spectra in Fig. 6 occur in well-defined regions associated with the OH-stretching and librational vibrations, i.e. in the 3800-3200 cm 1 and the 760- 960 cm 1 regions, respectively.

3800-3200 cm 1. This spectral region shows (i) intense IR absorption bands at ~3700 and 3620 cm 1 assigned to the stretching vibrations of internal and external OH groups respectively in 1 :1 dioctahedral layer-silicates (Farmer, 1974); (ii) a low-frequency shoulder on the 3620 cm i band appearing at 3600 cm t; and (iii) the occasional occurrence of OH-stretching vibrations absorbing at 3654 cm 1 (SN2, IR13, MU1, MK1) and 3527, 3450 cm i (SN4, SN5, SN2), the latter two bands being typical IR features of gibbsite. The 3654 cm i band is not present on the SN4, SN5 spectra, showing that halloysite is the dominant 1 : 1 layer-silicate in these samples (Farmer, 1974). The appearance of this band as well as the increase of the absorbance ratio A3700/A3620 (Parker, 1969; Nagasawa & Miyazaki, 1976) with increasing kaolinite content and soil weathering stage both agree with the X-ray data, i.e., the parallel decrease of the relative halloysite content (H/(H + K) (Fig. 4). The low-frequency shoulder on the 3620 cm -1 peak, localized at 3600 cm 1, is particularly distinct on the MK1 IR spectrum (Fig. 6) and may be assigned to octahedral A1Fe 3+ O H groups in 1 : 1 dioctahedral layer-silicates (Mendelovici et al., 1979; Petit et al., 1988). No typical IR feature in the 3800-3200 cm 1 can be unequivocally related to the presence of Fe-smectite in the samples studied. This is in line with the poor sensitivity of IR spectroscopy for detecting such minerals in 1 :1 /2 :1 mixtures or mixed-layer clays approaching the 1 : 1 pole (Rousseaux, 1978; Delvaux et al., 1989b),

960-760 cm -1. The main feature of this spectral region is the intense 912 cm 1 absorption band assigned to the librational vibrations of OH groups bound to A1 ions in 1 :1 dioctahedral layer-silicates (Farmer, 1974). The high frequency shoulder on this band, absorbing at 935 cm -1, is typical of kaolinite. This feature is lacking on the spectra


u~

z ~c oo

co

c'~ o r tad tt~

SN4

i eo

o; 7og~ SN5

co (xl co

Go (Xl r,-

0o I S N 2

IR13

t/)

. . M K 1

3 8 ~ 0 0 r J I J I I J p p I 3 2 0 0 9 5 0 7 5 0 8 5 0 8 0 0

W A V E N U M B E R C M - 1

FIG. 6. FTIR spectra performed on the deferrated clays after prolonged dehydration (180~ 9 h) in vacuum-tight cells with a residual pressure at 10 4 torr.

performed on the halloysite-rich clays SN4, SN5, and appears with increasing kaolinite content and soil weathering stage, in agreement with the X-ray data (Fig. 4). The 912 cm -1 band occasionally presents a low-frequency shoulder at 880-860 cm -1 assigned to A1Fe 3+- O H groups either in 1 : 1 layer-silicates (Mendelovici et al . , 1979; Petit et a l . , 1988) or in Fe-rich smectites ( G o o d m a n et al . , 1976). Some very weak absorption bands occasionally appear in the 850-785 cm-1 spectral region. They can be related to the librational vibrations of O H groups bound to the octahedral couples A1-Mg (830-850 cm - 1), Fe3+_Fe3+ (810-820 cm-1) , MgFe 3+ (785 cm 1) in 2 :1 dioctahedral layer-silicates (Farmer, 1974; G o o d m a n et al . , 1976). Nevertheless , their very weak intensity precludes their definite assignment, as also reported for synthetic kaolinite/nontronite mixed-layer clays of composit ions close to the kaolinitic pole (Delvaux et al . , 1989b).


E S R spectra

Unlike IR spectroscopy, ESR measurements proved to be a powerful iron probe for detect ing Fe-smect i te in 1 : 1 - 2 : 1 mineral mixtures or interstratif ied systems (Delvaux et al., 1989b).

ESR powder spectra were per formed (i) on the hydrated deferra ted clays, and (ii) on the same materials hea ted at 550~ and further defer ra ted with two consecutive t reatments (oxalate, DCB). Table 3 presents the Si, AI, Fe contents extracted by both t reatments . The oxalate extract is relatively richer in A1 which may originate from the dissolution of dehydroxyla ted gibbsite. The DCB extract shows a higher Fe extraction and Si/A1 atomic ratios approaching 1, suggesting the beginning of the dissolution of dehydroxyla ted 1 : 1 minerals. Higher Fe contents in the DCB extract may also be re la ted to a possible demixing of iron oxide from the layer-silicates. Table 3 indicates that both oxalate and DCB treatments extracted 30 to 58% of total iron.

Fig. 7 presents the ESR powder spectra per formed on (a) the hydrated defer ra ted clays, and (b) the same clay samples first heated at 550~ and further defer ra ted with oxalate and DCB. The ESR spectra per formed after heating at 550~ and the first subsequent oxalate t rea tment are similar to those obta ined for the hydrated defer ra ted clays (Fig. 7a) and are not presented. The E S R features of the clays studied (Fig. 7a, b) do not resemble those general ly repor ted for kaolini te (Angel et al., 1974; Jones et al., 1974; Meads & Malden, 1975; Herbi l lon et al., 1976; Mestdagh et al., 1980) and halloysite (Meads & Malden, 1975; Carr et al., 1977; Nagasawa & Noro, 1987; Chaikum & Carr , 1987). A broad g2 band is observed on all the spectra while a weak g4 band only appears on some ESR diagrams. The lat ter band appears to be resolved be t te r after the addit ional thermal and deferrification t reatments , part icularly for sample MU1. The weak g4 band can be a t t r ibuted to isolated Fe 3+ ions localized in the octahedral sheet of 1 :1 layer-silicates (see references listed above). The b roader g2 band is associated with Fe 3+ ions in adjacent structural sites in Fe-smecti tes as observed in other Fe bearing 2 : 1 clays (e.g. Olivier et al., 1975; G oodma n et al., 1988; Delvaux et al., 1989b). Because of the numerous deferrification t reatments , the large g2 ESR signal cannot be due to any free iron oxide phase.

The E S R features of the clays containing a mixed-layer 1 : 1/2 : 1 phase can usefully be compared to those re la ted to the kaolini te/Fe-smecti te mixed-layer clays studied by Delvaux et al. (1989b). It is indeed worth noticing the close similarity existing be tween the

TABLE 3. Si, A1, Fe contents extracted from the clays heated at 550~ and deferrated with 0-2 M oxalic acid- ammonium oxalate at pH 3, and Na dithionite-citrate-bicarbonate (DCB).

Oxalate DCB (Feox + FED)/

Fe Si A1 Fe Si AI Fetota 1 Clay % % % Si/A1 % % % Si/A1 %

SN5 0.37 0.50 2.07 0-23 1-61 0-61 0.72 0.84 57.5 SN2 0.14 0.23 1.27 0-17 0.99 0-47 0.54 0.84 37.0 IR13 0.19 0.28 1.53 0-17 1.38 0-60 0-54 1.06 41.8 MU1 0-16 0.29 1-89 0-14 0.70 0.53 0.59 0.86 30.4 MK1 0-24 0.26 1-51 0-16 1.64 0-49 0-65 0.72 37.1


IO00G

g-2

'] ~ / f ~ SN5

g ~ / f - - IR13

" ~ / ~ MUl

5 ~ f . ~ MK1

g-2

+ 1000 G

a b

FIG. 7. ESR spectra performed on (a) the hydrated deferrated clays and (b) the same materials after heating at 550~ and two subsequent deferrification treatments (oxalate + DCB).

ESR spectra presented in Fig. 7 and those reported for both synthetic kaolinite/nontronite and natural kaolinite/Fe-smectite mixed-layer clays. This observation as well as the chemical composition of the clays (Fig. 1) suggests the presence of Fe-rich smectite in the mixed-layer H/Sm phase identified from XRD.

Tentative crystal formulae of smectites

The results of the present investigation led to a further specification of the crystal formulae of the smectites contaminating hydrated halloysite in the H/Sm mixed-layer clays found in these samples. The evaluation was based on the following hypotheses: (a) gibbsite content was estimated from the A1 content extracted by oxalate after heating at 550~ (Table 3); (b) quartz impurities were neglected and Ti was not introduced in the crystal formulae of the 2 : 1 layers; (c) Fe content in the 1 : I layers (either halloysitic or kaolinitic)


was estimated from a substitution rate Fe/Fe + A1 = 4% (Herbillon et al., 1976); (d) the remaining Fe cations were assumed to be Fe In in octahedral coordination.

Then, after substracting the elements associated with the 1 : 1 layer-silicates, structural formulae for the 2 : I contaminants were computed (1) on the basis of a dehydroxylated 2 : 1 unit-cell having 44 negative charges, and (2) by assuming that the 2 :1 layer should be exactly dioctahedral (12 cations per unit-cell). The results of such computations showed that hypothesis (1) led to formulae of 2 : 1 clays exhibiting very low negative charges and octahedral cationic populations which were intermediate between the dioctahedral and the trioctahedral types of 2 : 1 layers. In contrast, hypothesis (2) ted to formulae of 2 : 1 clays with a layer charge close to that of micas. As a compromise, the formulae presented in Table 4 have been computed by establishing a good correspondence between calculated and measured CEC (see the CEC values in Table 1, Delvaux et al., 1990).

The inferred structural formulae are given in Table 4 for samples SN5, SN2 and IR13. In each of these clays, the total layer charge of the 2 : 1 layer is about 1 M cation/unit-cell and increases for sample SN5 to IR13, which is not in agreement with the X-ray data. Nevertheless, the proposed tentative formulae are interesting in two respects: (i) their tetrahedral charge decreases with increasing soil weathering stage and decreasing K + exchange selectivity (see next paragraph); (ii) the cationic population in the octahedral sheet decreases from the less weathered clay (SN5) to the most weathered one (IR13), suggesting the coexistence of trioctahedral phases or domains within the 2 : 1 dioctahedral clays of the less weathered soils. This observation is in line with the general weathering scheme of basic rocks: primary minerals --+ Fe saponite --+ Fe beidellite, nontronite --+ kaolinite + Fe oxide (Velde, 1985). Furthermore, the structural formulae presented in Table 4 show that smectites contaminating halloysite in the H/Sm mixed-layer clays have the characteristic compositions of the Fe-rich beidellite-nontronite series, as with most of the smectites found in the weathering products of basic rocks (Wilson, 1987).

TABLE 4. Tentative structural formulae of smectites contaminating hydrated halloysite in the clays studied.

SN5 (Si6.30All.70) TM

SN2 (Si7.05A10.95) TM

IR13 (Si7.40A104~0) TM

nl vI (Al2.20Fe t.42Mgo.97) O20OH4M~).90

111 v l (Al2.32Fe 1.32Mg0.52) O200H4M+0.99

111 vt (AlbloFee.37Mgo.53) 0200H4M+l. 13

Clay mineralogy and ion exchange properties'

In the first part of this study (Delvaux et al., 1990), it was shown that the K + selectivity of the clays (conventionally expressed by the value of Kc.0.04) and their internal surface charge density (oi) both decreased with increasing soil weathering stage. It can be seen from Fig. 8 that these parameters both decrease with increasing swelling of the K+-saturated smectite present in the H/SIn mixed-layer clay, following the clay sequence SN5 --+ SN2 --+ MK1. As discussed earlier, the shift of the combined reflection d 001(H)/d 001 (Sm-EG) after K+-saturation can be related to the charge properties of smectite associated with hydrated halloysite. Thus, Fig. 8 indicates that these charge properties related to the clay weathering stage, probably control the degree of K + affinity exhibited by the clays studied. In the less weathered soils, hydrated halloysite is associated with high-charge smectite probably of


1.0 %

0.8

~' 0.6

~ - 0.4

-J

80

60

40

o

I

10.0 10.2 101.4 10t.6 10t.8

d001/001 (K +, EG) ,~

�9 Tropepts �9 Udalfs ~ Udults

FIG. 8. Relationship between the position of the combined 001/001 reflection of the K+-saturated, EG-treated H/SIn mixed-layer and: (a) the estimated internal surface charge density oi, (b) the K + affinity of the clay exchanger, conventionally expressed by the value of the selectivity coefficient Kc measured for an equilibrium solution of a K + equivalent fraction = 0-04 (K~;, o.o4) (see Delvaux et al.,

1990).

beidellitic character, hence the clay exchanger exhibits a high exchange selectivity for K + ions. In more weathered soils, the lower amount as well as the lower charge of the 2 : 1 units contaminating halloysite in the H/Sm phase explain the lower K + affinity of the clay exchanger and may be related to the Al-hydroxy interlaying of smectite units. Thus, each weathering stage in the clay sequence considered shows a different type of smectitic layer associated with hydrated halloysite and hence a different exchange selectivity for K + ions, with different implications for soil fertility that were appraised elsewhere (Delvaux, 1988).

A model for the H/Sm mixed-layer clay

A hypothetical structural model depicting the association of hydrated halloysite and smectite in the H/Sm is proposed in Fig. 9. This model is inspired by the one suggested by Wada & Kakuto (1985) for embryonic halloysites. The H/Sm structural scheme proposed in Fig. 9 takes into account both the mineralogical characterization of the clays (present study) and their ion exchange behaviour (Delvaux et al., 1990), i.e. : (i) the definite presence of true 2 : 1 layers as identified from X R D and TGA, (ii) the loss of K + specific sites and the decrease of CEC, following the dehydration of halloysite, and (iii) the existence of edge K + specific sites which still remain operative after the collapse of H/SIn following its dehydration. This model shows also that the proportional relationship existing between


~ ~ ~ ~ ~\Y\/~ ~ ~ / X / J- /I ~ ~ ~ ~ ~ ~ ~ o o o

0 0 0 0 0 0 �9 ..... . . . . . . . .

1 8 { ] { { ,

~ ~ o o o ~ d2 : I: I/ li ~i: Ii : : I I ; . I ~ ~ ~ ~ ~ o

~o,~

O �9

~ ~ H20 K, FI6.9. Hypothetical structural scheme for the halloysite/smectite mixed-layer clay present in the soil

clays; oc: octahedral unit; te: tetrahedral unit.

relative halloysite content (H/H + K) and K + selectivity in these clays (Delvaux et al., 1990) as well as in other clays extracted from volcanic ash soils (Fontaine et al., 1989) is not haphazard but due to the close association of hydrated halloysite with smectite. Such a structural scheme might also prove to be useful to explain the high negative charge as well as the high exchange selectivity for dry counterions reported for some halloysites in other papers (Wada & Mizota, 1982; Okamura & Wada, 1984). Finally, the structural sketch in Fig. 9 is likely to describe, better than the one presented by Wada & Kakuto (1985), the hydrated soil clays which they called "embryonic halloysites". As noted by Parfitt & Churchman (1988), the X-ray patterns of these clays, as presented both by Wada & Kakuto (1985) and later by Wada et al. (1987), do exhibit several features indicating the contamination of halloysite units by 2:1 clay minerals. In agreement with Parfitt & Churchman (1988), it is believed, therefore, that these "embryonic halloysites" were actually misnamed and should, similarly to the halloysitic clays described, rather be identified as 2 : 1/1 : 1 mixed-layers.

G E N E R A L D I S C U S S I O N A N D C O N C L U S I O N S

The results presented in these two companion papers may have implications at three different levels. They may be useful (i) to discuss the charge properties of hydrated halloysite, (ii) to interpret the position of H/Sm mixed-layer clay in the clay weathering sequence involving the transformation of smectites to 1 : I layer-silicates in soils, and (iii) to discuss the significance of halloysite (10 A) in relation to criteria for soil classification in a system such as Soil Taxonomy (Soil Survey Staff, 1975).

(i) The halloysite-rich clays examined in the present study (SN4, SN5, SN2) proved to have all the properties that have been so far noted as characteristic for "high-charge" halloysite, i.e. a high CEC, a noticeable exchange selectivity for K +, and a chemical composition departing, in terms of Fe and Mg contents, from that of ideal kaolin. We have shown here that these properties could not be accounted for by halloysite per se, but by smectite associated with halloysite (10/k) within H/Sm mixed-layer clay. The interstratifica-

Hydra ted halloysitic soil clays: mixed- layer H / S m 157

tion of smectite with hydrated halloysite explains also the high internal surface area of these days, and the reduction of CEC and K + affinity after dehydration of the clay.

These results suggest that a thorough mineralogical study be required when a high K + affinity and/or high CEC are reported for halloysitic clays. It would therefore seem appropriate to investigate further the mineralogical composition of those "high-charge" halloysites whose occurrence brought about the controversy regarding the charge properties of halloysite (10 ~) . Without prejudging the results, it would not be surprising if the conclusion of the study of Lim et al. (1980) on kaolinites would apply to halloysites, i.e. whenever such 1:1 minerals exhibit a CEC over 10 mEq/100 g clay, contamination by higher charge 2 : 1 clays should be suspected.

(ii) It is worth noting that the soil clays investigated here belong to a clay weathering sequence that parallels a soil weathering sequence (Delvaux et al., 1989a), illustrated as follows:

Andepts --+ Tropepts, Udalfs ~ Udults

Allophane ~ H/Sm with high- ~ Kaolinite charge smectite H/SIn with lower

Kaolinite charge smectite

This clay sequence provides the opportunity to detail some of the steps causing, under humid tropical conditions, Fe-bearing 2 : 1 swelling clays to transform into 1 : 1 clays + Fe oxides (Craig & Loughnan, 1964; Kantor & Schwertmann, 1974; Velde, 1985). The intermediate steps of such a transformation consist of 1 : 1/2 : 1 mixed-layer clays in which the smectite content and layer charge both decrease as weathering proceeds. To the best of our knowledge, although 1 : 1/2 : 1 mixed-layer clays of kaolinitic nature are known to occur in soils (Wilson & Cradwick, 1972; Herbillon et al., 1981; Norrish & Pickering, 1983; Yerima et al., 1985), this seems to be the first time that halloysite (10 A,) is reported to be the major component of such 1 : 1/2 : 1 interstratified clay minerals in genetic soil horizons.

The occurrence of smectite in other genetic horizons of ash-derived soils has been reported in drier climatic conditions where hydrated halloysite is scarcely stable (Quantin, 1974; Van der Gaast et al., 1986). The present investigation shows that, in humid tropical conditions, minor amounts of smectite occur in the weathering products of basic rocks, but in very close association with hydrated halloysite. The persistence of smectite in soils at an advanced stage of weathering (Ultisols) may be explained by their octahedral composition and/or the presence of interlayer Al-hydroxy materials. Iron-rich smectites are indeed known to be more stable than montmorillonite in acid conditions (Robert & Veneau, 1979) and also from a thermodynamic point of view (Tardy & Garrels, 1974). Besides, Al-hydroxy interlayering reduces their susceptibility to alteration and increases their thermodynamic stability (Karanthasis et al., 1983).

(iii) Finally, emphasis has to be given to the taxonomic implications of the occurrence of H/Sm mixed-layer clay in volcanic ash soils. It has been pointed out that halloysite ( t0 /k ) appears as an earlier product of rock weathering than kaolinite, hence the occurrence of the former mineral is characteristic of a weathering stage that must be distinguished from that of kaolinitic soils (Quantin, 1974). Our results also show that, where halloysite is interstratified with smectite, these two categories of soils may also differ greatly in their surface properties and hence in their management (Delvaux, 1988), It is therefore justified (HerbiUon et al., 1989) that such differences be taken into account in a soil classification


system such as Soil Taxonomy (Soil Survey Staff, 1975) where a major taxonomic criterion relies on the activity (i.e. the CEC) of the clays.

A C K N O W L E D G E M E N T S

The authors thank Dr G. J. Churchman who provided the reference halloysite samples and made many useful suggestions for improving the original manuscripts of these papers.

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