Clay Minerals (1991) 26, 449-461

THE C H E M I C A L C O M P O S I T I O N A N D S T R U C T U R E OF A 14 A I N T E R G R A D I E N T M I N E R A L IN A K O R E A N

U L T I S O L

K. W A D A , Y. K A K U T O , M. A . W I L S O N * AND J. V. H A N N A t

Faculty of Agriculture, Kyushu University 46, Fukuoka 812, Japan, * Division of Coal and Energy Technology, CSIRO, Delhi Road, North Ryde, New South Wales 2113, Australia, and 7" Department of Chemistry, University

of New South Wales, Kensington, NSW 2033, Australia

(Received 21 December 1990; revised 14 May 1991)

A B S T R A C T: An intergradient 14 A mineral showing X-ray diffraction features of "chloritized" vermiculite in a Korean Ultisol was studied. The structural formula of the whole 2 : 1 layer-silicate (14 A mineral and mica) in the Na+-saturated 0.24).5 /~m fraction was obtained by elemental, thermogravimetric and 29Si and 27AI nuclear magnetic resonance spectroscopic analyses in combination with extraction of the interlayer material from the 14 A mineral by hot 1/3 M sodium citrate treatment. This formula showed: (1) the 2 : 1 layer contains nearly one AI 3+ in four tetrahedral positions, and AI 3+ is the dominant cation in the octahedral sheet; (2) K +, exchangeable Na + and sodium citrate extractable 1 : 1 layer occupy the interlayer space of the 14 & mineral in similar proportions. Very little interlayer K was replaced by Na + during the citrate treatment. Possible schemes of alteration of the 2 : 1 layer to the 1 : 1 layer as its interlayer material are discussed.

Many soils formed under leaching condit ions in the humid tempera te region contain a 14 intergradient mineral as a major clay component . It is called partially inter layered or "chlorit ized" vermiculite. "Chlori t ized" vermiculite is commonly identified by X-ray powder diffraction (XRD) analysis: it does not collapse readily when K-satura ted, but its 14 ~ reflection becomes diffuse and shifts toward 10 ~ on heating to 300 ~ and 550~ (Rich, 1968; Barnhisel & Bertsch, 1989). When A1 is removed by t rea tment with reagents such as hot 1/3 M sodium citrate, the mineral collapses on K-saturat ion but does not expand with glycerol solvation.

The following two observations on the 14 ~ mineral in several Korean Ultisols and Alfisols, however, quest ioned this identification: (1) the citrate t reatment which resulted in removal of the inter layer material also resulted in dissolution not only of A1, but of Si at the molar Si/A1 ratio of - 1 - 0 ; (2) the dissolution of the kaolin group mineral was indicated by difference infrared (IR) spectroscopy but not by X R D analysis for the c i t ra te- t reated and untreated samples. On the basis of these observat ions, Wada & Kakuto (1983a,b) suggested that the 14 & mineral is an intergradient vermiculi te-kaolin mineral , in which each 14 & layer part ial ly transforms into double kaolin layers. Later , u l t ramicrotomy and transmission electron microscopy (Wada & Kakuto , 1989) showed that the 14 ~ mineral particles are composed of stackings of more or less curved and often discontinuous silicate layers. On saturat ion with K +, the silicate layers had spacings of 14-10 ~ , with 14-12 being dominant . There was, however, no indication of the double kaolin layers with a spacing of 7 & close to the edge of each 14 & layer as suggested earlier.

In the present study, XRD, chemical, thermogravimetr ic and 29Si and 27A1 nuclear

�9 1991 The Mineralogical Society

450 K. Wada et al.

magnetic resonance (NMR) spectroscopic analyses were carried out for different particle- size clay fractions separated from a Korean Ultisol to calculate the structural formulae of the whole 2:1 layer-silicate including the 14 A mineral and mica, and to discuss the structure of the 14 A mineral on the basis of these formulae.

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

Clay samples

The soil selected is the same as that analysed by Wada & Kakuto (1983a,b; 1989). It is the IIB23t(g) horizon of a Typic Hapludult (Red-Yellow Podzolic soil) derived from old alluvium associated with granite and gneiss. The soil is acid (pH in H20 = 5.40; pH in 1 M KC1 = 3.80), poor in exchangeable bases, and rich in A1 (exchangeable AI = 6-8 mEq/100 g soil). The soil sample was treated with dithionite-citrate (Mehra & Jackson, 1960) for removal of iron oxides and then clay fractions (0.5-2; 0.2~).5 and <0.2 ~m) were separated by repeated dispersion at pH 10 (NaOH) with sonic vibration and sedimentation. The dithionite-citrate treatment prior to the clay separation was necessary to determine the chemical composition of the clay fractions free from iron oxide. It was also effective in preventing the mixing of the fine clay particles in the 0.5-2 and 0.2-0.5 ~tm fractions, as was found in earlier studies (Wada & Kakuto, 1983a, 1989).

Elemental analysis" and thermogravimetric analysis

The elemental composition of Na+-saturated clay fractions was determined by dissolution with aqua regia and HF according to the Bernas (1968) method. However, the dissolution of 100 mg of clay sample was carried out at 50~ for 4 h in a 100 ml polyethylene volumetric flask with an air-tight cap. The amounts of elements extracted were determined by atomic absorption spectroscopy. Weight losses at 200~ and higher temperatures were measured for -200 mg of clay sample by a step-wise heating to estimate the content of strutural OH groups.

Citrate treatment

The clay fractions were treated with 1/3 M sodium citrate at 100~ (Tamura, 1958) to remove interlayer material in a fluoropolypropyrene flask with a reflux condenser (solution/ clay ratio = 1 ml/1-1.25 mg). The treatment time was 16, 8-16 and 4-8 h for the 0.5-2, 0.2- 0-5 and <0.2 ~tm fractions, respectively.

X-ray powder diffraction analysis'

The clays with or without the citrate treatment (--10 rag) were saturated with Mg 2+ and K + and oriented on glass slides. XRD analysis was made after air-drying (25~ glycerol solvation or heating at 300 and 550~ for 1 h (Co-Ko~, 30 kV and 10 mA).

29Si and 27A1 nuclear magnetic spectroscopy

General principles of high-resolution solid state NMR of minerals are given elsewhere (Wilson, 1987). 29Si and 27A1 spectra were obtained on a Bruker MSL 400 Spectrometer

Chemistry and structure of an intergradient clay 451

operating at 79.5 and 104.3 MHz, respectively. Up to 6000 scans were collected in 2 K data points with line-broadening of 50 Hz (Si) and zero Hz (A1). Line-broadening at 200 Hz was used to obtain the 2-pulse TOSS spectra similar to that described by Hemminga & de Jager (1983). The sequence consists of a 90-tau 1-180-tau 2-180 acquire sequence. A recycle time of 60 s was used for 29Si experiments and 1 s for 27A1 experiments, and 90 ~ pulse times were 4 ~s and 3-5/xs, respectively. All experiments were performed with magic angle spinning (MAS). 27A1 spectra were obtained with high power decoupling of protons (dipolar decoupled (DD)). A 7 mm Zirconia rotor was used. Two types of 29Si spectra were obtained. In the first, DD spectra were obtained using a recycle delay of 60 s, which is sufficient to allow 1:1 and 2:1 layer-silicates to relax, but insufficient for quartz to be determined quantitatively (Barnes et al., 1986). In the second, cross polarization techniques were used to distinguish species in proximity to protons, e.g. 2 : I layer-silicates from quartz. A contact time of 5 ms was used in these experiments. Chemical shifts are quoted relative to tetramethylsilane (29Si) or aluminium chloride hexahydrate (27A1).

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

All three clay fractions contain the 14 • mineral, kaolin group minerals (7 A), mica (10 and 5.0 ~) , and gibbsite (4-8 A), and the 0.5-2 /~m fraction also contains quartz (4.25 A_) (Fig. 1). In addition, the 0.5-2 and 0-2-0.5/xm fractions contain small to very small amounts of a regularly interstratified mineral, possibly consisting of mica and the 14 Zk mineral layers in the ratio 1 : 1. When saturated with Mg 2+ or K + and air-dried, this mineral gave basal reflections at 24 and 12 A., although they are not visible in Fig. 1.

As described in the introduction, the 14 A_ mineral did not collapse on K-saturation and air-drying (Fig. 1). Subsequent heating of K-saturated clays resulted in a collapse, but the 10 A reflection showed broadening on the low-angle side even after heating at 550~ Nearly all interlayer material that inhibits the interlayer collapse was removed by the sodium citrate treatment. The 14 ~ mineral remaining after the citrate treatment did not expand with glycerol solvation (not shown in Fig. 1).

Table 1 shows the result of elemental analysis of Na+-saturated clay fractions. The differences in SiO2 and A1203 contents between different particle-size fractions are small, reflecting the quantities of quartz and gibbsite as indicated by the XRD analyis (Fig. 1). The Fe203 and MgO are low compared with A1203, indicating that the 2:1 and 1:1 layer- silicates contain A1 as the major octahedral cation. The analysed clay fractions were subjected to the dithionite-citrate treatment prior to their separation. The dithionite-citrate extraction has been widely used to estimate the content of secondary iron oxides in soils and soil clays (Schwertmann & Taylor, 1989), and has been shown to have little effect on iron layer-silicates (Mehra & Jackson, 1960). Earlier, Wada & Kakuto (1983a) showed that an untreated clay (<2/tLm) fraction contained 10-9% dithionite-citrate extractable Fe203 and that only 0.02% of Fe203 was extracted by subsequent citrate treatment for removal of interlayer material. The 2.33 to 3.00% Fe203 in the dithionite-citrate treated clay fractions (Table 1) was, therefore, considered to be present as Fe in the octahedral sheet of 2 : 1 layer- silicates that remain intact after the treatment. The K20 is highest in the 0.5-2/xm fraction which contains about one-third muscovite, and decreases with decreasing particle size. In contrast, the Na20 increases with decreasing particle size, and is present mostly as exchangeable Na. The Na20 increased and the K20 did not show a corresponding

452 K. Wada et al.

14

f

K-25

K-300

K-550

K-3oo#

K-55o#

0.5-2 0.2-0.5 <0.2 I~ra

4.8 7 i0

i I J !

Ng-25

1 1 i I i

20 I0

14

i

141 7 i0 I IL

4

/

j, I I I I I

20 I0 20 I0

20 ~

FIG. 1. X-ray diffraction patterns of three particle-size fractions; Mg and K indicate Mg 2+- and K § saturation and the following number indicates the temperature at which the sample was dried or heated. # indicates that the sample was pretreated with hot 1/3 r~ sodium citrate. Co-Ko: radiation.

Spacings in / i .

Chemistry and structure of an intergradient clay

TAaLE 1~ Elemental analyses of Na-saturated clay fractions.

453

Particle Wt% 1 size (/all) SiO2 A1203 Fe203 MgO K20 Na20 CaO H20(+) Sum

0-5-2 44.1 34.5 2-33 1.34 3.97 0-71 nd 2 11.1 98.1 0-2.0.5 41.5 36.7 2-61 0.96 2-78 0.88 0.06 13.6 99.1 0-2-0.5# 3 42,2 38-0 2-65 1.09 3,07 1.87 0.03 12.4 101 "3 <0-2 43.0 34"8 3-00 0.92 1.45 1.20 nd 12.9 97.3

I Oven-dry (105~ clay basis. 2 nd = not determined, 3 # = treated with hot 1/3 M sodium citrate.

d e c r e a s e in t h e 0 . 2 - 0 . 5 /~m f r a c t i o n a f t e r c i t r a t e t r e a t m e n t , i n d i c a t i n g t h e i n c r e a s e in

e x c h a n g e a b l e N a o n r e m o v a l o f i n t e r l a y e r m a t e r i a l o t h e r t h a n K § in t h e 2 : 1 l aye r - s i l i c a t e s .

T a b l e 2 s h o w s t h e r e su l t o f t h e w e i g h t loss d e t e r m i n a t i o n s o n h e a t i n g N a + - s a t u r a t e d c lay

f r a c t i o n s . T h e w e i g h t loss a t 550~ i n c r e a s e d a n d t h a t a t > 5 5 0 ~ d e c r e a s e d w i t h d e c r e a s e in

TABLE 2. Weight loss of Na-saturated clay fractions on step-wise heating at different temperatures.

Particle Weight loss (%)z at size (/am) 200 300 400 550 > 5 5 0 ~ 2

0-5-2 1.3 2.6 0.9 3.9 2.4 0.2.0-5 1.3 3.3 1.5 5.5 2.0 0.2.0-5# 3 0-7 3.1 0-95 5-0 2.6 <0.2 0-75 1-9 1.3 7.2 1-7

1 Oven-dry (105~ clay basis. 2 Weight loss at >550~ = loss on ignition - weight loss at 550~ 3 # = treated with hot 1/3 M sodium citrate.

TABLE 3. Allocation of Si and A1 to minerals 1 in clay fractions according to elemental analysis and weight loss on heating.

Si Al Particle allocated to allocated to

size Kn Qz 2 : 1 Kn Gb (/~m) (mmol/100 g) (mmol/100 g)

2:1

0.5-2 266 468 266 96 314 0.2.0.5 389 0 301 389 122 209 042-0.5# 2 331 0 371 331 115 299 <0-2 473 0 241 473 70 139

1 Kn = kaolin group mineral; Qz = quartz; 2 : 1 = 2 : 1 layer-silicates; Gb = gibbsite.

2 # = treated with hot 1/3 M sodium citrate.

454 K. Wada et al.

29Si DDMAS 27AI 2-Pulse TOSS

citrate citrate

untreated treated untreated treated

I 1 I I t I I I [ I I

-60 -80 -i00 -120-60 -80 -i00 -120 i00 0 -i00 I00 0 -i00

ppm ppm

FIG. 2, ~Si DDIVlAS and 27A1 2-pulse TOSS spectra of the 0.5-2 pm fraction (a) without and (b) with hot 1/3 M sodium citrate treatment.

particle-size, which may indicate an increase in kaolin group minerals and a decrease in 2 : 1 layer-silicates. The decrease in the weight loss of the 0-2q9.5/~m fraction at 400 and 550~ after the citrate treatment was in agreement with dissolution of a kaolin group mineral indicated by the analyses of dissolved Si and A1 and the difference IR spectroscopy for the citrate-treated and untreated clay samples (Wada & Kakuto, 1983a,b).

Table 3 shows the result of allocation of Si and AI to the kaolin group mineral, gibbsite, and 2 : 1 layer-silicates based on the data in Tables 1 and 2. In this allocation, the weight losses at 400 and 550~ and that at 300~ were assumed to be from the loss of structural O i l groups of the kaolin group mineral and gibbsite, respectively. The Si and/or AI allocated to the two minerals were then calculated based on the formulae of kaolinite, Si2A12Os(OH)4, and gibbsite, AI(OH)3. The remaining Si and A1 were allocated to 2 : 1 layer-silicates plus quartz in the 0.5-2/~tm fraction, and 2 : 1 layer-silicates in the 0-2~).5 and <0.2 ~tm fractions.

Fig. 2 shows 29Si D D M A S and 27A1 2-pulse TOSS NMR spectra of the 0.5-2/~m fraction. The 29Si r e s o n a n c e s at -91 .5 p.p.m, and - 8 7 . 2 p.p.m, were found for all clay fractions. Since the four-fold coordination of AI in the Si tetrahedral sheet of 2 : 1 layer-silicates causes a downfield shift of up to 5 p.p.m., the former resonance was attributed to the Si in 2 :1 layer-silicates with a molar Si/(Si + Ahv) ratio ->0.8 and/or kaolin group minerals, and the latter to the Si in the 2 : 1 layer-silicates with Si/(Si +Al Iv ) molar ratio = 0-75, e.g. muscovite (Kinsey et al., 1985; Wilson, 1987), where Ahv represents A1 in four-fold coordination. The resonance at -107-5 p.p.m, was found only for the 0.5-2 #m fraction and was assigned to quartz. These assignments were confirmed by cross polarization experiments.

The 29Si resonance at -87 .2 p.p.m, relative to that at -91-5 p.p.m, increased in intensity

Chemistry and structure of an intergradient clay 455

after the citrate t reatment . The 27A1 D D M A S spectra (not shown) are not useful for detecting te t rahedral AI in 2 : i layer-silicates due to prominent s idebands of the octahedral A1 resonance at 0 p .p .m. The 2-pulse TOSS technique (Hemminga & de Jager, 1983) has been developed for spinning sideband elimination from quadrupole nuclei such as 27A1 which have short spin-spin relaxation times. This is more useful than simply changing its spinning speed as it allows complete resolution of te t rahedral and octahedral A1. Al though 27A1 N M R may be non-quanti tat ive, it may be useful for comparat ive purposes. The 27A1 2- pulse TOSS spectra clearly showed that the 67 p.p.m, te t rahedral A1 resonance increases in intensity relative to the 0 p .p .m, octahedral A1 resonance due to the citrate t rea tment (Fig. 2). Al though the 29Si D D M A S and 2-pulse TOSS 27A1 spectra are not shown for the 0.2-0-5 and <0.2 gm fractions, similar differences were found between the samples t rea ted and not t reated with citrate. This indicates preferential dissolution of the 2 : 1 layer-silicates with a molar Si/(Si + Ahv) ratio ->0.8 and/or a kaolin group mineral by the citrate t reatment , but most probably the lat ter when the results of the other analyses are taken into consideration.

In principle, 298i N M R can be made quanti tat ive, although in practice the long spin lattice relaxation times of quartz makes this impossible for mixtures containing this mineral . However , it should be possible to distinguish the amounts of Si in te t rahedral sheets with A1 substitution from those without, and to make comparat ive studies on the mixtures containing quartz. Table 4 shows the allocation of Si to different mineral species according to the intensity (peak height) of the 29Si D D M A S resonances. The Si al located to the 2 : 1 layer-silicates with Si/(Si + Ahv) molar ratio = 0.75 in each particle-size fraction increases with its K20 content (Table 1). Also, in each fraction, the es t imated K in these 2 : 1 layer- silicates based on this Si and the muscovite composit ion of KSi3A13OI0(OH)2 (Table 4) was also comparable to the K allocated to 2 : 1 layer-silicates based on the elemental analysis (Table 5). The Si al located to the kaolin group mineral and the 2 : 1 layer-silicates with Si/(Si + Ahv) molar ratio ->0.8 (Table 4) was higher than that al located to the kaolin group mineral based on the weight losses at 400 and 550~ (Table 3). The difference gave an estimate of the Si in the 2 :1 layer associated with inter layer mater ial o ther than K + (Table 4).

Table 5 shows an allocation of elements to the 2 :1 layer-silicates including the 14 mineral and mica present in each clay fraction. The Si and A1 allocations were based on the data shown in Tables 3 and 4, and all Fe, Mg, K and Na (Table 1) were assumed to be

TABLE 4. Allocation of Si and K to mineralslin clay fractions according to elemental analysis and 29SiDDMAS spectroscopy.

29Si resonance Si in K in Particle peak height at

size -87 -92 -107 Qz 2:1' Kn + 2:1' 2:1"* 2:1' (/~m) p.p.m. (mmol/100 g clay)

0-5-2 79 137 37 110 228 396 130 76 0.24).5 58 142 0 0 200 490 101 67 0.24).5# 2 75 143 0 0 239 463 132 80 <0.2 23 144 0 0 100 616 143 33

1 Qz = quartz; Kn = kaolin group mineral; 2 : 1" = 2:1 layer-silicates with Si/(Si + Ahv) molar ratio = 0-75; 2 : 1"* = 2 : 1 layer-silicates with Si/(Si + Ahv) molar ratio/>0.8.

2 # = treated with hot 1/3 M sodium citrate.

456 K. Wada et al.

TABLE 5. Allocation of elements to 2 : 1 layer-silicates in Na-saturated clay fractions.

Particle size Si AI Fe Mg (#m) (mmol/100 g clay)

K Na H

0.5-2 358 314 29 33 84 23 266 0.2-0-5 301 209 32 24 59 28 222 0-2-0.5# 1 371 299 33 27 65 60 289 <0-2 243 139 38 23 31 39 189

1 # = treated with hot 1/3 M sodium citrate. i

present in the 2 : 1 layer-silicates. The amount of H was calculated from the weight loss at >550~ (Table 2).

Table 6 shows the structural formula of the whole 2 : 1 layer-silicate in each clay fraction calculated per O10(OH)2 unit. The calculated structural formulae show that the chemical composition of the 2 : 1 layer-silicates (Table 5) can be explained using idealized 2 : 1 layer structural concepts. There is no particular anomaly due to the presence of interlayer material that inhibits the interlayer collapse of the 14 A mineral, even although it was not removed from these samples (except for the citrate-treated 0.2-0-5 ~m fraction). As mentioned above, the material dissolved by the citrate treatment had the composition of the kaolin group mineral. Thus, the result may be taken as indicating that the effect of the interlayered kaolin group mineral was removed by calculating which parts of the analysed Si and A1 are allocated to the kaolin group mineral according to the weight losses at 400 and 550~

The structural formulae of the whole 2 : 1 layer-silicates (Table 6) are similar to those obtained by Barshad & Kishk (1969) for Na-saturated vermiculites associated with mica in five soils, although the analytical procedures and the assumptions used in the calculations are not the same. In the 0-5-2 and 0.2-0.5 ~tm fractions there is nearly one A13+ in four tetrahedral positions, and A13+ is the dominant cation in the octahedral sheet. The negative layer charge originates largely from a deficiency of positive charge in the tetrahedral sheet. These facts indicate that the whole 2 : 1 layer-silicate in each particle-size fraction is derived from muscovite.

Kirkland & Hajek (1972) analysed clay fractions separated from highly weathered Coastal Plain soils (Paleudults) in Alabama. They did not succeed in calculating structural

TABLE 6. Structural formula per O10(OH)2 unit of the whole 2 : 1 layer-silicate in Na-saturated clay fractions.

Tetrahedral Octahedral Interlayer Particle

size Net Net (#m) Si A1 charge AI Fe 3+ Mg H charge K Na Charge

0.5-2 2-97 1-03 -1.03 1-56 0.24 0.27 0-20 +0-14 0-69 0-18 +0.87 0.2--0-5 3-16 0.84 -0-84 1-35 0.34 0.25 0.33 -0.10 0-62 0.29 +0.91 0.2-0.5# ~ 3-02 0.98 -0.98 1-45 0.27 0.22 0.35 -0-05 0.53 0-49 +1.02 <0.2 3-22 0.78 -0.79 1-06 0.51 0.31 0.52 -0.15 0.41 0.52 +0-93

1 # = treated with hot 1/3 M sodium citrate.

Chemistry and structure of an intergradient clay 457

TABLE 7. Structural formula of the whole 2:1 layer-silicate in Na-saturated clay fractions according to the Kirkland & Hajek (1972) method.

Particle Tetrahedral Octahedral Interlayer size (~m) Si AI AI Fe Mg OH O K Na A1 OH OH/AI

0.5-2 3-03 0.97 1.48 0-24 0.28 2 10 0.71 0.19 0.19 0.24 1.26 0.2-0.5 3-27 0.73 1.40 0-34 0.26 2 10 0.64 0.30 0.14 0.41 2.92 0.2-0.5# 1 3.13 0.87 1.49 0-28 0-23 2 10 0.55 0.51 0.16 0.44 2.75 <0.2 3.40 0.60 1.15 0.53 0-32 2 10 0.43 0.55 0-21 0.64 3-05

1 # = treated with hot 1/3 M sodium citrate,

formulae of "Al- in ter layered" vermiculi tes in these clays using idealized 2 : 1 or 2 : 2 layer structural concepts, but modified the calculation procedure and calculated the formulae including inter layer material . A n average structural formula thus obta ined is

(Si3.24A10.76)(All.56Fe0.24Mg0.20)O10(OH)2Ex.Xo.41 +0"41 (All.45(OH)3.79) +0"46,

where Ex .X stands for monovalent exchangeable cations. Table 7 shows the structural formulae of the whole 2 : 1 layer-silicates in each particle-size

fraction calculated according to the Kirkland & Hajek (1972) method. These formulae, except for the 2 : 1 layer-silicates in the <0-2/~m fraction, in which the OH/A1 molar ratio exceeds 3.0, seem to indicate that they can also be "Al- in te r layered" vermiculites. However , the amount of inter layer A1 is small, and the inter layer A1 in the 0-2-0.5 /~m fractions t rea ted and not t rea ted with citrate are similar in amount . This demonst ra tes that the small amount of interlayer A1 calculated according to the Kirkland & Hajek (1972) method does not necessarily indicate its presence.

The structural formulae of the whole 2 :1 layer-silicates in the c i t ra te- t reated and untreated 0-2-0.5/~m fractions show the increase in inter layer Na by the citrate t rea tment (Table 6). Af ter the t reatment , the sum of inter layer K and Na becomes equal to the value of interlayer K in muscovite. Assuming the dissolution of the kaolin group mineral by the citrate t rea tment , as indicated by the different analyses, the amount of Si or Al extracted from the 0.2-0-5/~m fraction was calculated to be 101 mmol/100 g based on the amounts of Si and A1 al located to the kaolin group mineral in the 0.2-0.5/~m fractions before and after the citrate t rea tment (Table 3). Thus, the dissolved mater ia l is about one-fourth of the kaolin group mineral present before the citrate t reatment . Separa te determinat ions showed that the inter layer collapse was completed by extract ion of 94 to 111, and 88 to 110 mmol of A1 and Si/100 g clay, respectively, and that 2.0 out of 59 mmol of K and 1.2 out of 24 mmol of Mg were extracted with 100 mmol of A1/100 g clay by the citrate t rea tment . These results confirm the preferential dissolution of the kaolin group mineral .

On the other hand, Table 8 shows that the dissolution of the discrete kaolin group mineral by the citrate t rea tment was not evident in the X R D pat terns of Mg-25~ samples; the 7 A reflection did not decrease in intensity relative to the 10 ,~ reflection in the c i t ra te- t reated samples. A n exception was the dissolution of halloysite (10 A) from the <0.2 ~m fraction. We may, therefore, consider that the dissolved Si and A1 are mostly present as 1 : 1 layers associated with the 2 :1 layers of the 14 ,~ mineral , possibly as posit ively charged

458 K. Wada et al.

" inter layer material" . Then the structural formula of the whole 2 : 1 layer-silicate including interlayer mater ial in the 0.2-0-5/~m fraction can be written:

(Si3.02A10.9s) ~ ~176 (K0.53Na0.25) +~ 1-64)(H20)0.26 +0"25

where the amount of interlayer K is taken to be the same before and after the citrate t reatment . According to this formula, the disposit ion of interlayer K and Na in the inter layer space of the whole 2 : 1 layer-silicate, the propor t ion of the dissolved 1 : 1 layer to the 2 : 1 layer, and the increase in interlayer Na by dissolution of the 1 : 1 interlayer are shown schematically in Fig. 3.

The propor t ion of mica in the whole 2 :1 layer-sil icate in the 0.2-0.5 /~m fraction as indicated by the presence of interlayer K in Fig. 3 does not seem compatible with the X R D intensity of mica in Fig. 1; the 10 A reflection was very weak for the Mg-25~ sample compared with the K-550~ sample. The propor t ion of mica in the whole 2 : 1 layer-silicate was further es t imated from the intensities of the 10 A reflections from the ci t rate- treated, Mg-25~ and K-550~ samples (Table 8). The propor t ion of mica thus est imated, 20% of the whole 2 : 1 layer-silicate, is much less than 50% as indicated by the presence of interlayer K in Fig. 3. Only a small part of this difference could be explained by the presence of the regularly interstratified mineral consisting of mica and the 14 A mineral layers that give the 12 A reflection when saturated with Mg 2+ and air-dried (Table 8). i f this interstratified mineral is ignored, we may infer that 37.5% of the negative charge sites in the interlayer space of the 14 A mineral is occupied by K + and that the remaining 62.5% is equally occupied by exchangeable cations and the inter layer material . The lat ter keeps the interlayer open and maintains the 14 A. basal spacing when the mineral is saturated either with Na +, Mg 2+ or K + and air-dried. Similarly, the presence of interlayer K in the 14 A mineral can also be inferred for the 0.5-2/~m fraction (Tables 6 & 8).

Possible schemes for incorporat ing the 1:1 layer as the interlayer material which are

TABLE 8. Basal reflection intensities (peak heights) of mica, 14/~ mineral and their interstratification normalized against 7 A reflection of kaolin group mineral in clay fractions.

Pretreatment for XRD analysis

Mg-25~ K-25~ K-550~

Particle Reflection intensity at size (~m) 7 A 10 A 12/~ 14 ~ 7 A 10 ~ 10 A

0.24).5 50 24 (20) l 136 5(1 40 74 0-24).5# 2 50 22 (24) 235 50 75 109 0.5-2 46 44 (22) 81 46 50 72 0.5-2# 46 38 (30) 200 46 70 108 <0.2 68 433 -- 43 68 61 62 <0.2# 68 20 -- 80 68 73 89

1 Shoulder of the 14/k reflection. 2 # = treated with hot 1/3 M sodium citrate. 3 Includes the 10 A reflection from halloysite.

Chemistry and structure of an intergradient clay 459

- I/J T 2si,A1,

2 Si 1 :1 l a y e r e x t r a c t e d

Fro. 3. Schematic drawing of the interlayer space of the whole 2 : 1 layer-silicate and the proportion of the citrate-extractable 1 : 1 layer to the 2 : 1 layer in the 0.24/.5/ml fraction.

m 2 AI(Fe, Mg)

Na + | Na + (after citrate treatment)

~-~ 2 A1

compatible with the structural formulae before and after the citrate treatment are illustrated in Fig. 4. All the schemes include the inversion of Si tetrahedra stripped from the 2 : 1 layer and assume the formation of four unit-cells of i : i layer from two unit-cells of 2 : 1 layer (muscovite). The latter exceeds, however, what is expected from the stoichiometric equation:

2(K2Si6A12A14020(OH)4) + 4H + + 6H20 = 3(Si4A14010(OH)s) + 4K +

and further study is necessary on the chemistry of this transformation. The occurrence of the 1 : 1 layer as shown in these schemes would, however, explain the inhibition of interlayer collapse by saturation with K § if such 1 : 1 layers occur in every interlayer space. They can occur at any spot and on any scale, provided the proportion of the 1 : 1 layer to the 2 : 1 layer is not altered much and the two-dimensional continuity of the 2 : 1 layer is maintained even after removal of the 1 : 1 layer.

An inversion of Si-tetrahedra similar to scheme (a) in Fig. 4 was also suggested by Karathanasis & Hajek (1983) and Matsue & Wada (1988) to explain the 2 : 1 to 1 : 1 layer- silicate transformation and the structure of partially interlayered vermiculite, respectively. This scheme (a) cannot, however, explain the proportion of the 1 : 1 interlayer to the 2 : 1 layer, and the increase of interlayer Na by the dissolution of the 1 : 1 layer. The schemes (b) and (c) can explain these features, which are different from the scheme (a) in the disposition of the 1 : 1 layer derived from the 2 : 1 layer.

As described above, there was an increase in exchangeable Na but very little K replacement from the 2 :1 layer-silicates in association with removal of the interlayer material from the 14 A mineral by the citrate treatment. There was also no change in the 10 ,~ reflection intensity, but the 14 ~ reflection showed a remarkable increase in intensity when saturated with Mg 2+ and air-dried (Table 8). Thus, the K + remaining in the interlayer space gave no indication of the interlayer collapse due to its presence even after removal of the "interlayer" 1 : 1 layer. On the basis of these observations, it is inferred that the K + ions in the 14 ~ mineral are trapped in the holes of hexagonal, or more exactly smaller trigonal, oxygen rings of the tetrahedral sheet, so that they are not replaced by incoming Na + or

460 K. Wada et al.

- -1 14

14

--1 14

14

----~

q . . . .

Fro, 4. Possible schemes of incorporation of the 1 : 1 layer as the interlayer material in the 14 mineral. See Fig, 3 for legends.

Mg 2+. In Fig. 4, the K + are thus placed close to the base of Si(A1) oxygen tetrahedra, while the Na + are placed in the middle of the interlayer space.

C O N C L U S I O N S

The 14 ~ mineral showing XRD features of "chloritized" vermiculite in the Korean Ultisol studied is derived from muscovite and probably represents an intermediate phase during transformation of muscovite to vermiculite and kaolin group minerals in acid soils. This intergradient mineral contains non-exchangeat~le K +, exchangeable cations including AI 3+, and a citrate-extractable 1 : 1 layer in the interlayer space.

Chemis t ry a n d s tructure o f an in tergradient clay 461

R E F E R E N C E S

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