Clay Minerals (1986) 21, 85-92

E L E C T R O N D I F F R A C T I O N A N D T H E S T U D Y OF F E R R I H Y D R I T E C O A T I N G S ON K A O L I N I T E

A N G E L A A. J O N E S AND A. M. S A L E H

Department of Soil Science, University of Reading, Reading, Berkshire RG1 5A Q

(Received 9 September 1985; revised 1 November 1985)

A B S T R A C T : Selected-area electron diffraction has been used to examine ferrihydrite coatings on kaolinite crystals and is shown to provide a sensitive means of detection. It also gives better diffraction patterns of ferrihydrite than does XRD and the patterns sometimes give additional indications of the crystal size. It is suggested that the technique may be useful in examining other coatings on soils and clays.

Elucidating the nature of poorly crystalline ferric oxides and ferrihydrite, and detecting their presence amongst other soil clay minerals, is notoriously difficult (Brown, 1980) because the XRD pattern of even a synthetic ferrihydrite is characterized by only five or six broad maxima (or peaks), the equivalent spacings of which, at 2.54, 2.24, 1.98, 1.73, 1-51 and 1.47 A, broadly coincide with common spacings of phyllosilicates and other iron oxides and hydroxides. To meet this problem, differential X-ray diffraction (DXRD) methods have been developed which depend on examining the effect of the removal of ferrihydrite and its broad diffraction peaks from the total diffraction pattern of a soil clay by ammonium oxalate treatment (Schulze, 1981; Schwertmann et al., 1982; Brown & Wood, 1985). Even this elegant method may be relatively insensitive in a situation where small amounts of ferrihydrite or other iron oxide/oxyhydroxide minerals affect the surfaces and surface behaviour of other soil minerals. Other means of determining ferrihydrite such as by IR spectroscopy (Russell, 1979; Schwertmann & Fischer, 1973), M6ssbauer spectroscopy (Murad & Schwertmann 1980; Childs & Johnston, 1980; Schwertmann et al., 1982; Childs et al., 1984) and DTA (Towe & Bradley 1967; Chukhrov et al., 1972; Jackson & Keller, 1970; Schwertmann & Fischer, 1973) have particular applications. However, the definitive chemical determination of ferrihydrite by dissolution in ammonium oxalate (Schwertmann, 1964) remains the most sensitive, and often the only practical, method of establishing the presence of ferrihydrite in a mixture of soil minerals. This paper presents the results of an investigation of small amounts of ferrihydrite in association with kaolinite (Saleh & Jones, 1984) where the use of selected-area electron diffraction in association with electron microscopy has confirmed the presence of thin layers of ferrihydrite on the surfaces of kaolinite crystals and provided better diffraction data on ferrihydrite than has been described previously.

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

Synthetic ferrihydrite was produced by the method of Towe & Bradley (1967). 10 g Fe(NO3) 3. 9H20 was hydrolysed in 1 litre distilled water at 70~ the suspension was then

�9 1986 The Mineralogical Society

86 A. A. Jones and A. M. Saleh

dialysed against distilled water until there was no further change of pH. The ferrihydrite was collected and freeze-dried. Si-ferrihydrite containing 0-08 Si/Fe was prepared by introducing an appropriate concentration of sodium silicate to the hydrolysed ferric nitrate. This was dialysed and freeze-dried in the same way as the pure ferrihydrite. Natural ferrihydrite from a cold spring in Finland (Carlson & Schwertmann, 1981) was supplied by Professor U. Schwertmann. Kaolinites from Pugu, Tanzania (Robertson et al, 1954), and Georgia, USA were fractionated and the <2 /~m fraction washed to remove all exchangeable ions and then dried at 70~ The kaolinites were known to have a small permanent charge on their basal surfaces to which, in mixtures at low pH, ferrihydrite is attracted (Saleh & Jones, 1984). The kaolinites were mixed with 5% synthetic ferrihydrite at pH 3 and the mixture shaken for 24 h. Subsequently, the pH of mixtures in suspension was altered by additions of 0.1 M NaOH to a pH slightly greater than 6 to allow observation of the effect of change of pH on the interaction of the two materials (Saleh & Jones, 1984).

Suspended material was placed on grids coated with cellulose nitrate for examination in a Hitachi H800 electron microscope in both microscopic and diffraction modes. Preparations were examined in microscopic mode at 200 kV and at magnifications of up to 200 000; electron diffraction patterns were obtained at 200 kV and at a magnification of 30 000. The diffraction patterns were calibrated with the diffraction pattern produced by a 10 nm thick preparation of gold in exactly similar conditions of diffraction. XRD traces of back-loaded powder mounts were obtained using monochromatic Cu or Co radiation at a scanning rate of 1 ~ Peak width at half-height of the most intense, 110, diffraction peak was used as a measure of broadening (WHH).

R E S U L T S

Electron micrographs revealed that the particles of ferrihydrite were fairly uniformly sized, irregular spheres of 4 to 5 nm diameter (Fig. la). Electron diffraction of the pure ferrihydrite produced a sharp pattern with as many as 11 maxima (Fig. Ib; Table I). The geometrical limitations of the microscope meant that the selected area for diffraction (~450 nm diameter in the conditions of use) was much larger than the size of an individual ferrihydrite crystal and it was therefore impossible to obtain a diffraction pattern from a single crystal. The electron diffraction pattern obtained from the synthetic ferrihydrite of nearly continuous rings containing only a few spots was consistent with a pattern from many crystals of very small particle size. The patterns produced by natural ferrihydrite (Fig. 1 d; Table 1) and by the Sbferrihydrite (Fig. l f, Table 1) are of significantly smoother, continuous rings with few or no spots, indicating that these materials bad an even smaller particle size than the pure ferrihydrite. Electron micrographs appear to confirm this (Fig. I c and e), although estimating the size of the particles from the micrographs is difficult.

XRD traces of the pure ferrihydrites consisted of a pattern of six broad maxima with WHH = 5.0~ (Fig. 2a). When Si was incorporated into the ferrihydrite, at a concentration of 0.08 Si/Fe, the pattern deteriorated to the extent that the less intense maxima, (113) and (114), were barely discernible and WHH = 7.4~ (Fig. 2b). The pattern from natural ferrihydrite was even less distinct and in this WHH = 10.0~ (Fig. 2c).

Electron micrographs of the kaolinite showed that they consisted of well-formed, hexagonal crystals (Figs 3 and 4); Pugu D kaolinite crystals had the smaller cross-section

Electron diffraction of ferrihydrite and kaolinite 87

FI6. 1. Transmission electron micrographs (left) and electron diffraction patterns (right) of: (a, b) pure synthetic ferrihydrite; (c, d) natural ferrihydrite; (e, f) Si-ferrihydrite (0.08 Si/Fe).

Arrows in (a), (c) and (e) indicate the centre of the selected area for electron diffraction.

of < 0 . 5 / a m while the Geo rg i a kaol ini te cons is ted of c rys ta l s < 1 /am across . I t was poss ible to determine f rom crys ta ls or iented on edge that the Pugu and Geo rg i a kaol ini te c rys ta l s were about 15 and 80 nm thick, respect ively. Elect ron diffract ion pa t te rns were readi ly

88 A. A. Jones and A. M. Saleh

TABLE 1. Electron diffraction spacings of ferrihydrites.

Observed

Pure Calculated synthetic 0.08 Si/Fe Natural

hkl d(A) d(h) d(A) d(A) 110 2,532 2.54 2.54 2,56 111 2.444 2.46 - - - - 112 2.229 2.24 2.24 2.24 113 1.969 1.97 1.97 1.98 114 1.722 1.74 - - - - 115 1,510 1.51 - - - - 300 1.460 1,49 1.49 1.49 116 1.332

- - - - 1.28 204 1.241 117 1.186 1.18 1.18 1.18 306 1.069

1.06 1.05 - - 118 1.066 119 0.965 0.97 - - - - 11,10 0.881 - - 0.89 0.89 00,11 0.854

330 0.844 0.85 0.85

300 115 1 1.491 .53 114 113 2.

110 2.54 ~ W H H

5 . 0

7 . 4

10.0

"20 6'0 5'0 4'0 3'0 2'0

FIG. 2. X-ray diffraction patterns of (a) pure, synthetic ferrihydrite, (b) Si-ferrihydrite (0.08 Si/Fe), (c) natural ferrihydrite.

o b t a i n e d f rom single kao l in i te c rys ta l s , t hese showing the cha rac t e r i s t i c h e x a g o n a l a r r a y o f

spo ts revea l ing the o r i e n t a t i o n o f the r ec ip roca l la t t ice (Fig. 4b). The c rys ta l s of ten over-

l apped or the o r i en t a t i on o f a c rys ta l was such t h a t the d i f f rac t ion p a t t e r n s h o w e d a

n u m b e r of spots f rom wh ich layer lines were on ly d i sce rn ib le wi th difficulty (Fig. 3b).

Electron diffraction of ferrihydrite and kaolinite 89

FIG. 3. Transmission electron micrographs and electron diffraction patterns of Pugu D kaolinite: (a, b) without addition; (c, d) with 5% ferrihydrite at pH 3; (e, f) with 5% ferrihydrite

at pH 6.5.

In certain conditions the basal surfaces of kaolinite crystals are known to acquire a coating of iron oxide (Greenland & Wilkinson, 1969) and, particularly, of ferrihydrite (Saleh & Jones, 1984). Mixtures of the kaolinites and 5% pure ferrihydrite at pH 3 gave electron micrographs showing such a coating (Figs 3c, 4c). The presence and the identity

90 A. A. Jones and A. M. Saleh

Fro. 4, Transmission electron micrographs and electron diffraction patterns of Georgia kaolinite: (a, b) without addition; (c, d) with 5% ferrihydrite at pH 3.

of this coating was strikingly confirmed by the electron diffraction patterns from the coated kaolinite crystals: frequently the spots from the kaolinite crystal were perfectly distinct from the rings produced by the ferrihydrite (Figs 3d and 4d). When the attraction between the surfaces of the two minerals was weakened by raising the pH, the effect could be seen in both electron micrographs and by a change in the typical electron diffraction patterns. Thus, at a pH slightly greater than 6, the coating of ferrihydrite is seen in the micrographs to be separating from the surface of the kaolinite (Fig. 3e) and electron diffraction patterns show only spotty rings of ferrihydrite, along with those from the kaolinite (Fig. 3f). Electron diffraction from any uncoated kaolinite surfaces showed no ferrihydrite pattern. On returning the mineral mixture to pH3 again, ferrihydrite coated the kaolinite surfaces as before and the smooth rings of the ferrihydrite pattern reappeared in electron diffraction patterns of the kaolinite crystals.

D I S C U S S I O N

It is generally considered that electron diffraction data provides no more, and often less, information about crystalline material than does X-ray diffraction data (Gard, 1971).

Electron diffraction of ferrihydrite and kaolinite 91

However, with all the samples of ferrihydrite examined in the present work electron powder diffraction gave better results than X-ray powder diffraction. This is in accord with observations by Chukhrov et al. (1972) that selected-area electron diffraction was the most effective tool in examining ferrihydrite. A typical X-ray diffraction trace of pure, synthetic ferrihydrite (Fig. 2a) shows not more than six broad maxima, the exact positions of which can be determined only to within 1~ at 20 angles >70 ~ the peaks broaden and it is impossible to distinguish them even at low scanning speeds and with increased slit widths. However, electron diffraction patterns from the ferrihydrites allow measurements, on a negative or print, of 11 diffractions with 0.1% accuracy (Andrews et al., 1971; Gard, 1971) (Table 1). These can be indexed on the basis of the rhombohedral cell with a 0 = 5.08

and c 0 = 9.4/~ proposed by Towe & Bradley (1967), and indicate that the form {1 ll} dominates the pattern even more than in Towe & Bradley's ferrihydrite. The greater accuracy of measurement of the d-spacings by electron diffraction than by X-ray diffraction is due in part to the lower angles at which the electron diffractions occur and probabiy also to the electron beam being diffracted from a much smaller area, or volume, of material than the X-ray beam. With natural ferrihydrite, where the particle size is particularly small, only two broad X-ray diffractions can be measured, at 2.5 and 1.5 A (Fig. 2c), and the breadth of these diffractions allows their positions to be determined only to within 1 or 2~ at best. The electron diffraction pattern of this material, however, shows five diffractions, the positions of which can be determined to within 0.1% (Table 1). The smooth, continuous electron diffraction rings produced by the natural ferrihydrite and by the Si-ferrihydrite strongly suggest that in these the crystal size is smaller than in the synthetic, pure ferrihydrite, i.e. <4 nm diameter.

In using electron diffraction to examine the ferrihydrite coating on kaolinite crystals it has been shown here that the technique is useful both in detecting a thin coating and in identifying its crystalline phase. This capability has almost certainly been recognized previously but seems to have been neglected as ameans of examining coatings in soils and clays. Although electron diffraction can only respond to the presence of crystalline material, it is much more sensitive to small amounts of such material than is X-ray diffraction so may in some instances be able to indicate crystalline components in coatings previously considered to be amorphous. Furthermore, the small crystal size which is frequently characteristic of X-ray amorphous materials does not interfere with the production of electron powder diffraction patterns. As well as confirming and amplifying the evidence of electron microscopy and X-ray powder diffraction, electron diffraction may provide information about the number and orientation of the crystals of the coating material. Thus, it is estimated that the ferrihydrite coating which produced the pattern of smooth diffraction rings along with the pattern from a kaolinite crystal was less than 40 nm thick (Saleh & Jones, 1984); when this coating thinned the pattern altered and the ferrihydrite diffraction rings became spotty.

ACKNOWLEDGMENTS

We are indebted to Dr J. R. Barnett of the Electron Microscope Unit, Botany Department, University of Reading, for his painstaking help and interest. We are also grateful to Mr R. H. S. Robertson for the sample of Pugu kaolinite and Professor U. Schwertmann for the natural ferrihydrite.

92 A . A . J o n e s a n d A. M . S a l e h

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