THE CHARACTERIZATION BY MÖSSBAUER SPECTROSCOPY OF THE SECONDARY IRON IN PANS FORMED IN SCOTTISH PODZOLIC SOILS

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THE CHARACTERIZATION BY MÖSSBAUER

SPECTROSCOPY OF THE SECONDARY IRON IN

PANS FORMED IN SCOTTISH PODZOLIC SOILS

B. Goodman, M. Berrow

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppltment au no 12, Tome 37, Dtcembre 1976, page C6-849

%HE CHGRACCERIZATION

BY

MOSSBAUER SP@CTROSCOPY

QP

THE

SECONDARY I R ~ N

IN

PANS FORMED

IN

SCOTTISH PODZOLIC SOILS

B. A. GOODMAN and M. L. BERROW

The ~ a c a u l a ~ Institute for Soil Research, Craigiebuckler, Aberdeen AB9 245, Scotland

R6sum6. - On a Btudie douze alios ferrugineux bien developpes des sols d'origines differentes par la methode spectroscopique Mossbauer a une temperature de 77 K et h la temperature de salle.

Onze aes echantillons provenaient des profils podzoliques sous l'horizon A2 et l'autre provenait d'un gley de nappe sous l'horizon B2. A une tempkature de 77 K ce dernier a donne un spectre compose d'une structure magnetique bien determink aux paramhtres semblables

a

ceux de a-FeOOH ; par contre a la temperature de salle on a obtenu un spectre compose d'une structure magnetique en partie desagregke. Les autres ont produit h une temperature de 77 K comme ala

temperature de salle, seulement un doublet large qui ne correspondait pas d'une mani6re satls- faisante aux pointes de Lorentz. En deux cas il y avait des correspondances acceptables mais des spectres supplkmentaires resultants d'etudes aux temperatures variables (4,2 h 50 K) indiquaient une structure hyperfine. Les pointes Btaient larges meme

a

une temperature de 5 K, indiquant qu'il y avait une dispersion de champs magnetiques dans lY8chantillon. Les valeurs obtenues pour ce champ magnetique ktaient telles qu'elles rendent possible la presence de tous les differents oxyhydroxydes de fer, mais on pouvait aussi dire que ce spectre representait un genre unique, a-FeOOH, compose d'une skrie de particules microscristallines de tailles differentes.

On propose une structure qui tient compte de l'accumulation du fer avec la matikre organique aux groupes carboxyliques nombreuses et la combinaison qui suit avec la surface des particules de FeOOH. En ce cas la matihre organique fait fonction d'agent cimentant qui produit des alios ferrugineux, durs et relativement impermeables.

Abstract.

-

Twelve well-developed iron pans, formed in soils derived from widely different parent materials, have been investigated at room temperature and 77 K by Mossbauer spectroscopy. Eleven of the samples were from podzol profiles with iron pans developed under the A2 horizon, whilst the twelfth was formed below the B2 horizon of a ground water gley. This latter sample at room temperature gave a spectrum which consisted of a partly collapsed magnetic structure and at 77 K the spectrum consisted of a well-defined magnetic structure with parameters similar to those for a-FeOOH. The eleven podzol iron pan samples at room temperature and 77 K produced only a broad doublet (apart from peaks which could be assigned to minerals present throughout the soil profile), which could not be satisfactorily fitted to a pair of Lorentzian peaks. Statistically acceptable fits were obtained with two Lorentzian doublets, but additional spectra from variable temperature studies of some of the samples between 4.2 and 50 K showed magnetic hyperfine structure. Peaks were broad, even at 5 K indicating a distribution of magnetic fields in the sample. Although the range of values for this internal magnetic field is such that all of the various iron oxyhydroxides could be present, the spectra can also be interpreted in terms of a single species, a-FeOOH, with a range of microcrystalline particle sizes.

A structure is proposed which takes account of the accumulation with the iron of organic matter high in carboxyl groups which combine with the surface of the FeOOH particles. The organic matter effectively acts as a cementing agent, thereby producing the hard relatively impervious iron pans.

1. Introduction.

-

Podzolic soils are found exten- sively in cool-temperate to temperate, humid climates and are characterized particularly by a highly-leached whitish-grey A, horizon. Directly beneath this, especially when there is a peaty topsoil, a red-brown layer rich in iron, organic matter and sometimes manganese is often found. This may take the form of a thin iron pan which is relatively impervious t o water leading t o impeded drainage, and which can be penetrated only

with difficulty by plant roots. A typical podzol profile containing a thin iron pan is illustrated in figure 1, where the highly leached A horizon and the iron pan are clearly shown.

This paper describes the investigation by Mossbauer spectroscopy of twelve well-developed iron pans formed in soils derived from widely different parent materials. Similar studies have been carried out by other workers o n a wide range of natural systems in

54

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c6-850 B. A. GOODMAN AND M. L. BERROW Vegetation Callunetum Peaty top H

-

Whitish-grey + Horizon A2 -4

Thin iron pan B1 +

Below pan B3

-

FIG. 1. - A typical peaty podzol soil profile with thin iron pan. (R. Grant, Department of Soil Survey, Macaulay Institute for Soil Research.)

which iron is found in a secondary form, such as marine nodules [l-51, fresh water nodules [6, 71 and soils [S, 91.

2. Experimental.

-

The iron pans were separated from the air-dried soils by hand picking under a magnifying lens. They were then broken down with a wooden roller, passed through a 2 mm sieve to remove stones and gravel, and then lightly crushed in an agate mortar. Details of the iron pans and the soils in which they were formed are presented in table I. Moisture contents were obtained by drying a sub-sample at 80 O C and loss on ignition by igniting this dried mate- rial at 450 OC. The analytical results are all expressed relative to air-dried samples even though the Fe and Mn determinations were made on ignited samples. Absor- bers for the ~ o s s b a u e r experiments were prepared from the air-dried materials so that they contained 3-5 mg Fe per cm2, analytical grade A1203 being added where necessary to give sufficient bulk for convenient handling.

Mossbauer spectra at room temperature and 77 K were obtained with a spectrometer supplied by Harwell Scientific Services, Didcot, Berks which incorporated an Ortec Model 6200 1024-channel analyzer. Spectra below 77 K were obtained with a spectrometet using an Intertechnique SA41 400ichannel analyzer, an electromechanical drive system similar to that of Clark et al. [l01 and an Oxford Instruments MD4A variable temperature cryostat. Sources of 57Co in Pd or Ir, both of nominal strength 25 mCi (9.25 X 106

Bq)

were used

with argon-methane proportional counters as y-ray detectors. Velocity calibration was carried out with high purity metallic iron foil using the data of Preston

et al. [ll]. The spectra were fitted by a least squares

computer programme to a sum of doublets having Lorentzian peak shapes. The peaks of each doublet were constrained to have equal areas and widths and, if required, any of the variables could be constrained either to a fixed value or to be equal to a multiple of any other variable (e. g. relative areas of the 3 doublets from a magnetically ordered spectrum could be constrained to be 3 : 2 : 1). A parabolic baseline was assumed for all spectra.

3. Results.

-

The room temperature Mossbauer spectrum of the Ardalanish iron pan which was developed in a ground water gley is shown in figure 2a and is characteristic of a partially collapsed magnetic structure. At 77 K a well resolved 6-peak spectrum is obtained (Fig. 2b). An X-ray diffraction study of this iron pan has established that it consists largely of goethite, a-FeOOH (unpublished results). The Moss- bauer parameters at 77 K, however, have a value of the internal magnetic field, H, of 486 KOe which is some- what lower than the value of 504 KOe previously reported for a-FeOOH (Table 11), but not greatly different from a micro-crystalline sample of the same mineral. The absence of a completely developed magnetic structure in the room temperature .spectrum supports the hypothesis that the sample is composed of small particles, whose behaviour in producing paramagnetic spectra below the N6el temperature is well known. Some substitution of aluminium for iron in the goethite probably occurs and this could also have some effect on the internal magnetic field and the paramagnetic behaviour.

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THE CHARACTERIZATION BY MOSSBAUER SPECTROSCOPY OF THE SECONDARY IRON C6-851

TABLE I

Description of the iron-pans and of the soils from which they were isolated

(*) Estimated as C X 1.72.

k

-8 -4

,

..

i

4 I 8 l

Velocity mm S-'

Soils

FIG. 2.

-

Ardalanish ground water iron pan (a) at room temperature, (b) at 77K. Description

Dense mass dark metallic tubes throughout, friable yellow ground- mass, 5 cm thick

Thin ironpan, metallic well-developed, dark red-brown, 1-3 mm thick Thin iron pan, brown, 3 mm thick

Thin iron pan, well-developed, brown, 3 mm thick

Thin iron-manganese pan, firm, dark-brown, 3 mm thick

Thin iron pan, well-developed red brown, 1-3 mm thick

Humus iron pan, discontinuous Thin dark metallic iron pan, hard,

red-brown, 1-3 mm thick

Thin iron pan, red-brown, 3 mm thick

Thin dark metallic iron pan, red- brown, 1-4 mm thick

Thin iron pan, hard, continuous, dusky red, 1-3 mm thick

Thin iron pan, well developed, red- brown, 2 mm thick Major Soil Group Ground water gley Peaty podzol Peaty gleyed podzol Peaty pod- 201 Peaty pod- 201 Peaty pod- 201 Iron-humus podzol Peaty podzol Peaty podzol Peaty gley Peaty podzol Peaty pod- 201 Profile name and Map reference Ardalanish NM 375189 Blackhope NT 344525 Cairn o' mount NO 650842 Cantraydoune NH 788438 Castlelaw NT 227649 Cruachan NM 421490 Faille NH 689379 Glengorm NM 464564 Glenkirk NH 845324 Killiechronan NM 509411 Listonshiels NT 130614 Munduff hill NO 193036 Iron-Pans

Analyses on air-dry basis as %

Parent rock Siliceous sand Silurian grey- wacke Granite Schist and gra-

nite

Trachyte and rhyolife Basalt Schist and gra-

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B. A. GOODMAN AND M. L. BERROW

Mossbauer parameters for magnetically ordered components from iron pans and related minerals

Sample

-

Ardalanish iron pan

Faille iron pan component (i) (ii) Glengorm iron pan component (i)

(ii) Cruachan iron pan component (i) (ii) Killiechronan iron pan (3)

a-FeOOH a-FeOOH (microcrystalline) P-FeOOH y-FeOOH a-Fe2O, Temperature (K)

_-

77 5.4 Ref.

-

This work

(1) Isomer shifts, 6, relative to iron metal. (2) Parameters unable to be determined accurately.

(3) A minor component which accounts for 12 % of the area of the spectrum. (4) B. A. Goodman - unpublished results.

at room temperature (Fig. 3) and 77 K and although other weak peaks were observed with some of the samples, these could be assigned to well defined minerals present throughout the soil profile and are not considered in this paper. Attempts to fit these spectra to a doublet of Lorentzian peaks having equal areas and widths were unsuccessful, with unreasonable values of

x2

being obtained (Fig. 3a). However, statistically

I I I I

-1 0 1

Velocity mm 8

FIG. 3.

-

Glengorm podzol iron pan at room temperature (a)

fitted to one Lorentzian doublet, (b) fitted to two Lorentzian doublets.

acceptable fits were produced for most of the samples when 2 doublets were used (Fig. 3b) and the results are summarized in table 111. The great similarity of the parameters for the component doublets from different samples might seem to indicate that each of these pans has a similar composition. However, a-,

p-

and y- FeOOH as well as a-Fe20,, all possible constituents of the iron pans, have similar parameters in their paramagnetic forms to those of doublet (i) (Table 111). Doublet (ii) has a value of A similar to that obtained from the surface regions of very small particles of a-Fe20, [16], but iron oxyhydroxides of similar size would probably also have a component with similar parameters. Consequently it is thought that any inter- pretation of these spectra as specifically identifying one or a mixture of the above species is unjustified, especially since, with the exception of the Cruachan and Killiechronan iron pans which showed evidence for the presence of small amounts of poorly crystalline goethite and hematite respectively, iron-containing minerals could not be identified by X-ray diffraction. At 77 K

the Mossbauer spectrum of the Killiechronan pan contained a magnetically ordered component (Table 11) which accounted for about 12

%

of the iron in the pan. Four of the podzol iron pan samples, from Cruachan, Glengorm, Faille and Cairn oYMount, were selected for study in the temperature range 4.2 to 50 K. At 4.2 K

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THE CHARACTERIZATION BY M~~SSBAUER SPECTROSCOPY OF THE SECONDARY IRON C6-853

Computed Mossbauer parameters for the paramagnetic components from iron pans and related minerals

Sample Blackhope Cairn o' Mount Cantraydoune (l) Castlelaw Cruachan Faille Glengorm Glenkirk Killiechronan Listonshiels Munduff Hill a-FeOOH above temperature a- Fe00H-natural sample P-FeOOH y-FeOOH a-FezO3 Doublet (i) A

a

r

0.53

(ij

0.36

iij

Neel 0.6 (2)

-

Doublet (ii) 6

r

%

Quadrupole splitting, A, isomer shift, 6, and peak width, T, in mm/s-1 with 6 relative to iron metal. Numbers in brackets indicate the computed standard deviation on last significant figure. All parameters refer to spectra recorded at room temperature. (Similar spectra were obtained at 77 K, but with isomer shifts increased by approximately 0.10 mm/s-1.)

(1) An additional component was present with A = 2.73, 6 = 1.05,

r

= 0.36,6 %. (2) Reference [12].

(3) B. A. Goodman unpublished results.

(4) Reference [13]. (5) Reference [17].

( 6 ) Reference [14].

(7) Reference [16].

gorm samples, with the exception that ordering commenced at a slightly higher temperature with Cruachan and at a slightly lower temperature with Glengorm. The Cairn o'Mount sample showed signs of collapse of the magnetic structure commencing at 4.2 K. Only a poorly defined spectrum was obtained from this sample because of its low iron content. Accu- rate estimates of the Mossbauer parameters could not therefore be obtained and have not been included in Table 11, although the value for the internal magnetic field H could be seen to be similar to that obtained from the other iron pans.

4. Discussion.

-

The spectra obtained at room temperature from the podzol iron pans are similar to these reported by Karpachevskii et al. [8] and Logan et al. [g]. In the former publication, in which the paramagnetic spectra were analyzed as a single doublet, evidence was presented which favoured a-FeOOH as the major iron-containing constituent, but in the latter paper, in which two-doublet fits were used, the outer doublet was assigned to P-FeOOH. The evidence for

this assignment was based on the size of the quadrupole splitting at room temperature and the fact that magnetic ordering commenced between room temperature and 77 K. However, as shown by van der Kraan [l61 for a-FezO, the quadrupole splitting becomes larger when atoms are near to the surface of a particle and this can contribute significantly to the Mossbauer spectrum when particle sizes are extremely small. In the podzol iron pans the particle sizes of most of the iron containing minerals are so small that they cannot be identified by X-ray diffraction, and therefore it appears reasonable to assign the outer doublet in the paramagnetic spectra to surface atoms in an oxide or oxy- hydroxide structure.

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C6-854 B. A. GOODMAN AND M. L. BERROW

I

I I I I I

-8 -4

Velocity O mm 8

FIG. 4.

-

Faille podzol iron pan (a) at 5.4 K, (b) at 25 K, (c)

at 30 K, (d) at 35 K.

in P-FeOOH is intermediate between those of the a- and y-forms its presence is certainly not excluded by these spectra. Also the variable temperature spectra show that magnetic ordering takes place over a fairly large temperature range, which is a behaviour to be expected from the presence of ultrafine particles having a range of sizes [IS] and which is entirely consistent with the conclusion, based on the room temperature data, that a considerable proportion of the iron is located near the surfaces of particles. Thus, by analogy with the results of van der Kraan [16], the maximum observed magnetic splitting in the low temperature spectra could correspond to the bulk structure with the smaller splittings corresponding to surface regions. The results in table I1 show that the maximum magnetic splitting from the iron pan samples is similar to that of a-FeOOH, significantly greater than those of

P-

and y-FeOOH, and smaller than that of a-Fe20,. Thus

P-

and y-FeOOH are eliminated as unique iron-containing components of the pans (although they could still be present in significant amounts), and a-Fe20, can only be a minor component in those pans studied below 77 K.

It is possible therefore that the podzol iron pans have a similar chemical composition to the Ardalanish ground water iron pan, where it has been established that a-FeOOH is the principal component, and that the major difference is one of particle size. This conclusion is supported by the identification by X-ray diffraction of some poorly crystalline a-Fe00H in the Cruachan iron pan. Evidence for the presence of a-FeOOH in a number of the pans has also been obtained by electron diffraction. On the other hand X-ray diffraction of the Killiechronan iron pan has indicated the presence of poorly crystalline a-Fe20, and electron diffraction studies have indicated the presence of y-FeOOH in some of the iron pans.

Appreciable quantities of organic matter are found intimately associated with the podzol iron pans. Infrared spectra show that the main functional groups are carboxylate, and differential thermal analysis in an oxygen atmosphere gives curves very similar to those for the high ferric-fulvic acid complex of Schnitzer and Kodama [19], indicating combination of some iron with the organic matter. Using the analytical data of table I, Fe : C atomic ratios can be calculated for the iron pans. For the podzol pans this ratio is between 0.2 and 0.8; whereas the ground water pan has a value of

-

3.5. Published values for the carboxylate contents of soil organic materials are of the order 1.5-9 meq/g [20], which represent about 3-18

%

of the carbon. Therefore, in all of the pans there is probably a greater number of iron atoms than there are carboxylate groups and, since a carboxylate group only satisfies 1 of the 6 coor- dination sites of Fe3

*,

the fraction of the iron in organic combination is likely to be small. This is especially true of the ground water pan where organically-bound iron must represent a very minor component. There- fore, although there is evidence that most of the organic matter is bound to iron, a large proportion of the iron occuss in inorganic forms of very small particle sizes. Since the pans are usually extremely hard and form continuous layers through the soil (Fig. 1) a structure is envisaged in which organic molecules link together a number of FeOOH particles, the size of the inorganic component being influenced by the Felcarboxylate ratio. This structure is not greatly different from that in ferritin, which has a core of composition (FeO0H)n and dimensions of up to a few tens of

a

and a surface coating of protein molecules.

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THE CHARACTERIZATION BY MOSSBAUER SPECTROSCOPY OF THE SECONDARY IRON C6-855

References

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l 0 (1972) 116.

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Soil Res. in press.

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