amorphous clay minerals in some scottish...

AMORPHOUS CLAY M I N E R A L S I N SOME SCOTTISH SOIL PROFILES

By B. D. MITCI-~LL and V. C. FARMER

The Macaulay Institute for Soil Research, Aberdeen.

[Read 4th November, 1960]

ABSTRACT Examination of clays from surface and basal horizons of four Scottish soils by differential thermal analysis, before and after heating to 600~ and by infra-red spectroscopy indicates that these contain a proportion of highly hydrated amorphous material, resembling allophane in its properties, and that this component is particularly high in clays from the surface horizons with the greatest organic matter content. This conclusion is consistent with the cation-exchange capacities of the clays and with the amounts of silica and alumina extractable from them by hot sodium carbonate solutions. The ratio of silica to alumina in these extracts indicates that the amorphous material is highly siliceous.

INTRODUCTION

Before the discovery of crystalline clay minerals pedologists regarded the clay fraction of the soil as a colloidal complex and completely amorphous. With the advent of the concept of crystalline clay minerals ideas concerning amorphous inorganic material in clays were virtually abandoned. Recently soil-clay mineralogists, while supporting the view that many soil clays are essentially crystalline, have realised that this fraction may contain an appreciable amount of amorphous inorganic material. It has been demonstrated (Birrell and Fieldes, 1952; Fieldes, 1955, 1957) that allophane, or a similar association of hydrous oxides of silicon and aluminium, is a common constituent of New Zealand soils developed on volcanic ash. Sudo (1954), Ishii and Mori (1959), Kanno (1959) and Matsui (1959) have also shown that allophane or allophane-like material predominates in many of the volcanic ash soils of Japan. Clays from soils developed on recent or late Pleistocene marine sands of the Mississippi Coastal Terrace were found by De Mumbrum (1960) to contain a high proportion of amorphous ahiminosilicate.

An examination of the results obtained from differential thermal analysis, in an inert atmosphere, of a large number of Scottish soil clays has revealed that the dehydroxylation peaks, particularly of clays from highly organic upper horizons of the profile, were often smaller than would be expected from the results of X-ray analysis; also, the

128

AMORPHOUS CLAY MINERALS IN SOILS 129

sorbed moisture associated with these particular soil clays was, according to their differential thermal curves and infra-red absorption spectra, much larger than normal. Since X-ray analysis shows only the crystalline components of the clay it seemed possible that the differential thermal and infra-red absorption results were due to the presence of amorphous inorganic material. Evidence presented here from a more detailed study of clays from four soils supports this interpretation.

MATERIALS AND METHODS

The clay fraction (< 2/~ equivalent spherical diameter) from A and C horizons of four soil profiles from North Ayrshire (Mitchell and Jarvis, 1956) were used in this investigation. Two of the soils examined were shallow and residual. Auchans, a brown forest soil of low base status possessing a silicate moder humus* was developed on teschenite-olivine dolerite sill material of Carboniferous Age and consisted only of an A horizon over unweathered rock. Littlehill, a brown forest soil of low-base status with a well-formed medium crumb structure in the A horizon, was developed on a basaltic ash of Post Carboniferous Age. The other two soils were deeper and formed on glacial till: Lanfine, a grey-brown podzolic soil, was developed on a moderately fine textured glacial till derived from a mixture of basalt, andesite and sandstone of Old Red Sandstone Age, whilst Picket Loch, a brown forest soil, was developed on a medium- textured till derived from a mixture of basic and intermediate lavas of Calciferous Sandstone Age. The surface horizon of both of these profiles conformed to the silicate-moder humus type.

Detailed morphological characteristics of the soil profiles are as follows:

Depth or Horizon Thickness

L 1" A 0-14"

Dr 14"+

AUCHANS

Litter. Brown--dark brown (10YR:4/3) loam; moderate fine crumb; friable; organic matter high; stones few; roots abundant; no mottling. Rock; unweathered teschenite-olivine 6olerite.

Depth or Horizon Thickness

A 0-4"

C 4-11"

LITTLEHILL

Dr 11"+

Reddish brown (2-5YR:5/4) sandy clay loam; strong medium crumb; firm; organic matter moderate; stones few; roots abundant; no mottling; merging into Reddish brown (2.5YR:5/4) clay loam; moderate fine subangular blocky; firm; organic matter low; stony; roots frequent; few fine faint ochreous mottles. Rock; basaltic ash Post Carboniferous Age.

*Silicate moder is a humus type in which the minerals and detrital vegetation of the A horizon are intimately mixed but the constituent mineral and organic parts can be identified by a good lens.

130 B. D. MITCHELL AND V. C. FARMER

Hot izon L AI

Aj2

B2

B3

C

Dr

Horizon L A

B2(g)

B3(g)

Depth or Thickness

2" 0-6"

6-12"

12-20"

20-24"

24-29"

29"+

Depth or Thickness �89

0-8"

PICKET LOCH

Litter. Dark brown (10YR:4/3) loam; weak fine crumb; loose; organic matter high; stones few; roots abundant; no mottling; merging into Dark brown (10YR:4/3) loam; moderate fine crumb; friable; organic matter high; stones few; roots abundant; no mottling; sharp change into Yellowish brown (10YR:5/4) sandy clay loam; weak fine subangular blocky; friable; organic matter low; stones few; roots occasional; no mottling; merging into Light brown (7-5YR:6/4) sandy clay loam; moderate medium platy; firm; moderately indurated; organic matter low; few stones; roots occasional; few fine faint ochreous mottles; sharp change into Brown (7'5YR:5/4) sandy loam; moderate medium platy; firm; moderately indurated; organic matter low; stones few; roots rare; few fine faint ocbreous mottles. Rock; basalt Calciferous Sandstone Age.

LANFINE

Litter. Dark yellowish brown (10YR :4/4) sandy loam; weak fine crumb; friable; organic matter moderate; stones absent; roots abundant; no mottling; merging into

8-21" Brown (10YR:5/3) sandy clay loam; fine subangular blocky; loose; organic matter low ; stones absent; roots frequent; ochreous mottling along root channels; sharp change into

21-30" Brown (10YR:5/3) sandy loam; weak subangular blocky; loose; organic matter low; stones absent; roots frequent; marked ochreous mottling along root channels; sharp change into

C(g) 30-45" Yellowish red (5YR:4/6) sandy clay loam; massive; organic matter low; stony--some highly weathered; roots occasional; marked ochreous mottling along root channels.

In order to assess the distribution of sorbed water between the mineral and the organic components of the clay fraction of these soils a portion was treated with hydrogen peroxide followed by washing with hot water. This procedure removed the bulk of the organic matter and that which remained did not interfere significantly.

The differential thermal curves of the untreated and peroxidized clays were determined in nitrogen with the controlled-atmosphere thermal analysis apparatus described by Mitchell and Mackenzie .(1959). The infra-red absorption spectra of the clays were obtained m potassium bromide pressed discs using a Grubb-Parsons $4 double-beam spectrometer equipped with a grating for the 3tz region and a sodium chloride prism for the 5/~ to 16tz region. Loss of water from the clays was followed in spectra obtained after heating the pressed discs for 10 hours at 100~ 200~ and 300~ (Farmer, 1958).


Diffraction patterns of the soil clays were obtained on film with a Raymax 60 X-ray generator. The micro method of Mackenzie (1951) was employed to determine the cation-exchange capacity of the clays, and their free iron oxide content was found by the dithionite method of Mitchell and Mackenzie (1954). Selective dissolution with alkali of amorphous and crystalline alumino-silicates in the clay fraction was effected by successive four-hour digestions of the sample in platinum with 50 mt of 5 per cent. sodium carbonate solution on

'r' t_-'t I " - ~ f - ' t J - ' ~

' | J

/____~._/ ' i '

I - - - 2 g / _ _ . t - / , , !/~ .

I

Fio. 1--Differential thermal curves for surface soiI ciays: unperoxidized, a--- Auehans, b--Littlehill, c--Picket Loch and d--Lanfine; peroxidizefl, e--

Auchans, f--Littlehill, g--Picket Loch and h--Lartfine.

the steam bath. The initial weight of the sample was 100 rag, water was added at intervals to maintain the volume of solution and the digestions were continued until the extracts contained less than 1 mg silica and about 0"3 mg alumina: this usually required about twelve extractions.

RESULTS AND DISCUSSION The clays from the A and C horizons were studied because the A

132 Be D. MITCHELL AND V. C. FARMER

horizon is that part of the profile most exposed to pedological weathering whereas the C horizon represents the soil stratum virtually unaffected by pedogenic processes. A comparison between the clay mineralogy of the surface and basal layers can therefore provide an index of weathering within the profile. Since the soils are freely drained their A and C horizons have undergone the maximum and minimum of pedological weathering that occurs within the zonal soils of a temperate region.

Differential thermal, X-ray and infra-red studies. The differential thermal curves, obtained in an atmosphere of nitrogen, of the untreated clay fractions from the surface and basal horizons of the profiles are shown in curves a-d, Fig. l, and curves a-c, Fig. 2. When

dT

0 400 800 O00 ~fl00

FIG. 2--Difterential thermal curves for basal soil clays: unperoxidized, a--Li t t le- hill, b--Picket Loch and c--Lanfine; peroxidized, d--Littlehill, e--Picket Loch

and f--Lanfine.

the content of organic matter in the soil clay is high, as in the Auchans and Picket Loch A-horizon material, the hygroscopic moisture peak at 120~ is large and the endothermic peak between 500~ and 700~ the range within which dehydroxylation of layer silicates commonly found in soil clays occurs, is either very small or absent. The thermal curves of clays from basal horizons and from the surface horizons of the Littlehill and Lanfine soils, which have a moderate to low organic matter content, exhibit a less pronounced dehydration peak than the highly organic soil clays. They also show the typically well-defined endothermic peak in the 500-600~ range. It will be shown here that


the unusual pattern of thermal curves given by the clays from the horizons rich in organic matter cannot be accounted for by either their organic matter content or their crystalline clay-mineral composition. Treatment of the soil clays with hydrogen peroxide reduced their carbon content to less than one per cent., even in the clays from the highly organic surface horizons, but produced no appreciable change in their differential thermal curves (Curves e-h, Fig. 1, and

/L_,

/

i i~ lp iw~r -t

Fz~. 3--Differential thermal curves for: a--vermiculite, Kenya; b--the same after heating to 600~ e--illite, Fithian; d--the same after heating to 600~ e--allophane, Tirau clay, New Zealand; f---the same after heating to 600~C; soil clays after heating to 600~ g--Auchans A horizon, h--Picket Loch A holizon, i--Lanfine A hoIizon, j--Littlehill C horizon and k--Picket Loch

C horizon.

Curves d-f, Fig. 2). In curves for the clays from the Auchans and Picket Loch A horizons, the hygroscopic moisture peak is still abnormally large and the endothermic peak at approximately 550~ has not significantly increased. The thermal curve of the Auchans soil clay shows an endothermic peak at about 300~ which is a result of the incomplete removal of complex oxalates formed during the per- oxidation treatment. The endothermic peaks at 200~ 300~ and


450~ on the thermal curve for the peroxidized and water-washed clay from the Picket Loch A horizon indicate that some calcium oxalate and complex oxalates are present.

The X-ray diffraction patterns of the clays from the four soils show that there is very little variation in the nature or relative amounts of crystalline clay minerals within a profile and no great variation between profiles. Vermiculite is the principal crystalline clay mineral in the A horizon of the Auchans profile, but a little kaolin is also observable. The clays from the surface and basal horizons of the Picket Loch soils contain vermiculite-chlorite mixed-layer mineral, plus a little kandite, there being more of the latter in the C-horizon clay than in the A-horizon clay. The Littlehill and Lanfine soil clays also contain predominantly vermiculite-chlorite mixed-layer mineral with kandite as a minor constituent.

Illite, kandite and some samples of clay vermiculite (Fieldes, 1957), which are the crystalline components of these clays, all show endothermic effects between 500~ and 600~ and their presence can explain those features of the differential thermal curves given by the clays from the basal horizons and from the surface horizons of the Littlehill and Lanfine soils. The X-ray results, however, do not account for the extreme weakness of the endothermic effects near 550~ and for the large hygroscopic-moisture peaks near 120~ shown by the thermal curves of the clays from the highly organic surface horizons of the Auchans and Picket Loch soils. Differential thermal curves obtained so far for soil vermiculite and other crystalline clay minerals do not have peaks due to dehydration in any way comparable in size with these. The dehydration peak on the thermal curve for a pure macrocrystalline vermiculite, NH4-saturated like the soil clays examined here, is of only moderate size (Curve a, Fig. 3), and the presence of chlorite layers in the vermiculite can only reduce the water holding capacity.

The differential thermal curves can best be explained by the presence of a high proportion of allophane or allophane-like amorphous material in the clays from the highly organic surface horizons. X-ray diffraction cannot detect this amorphous material, and does not readily give information on the absolute amounts of the crystalline components present. The differential thermal curves of allophanes and silica gel (Fig. 4) feature a very large, broad endothermic peak between 100~ and 200~ caused by the loss of water sorbed on the internal and external surfaces (Fieldes, 1957). The loss of structural hydroxyl groups is gradual; consequently, there is no distinct endothermic peak corresponding to those shown by most crystalline clay minerals in the 500~ to 700~ range. The presence of a high proportion of such highly hydrated amorphous material in the clays from the surface horizons of the Auchans and Picket Loch profiles can therefore account for their differential thermal behaviour.

Examination of the NHa-saturated peroxidized clays by infra-red


~r

f

j b

/ c

e

Ft6.4---Differential thermal curves for: a--allophane, Tirau clay, New Zealand; +--allophane, Sta. Creu Elorde, Barcelona, Spain; c--allophane, Heathcote,

Derbyshire; d--allophane, Woolwich, Kent; e--silica gel (synthetic).

(b)

_ J _J

Aras"

c too"

FIG. 5--Infra-red absorption due to ionic hydroxyl groups, water, and ammonium in NH4-saturated clays: a--allophane; b--vermiculite; c--illite; d---clay from Picket Loch A horizon; e--clay from Picket Loch C horizon; f--clays from Lanfine A and C horizons. The clays were heated in KBr pressed discs to the

temperatures indicated.


spectroscopy agrees with this interpretation. Sorbed water in clays gives absorption bands near 3tz and near 6/z arising from its stretching and bending vibrations respectively. Comparison of the spectra of the ammonium-saturated clays after drying in potassium-bromide pressed discs at 100~ showed that all the soil clays retained more water than did vermiculite or illite, the principal hydrated crystalline clay minerals present. Typical spectra obtained in the 3tz region are shown in Fig. 5 for ammonium-saturated allophane (a New Zealand soil clay), vermiculite, illite, and soil clays from Picket Loch A and C horizons, and Lanfine A and C horizons. In these spectra, ionic hydroxyl groups in the crystalline clay minerals give a fairly narrow absorption band between 2-70/~ and 2.75/z, water a broad band with a maximum near 2.85tz, and NH4 + a broad band with a maximum near 3-1/z. The 2-85/z band possibly includes absorption by hydrogen-bonded hydroxyl groups on the surface of amorphous components (McDonald, 1958), but as it is lost after heating to 300~ it will be treated as arising from sorbed water. The allophane gave the strongest water absorption bands, followed by clays from the highly organic A horizons of the Picket Loch and Auchans profiles. Successively lower water contents were found in the clays from Picket Loch C, Lanfine A, Littlehill A and C, and Lanfine C horizons. Illite and vermiculite retained more water than did a NH4-saturated montmorillonite examined, which was almost anhydrous after heating at 100~ Almost complete dehydration of the clays was achieved by heating the pressed discs at 300~ and their spectra then showed that absorption due to ionic hydroxyl groups was twice as strong in the clay from the Picket Loch C horizon as in that from A horizon (Curves d and e, Fig. 5). This is consistent with the marked differ- ence in the heights of the peaks at 550~ on the thermal curves of these clays, since these peaks correspond to loss of ionic hydroxyl groups from the crystalline clay minerals. Spectra given by the clay from the Auchans A horizon closely resembles those given by the clay from the Picket Loch A horizon, but the allophane showed still less ionic hydroxyl absorption after heating to 300~ This arises from a trace of kandite in the material (Curve a, Fig. 5). It is note- worthy, too, that the allophane and these two most highly hydrated soil clays lost all or nearly all their ammonium content after heating to 300~ whereas little was lost by vermiculite and illite, and a signi- ficant amount was retained by the clay from the Picket Loch C horizon, as indicated by the intensities of ammonium absorption at 3.1/~ (Fig. 5). This behaviour, together with the high state of hydration and low ionic hydroxyl content of the clays from the A horizons of the Auchans and Picket Loch profiles point to their having a high content of hydrated amorphous material resembling allophane in its properties. All the soil clays, however, appear to contain a proportion of such amorphous material.

In the 7-16/~ region, the spectra of the two most highly hydrated soil clays, which were almost identical, showed a principal absorption


band at 10/z and a subsidiary band at 10.9/~ (Curve a, Fig. 6), both of which can be ascribed to the crystalline components, vermiculite and/or kandite. These bands were weaker than the corresponding bands in the spectra of the other soil clays, for example, that from the Picket Loch C horizon (Curve b, Fig. 6). On the other hand, a broad shoulder between 9/z and 9-5/~ is a much more prominent

7 �9 | tO n 12 t i N S N

(a)

f . _ _ - _ . J

J

(0)

f ~ . - . f

/ - \ j / \ J J r

; 4 �9 I0 It 11 13 H IS Ig

FIc. 6---Infra-red absorption spectra, 7-16~, of clays from: a--Picket Loch A horizon; b---Picket Loch C horizon; c--allophane, Tirau clay, New Zealand.

feature of the spectra of the more highly hydrated clays. Although quartz and kaolinite both absorb in this region, they are not present in sufficient amount to account for the intensity of this feature. Allophanes which have been examined give broad featureless absorption bands with maxima at longer wavelengths than these--- between 9.5/z and 10.5/z (Curve c, Fig. 6). Amorphous silica gels

138 B. D . MITCHELL AND V. C. FARMER

do absorb in this region (Hunt, Wisherd and Bonham, 1950) and it seems likely that the highly hydrated soil clays contain amorphous silica, or an allophane with a high silica to alumina ratio. It is uncertain whether the soil clays contain allophanes with absorption maxima between 9.5~ and 10.5~, since this region is dominated by the more distinctive absorption of the crystalline components of the clays. It is to be expected that the maximum of the Si-O absorption band in allophanes will shift to shorter wavelengths with increasing degree of polymerization of the SiO4 tetrahedra in the structure, i.e., with increasing silica to alumina ratio, from perhaps between 10t* and lit , for isolated tetrahedra, to 9/* for highly polymerized tetrahedra, as in silica gel (Launer, 1952). Al-for-Si substitution in the tetrahedral network should cause a displacement to longer wavelengths for any given degree of polymerization (Milkey, 1960), but neither of these effects has been studied experimentally.

The spectra of silica gels examined do not show such strong water sorption as the soil clays in which they have been detected, so it seems likely that the high state of hydration of these clays resides in some other amorphous component, probably in highly hydrated sesquioxides, or allophanes with lower silica to sesquioxide ratios. In support of this, it was found that extraction of sesquioxides from these clays by treatment with 5 per cent. solutions of E.D.T.A. (ammonium salt, at pH 7) for one hour on the steambath left residues, the spectra of which showed very much less sorbed water, although absorption bands due to amorphous silica and crystalline clays in the 9-12tz region were greatly enhanced.

Heating to 600~ is known to cause the collapse of expanding layer silicates with consequent loss of interlayer water (Brown, 1961) but it has been observed here that allophane rehydrated to a considerable extent after this treatment as indicated by the low-temperature peak on its differential thermal curves (Curve J; Fig. 3). It seemed possible, therefore, that the degree of rehydration of the soil clays after heat treatment might provide a further basis for differentiating between amorphous and crystalline minerals in these clays. Samples of vermiculite, chlorite, illite, and allophane, and of peroxidized clays f om the A and C horizons of the four soil profiles were heated to 600~ for six hours, then equilibrated at 56 per cent. relative humidity for four days and their differential thermal curves were determined (Fig. 3). Of the mine'als so treated only illite and allophane were found to rehydrate, and it was also noted that the illite recovered some of its structural hydroxyl groups. The area of the peak due to dehydration on the thermal curve of the heat-treated illite was, however, 70 per cent. of the area of the corresponding peak on the curve for the untreated illite. The heat-treated allophane was also less hydrated than the original material, the dehydration peak area of the former being 60 per cent. of that of the untreated sample. The degree to which the allophane rehydrates is considerably greater than that observed by White (1953) who, however, simply


left the heat-treated allophane standing in air for periods of from 12 hours to 80 days. The thermal curves of the preheated soil clays indicate that their ability to rehydrate varies considerably. Never- theless the following general trends are observable: (a) the areas of the dehydration peaks of heat-treated clays high in amorphous colloids are 60-70 per cent. of those of the corresponding untreated clays, and (b) the size of the dehydration peaks of the preheated clays containing a high proportion of crystalline material are 30-50 per cent. of those of the corresponding untreated clay. Of the crystalline minerals identified in the soil clays only illite rehydrates after prolonged heating at 600~ This mineral is present in four of the soil clays examined, but as it is only a minor component its con- tribution to the rehydrated state of these clays may be disregarded. The pronounced dehydration peaks given by the heat-treated clays from the surface horizons of the Auchans and Picket Loch soils are in agreement with the conclusion that they contain a high proportion of amorphous material. The clays from the other soil horizons rehydrate more than would be anticipated from their crystalline mineral complement and this indicates that they too contain a proportion of hydrated amorphous material.

The results obtained from differential thermal, X-ray and infra-red investigations do not lend themselves to rigorous quantitative treatment nor do they necessarily fully express the degree of similarity between the amorphous material in the soil clays and allophane. With a view to obtaining such information the cation-exchange capacities of the clays and the differential solubility of their components in sodium carbonate solution were investigated.

Cation-exchange capacity. Because of differences in particle size and in degree of development of structure in naturally occurring allophane-rich clays and synthetic aluminosilicate gels it is not sur- prising that the cation-exchange capacities recorded for such materials vary considerably. Birrell and Fieldes (1952), for example, obtained a value of 54 m-eq/100g for an allophane from soil and Mattson (1931) recorded values of 47 m-eq/100 g and 30 m-eq/100 g for two chemically distinct aluminosilicate precipitates. The cation-exchange capacities of the allophane-containing soil clays of this investigation, determined by the micro-method of Mackenzie (1951) were 26 m-eq] 100 g (Auchans) and 36 m-eq/100 g (Picket Loeb), agreeing well with a value of 29 m-eq/100 g obtained by the same method for an allophane-rich soil clay from New Zealand. The essentially crystalline soil clays examined, containing principally vermiculite-chlorite mixed- layer mineral have an exchange capacity in the range 34-55 m-eq/100 g, the mean value being 41 m-eq/100 g. Exchange capacities of up to 54 m-eq/100 g have been found for vermiculite clays from other Scottish soils. Compared with an exchange capacity of 150-160 m-eq/100 g for a well-crystallized vermiculite the value for vermiculitic soil clays is low. The greater degree of sub-division of the soil vermiculite compared with that of the pure mineral and,


perhaps of greater significance, the presence of amorphous material in the form of coatings on the primary soil particles could contribute to a reduction in cation-exchange capacity. Since the exchange capacity of chlorite minerals may be anything from 10 to 40 per cent. of those of vermiculite (Grim, 1953) an average value of 45 m-eq/100 g for soil clays composed largely of vermiculite-chlorite mixed layer minerals seems reasonable but does not exclude the possibility of amorphous aluminosilicate in those clays. The exchange capacities of the soils investigated are therefore consistent with their mineralogical composition as determined by the instru- mental techniques.

Selective dissolution of components. Allophane varies considerably in chemical composition. Ross and Kerr (1934), who proposed that the name be applied to all mutual solutions o f silica, alumina and water with minor amounts of bases, found that the silica to alumina molar ratios for twelve samples of allophane which they examined ranged from 0.78:1 to 1-98:1. Birrell and Fieldes (1952) observed that SiO2:AI20 ~ ratios for allophane-rich New Zealand soil clays developed on volcanic ash deposits varied from 1.01:1 to 1.78:1. In this investigation the soil clays with a high content of amorphous aluminosilicate have SiO2:A120 3 ratios of 1.31:1 (Picket Loch) and 2"03:1 (Auchans) and for the others, i.e., those with a high proportion of vermiculite-chlorite mixed-layer mineral, the ratios vary from 2.04:1 to 2.52:1 (Table 1). These ratios are in accord with the mineralogical composition of the clays.

For many years the usual method of determining the soluble silica in soils and minerals has been treatment with hot sodium carbonate solution (Salv6tat, 1851). This technique of alkali dissolution using hot 5 per cent. sodium carbonate was applied to the soil clays with a view to effecting a separation of the amorphous silicate from the crystalline silicate component. The molar ratio of the silica to alumina in the extracted fraction was 3-42:1 and 1.93:1 for the Auchans and Picket Loch clays with a high amorphous silicate content and varied from 2-27:1 to 3.55:1 for the clays containing a higher proportion of crystalline material. All the Scottish soil clays gave a higher SiO2:A1203 ratio in the sodium carbonate soluble fraction than in the original clay. However, an allophane-rich New Zealand soil clay with a SiO2:A120 3 ratio of 1.54:1 gave a sodium carbonate soluble fraction with a ratio of 1-24:1 which is within the range quoted by previous workers for aUophane. The sodium carbonate-soluble fractions of the Scottish soil clays are thus by comparison very rich in silica.

The amount of sodium carbonate-soluble silica in the predominantly amorphous soil clays varied from 72 to 86 per cent. of the total and the soluble alumina from 43 to 58 per cent. The cays containing predominantly interstratified vermiculite-chlorite have 55-72 per centL of their silica soluble in alkali and 40-55 per cent. of their alumina.


0

0 C

e~

o

~'~ ~ 0 o '~.

,=:~,

0

..~

1

�9

-g �9

�9 o C ~ O 0 N N

~ 0 0 0 "~~=~.~

~ 0 0 ~

. ~ . ~ . u ~


The possibility of dissolution of the crystalline mineral components of the clay by this sodium carbonate digestion cannot be ignored. The New Zealand soil clay which originally contained too little kandite to be detected by X-rays yielded, after ten sodium carbonate digestions, a very small residue which gave the X-ray diffraction pattern of a well-crystallized kandite. Clearly the alkali treatment has much less effect upon the kandite than upon the amorphous component. The crystalline components of the Scottish soil clays of this investigation are, however, essentially vermiculite-chlorite minerals and in order to assess the solubility of such materials in hot sodium carbonate solution a well-crystallized vermiculite filed to - 100 mesh and a chlorite were used as controls. It was found that 28 per cent. of the silica and 11 per cent. of the alumina in the vermiculite is dissolved in hot alkali solution, but that only 16 per cent. of the silica and 2.5 per cent. of the alumina in the chlorite is removed. These results suggest that vermiculite-chlorite minerals in soil clays, especial- ly as they are finely particulate, may undergo a certain amount of dissolution by alkali. There was, however, no indication from tho X-ray diffraction diagrams of the residues of the sodium-carbonate- treated soil clays of any marked breakdown of the crystalline mineral having taken place. Infra-red examination of the residues from the two most highly hydrated soil clays showed that these contained much less sorbed water and amorphous silica, and a higher proportion of crystalline clay minerals absorbing at 10/z, than did the untreated clays. The residues from the other clays showed much less change, although in some there was a decrease in the proportion of kandite minerals.

In this investigation attention has been focused on the identification and estimation of the amorphous inorganic fraction of the soil clays in terms of silica and alumina. Since, however, free iron oxides are of common occurrence in Scottish soil clays they too have been determined. Using the dithionite method it was found that 70 per cent. of the iron in the clays from horizons with high organic matter content was in the forms of free iron oxides and 53-61 per cent. in the clays from the other horizons (Table I). Although the distribution of free iron oxides in these soil clays is similar to that of the amorphous alumino-silicate, it is uncertain whether these free iron oxides are discrete or form part of the amorphous aluminosilicate complex.

CONCLUSIONS

The amount of sorbed water in the mineral component of the soil clay as observed from its thermal curve and infra-red spectrum has been taken as an indication of the amorphous aluminosilicate content, and the results have shown that whilst all the clays examined appear to contain a proportion of hydrated amorphous material, this component is particularly high in clays from surface horizons having


high contents of organic matter. This conclusion is supported by differential thermal observations on the dehydroxylation peaks given by these clays and on the degree to which they rehydrate after heat treatment, and is consistent with their observed cation-exchange capacities, and the appreciable quantities of alkali-soluble silica and alumina which they contain. Although tests indicate that vermiculite- chlorite mineral, which is the principal component of these clays may be to some extent attacked by the hot sodium carbonate solution, it is considered that the greater part of this soluble silica and alumina is derived from the amorphous component.

The molar ratios of silica to alumina in the sodium carbonate fraction of the clays show that in g~neral, the amorphous material is much more siliceous than the allophane in the volcanic ash soils of New Zealand, and this can be correlated with evidence, from the infra-red spectra, of silica gel in the clays with highest amorphous content. Glenn et al. (1960) obtained SiOz:Al203 ratios as high as 8-5:1 for material dissolved from soil clays by rapid boiling with caustic soda. The highest ratio was associated with the coarse clays (2-0"2t~) from the A horizon of a Prairie soil and was attributed to the presence of siliceous residues of weathered layer silicates. Recently, Duff, Webley and Scott (1962) have shown that such residues can be produced by the action of micro-organisms on soil minerals. Other possible sources of amorphous silica include precipitation of silica from solution, or the siliceous residues of graminaceous plants (Lovering, 1959). It is not known whether the amorphous alumina in these clays is free, or chemically combined with silica as in allophanes (Fieldes, 1955).

The soils used in this investigation are freely drained and lie in an area with an annual rainfall exceeding 35 in. They may be regarded, therefore as highly leached and actively weathering. Although the pedological processes which may account for the clay mineralogy of these soils is outwith the scope of this communication, it is believed that the presence of a high proportion of hydrated amorphous aluminosilicates in soil clays is associated with an organic cycle of weathering. Although this investigation has been restricted to clays from four soil profiles from North Ayrshire, a broad survey of differential thermal and X-ray results of clays from soils throughout Scot- land suggests that the results may be indicative of a general pattern in Scottish soil-clay mineralogy, as clays from highly organic surface horizons have been commonly found to show large hygroscopic moisture peaks and weak dehydroxylation peaks on their differential thermal curves.

Acknowledgements.--The authors desire to thank Mr W. A. Mitchell of the Macaulay Institute for supplying the X-ray results. They are indebted to Dr M. Fieldes of the Soil Bureau, Department of Scientific and Industrial Research, New Zealand, and the Geological Survey and Museum for the samples of allophane.


REFERENCES

BIRRELL, K. S., and FIELr)ES, M., 1952. J. Soil Sci., 3, 156. BROWN, G. (editor), 1961. The X-ray Identification of Clay Minerals.

Mineralogical Society, London. DE MUtvmRUM, L. E., 1960. Proc. Soil Sci. Soc. Amer., 24, 185. DVrF, R. B., WEBLEY, D. M., and SCOTT, R. O., 1962. SoiISei., in Press. FARMER, V. C., 1958. Miner. Map., 31, ~29. FIELDES, M., 1955. N.Z.J. Sci. Teeh., B37, 336. FIELDES, M., 1957. N.Z.J. Sci. Tech., B38, 534. GLENN, R. C., JACKSON, M. L., HOLE, F. D., and LEE, G. B., 1960. Clays and

Clay Minerals (A. Swineford, editor). Pergamon Press, London, p. 63. GR1ta, R. E., 1953. Clay Mineralogy. McGraw-Hill, New York. HUNT, J. M., WISHERD, M. P., and BONHAM, L. C., 1950. Analyt. Chem., 22, 1478. IsnIt, J., and MORI, T., 1959. J. geol. Soc. Japan, 65, 357. KANNO, J., 1959. Nendokaguku no Shimpo, 1, 213. LAtmER, P. J., 1952. ,4met. Min., 37, 764. LOVERING, T. S., 1959. Bull. geol. Soc. ,4mer., 70, 781. MATTSON, S., 1931. Soil Sei., 30, 459. MATStn, T., 1959. Nendokaguku no Shimpo, 1,244. McDONALD, R. S., 1958. J. phys. Chem., 62, 1168. MACKENZIE, R. C., 1951. J. Colloid Sr 6, 219. MILKEY, R. G., 1960. Amer. Min., 45, 990. MITCIaELL, B. D., and JAaVlS, R. A., 1956. The Soils of the Country round

Kilmarnock. Mem. Soil Surv. Scot., H.M.S.O., Edinburgh. MITCHELL B. D., and MACKENZIE, R. C., 1954. Soil Sei., 77, 173. MITCHELL, B. D., and MACKENZIE, R. C., 1959. Clay Min. Bull., 4, 31. Ross, C. S., and KERR, P. F., 1934. Prof. Pap. U.S. geol. Surv., No. 185-G, p. 135. SALVETAT, L. A., 1851. Ann. Chim. (Phys.), 31, 102. SVDO, T., 1954. Clay Min. Bull., 2, 96. WroTE, W. A., 1953. Amer. Min. 38, 634.

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