Huggett and Cuadros 2010 Non-marine glauconite

Glauconite formation in lacustrine/palaeosol sediments, Isle of Wight

(Hampshire Basin), UK

J. M. HUGGETT1 , 2 , * AND J . CUADROS1

1 Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK, and2 Petroclays, The Oast, Sandy Cross, Heathfield, East Sussex, TN21 8QP, UK

(Received 27 August 2009; revised 23 November 2009; Editor: John Adams)

ABSTRACT: The clay mineralogy and chemistry of a green lacustrine marl that has been

pedogenically modified in the upper part was investigated in order to better understand the formation

of low-temperature Fe-rich 10 A clay. Twelve samples in a vertical sequence have been investigated

using X-ray diffraction (XRD), chemical analysis, scanning electron microscopy (SEM) and laser

particle size analysis. The clay assemblage has a range of overall illite-smectite (I-S) compositions

(64�100%) resulting from several I-S phases that, for the sake of modelling, have been simplified to

one to three I-S phases of increasing illitic content. Where the lacustrine marl has been pedogenically

modified, the smectite-rich I-S is much reduced in abundance or absent and the 10 A-rich component

is both more abundant and more illitic. These assemblages are a consequence of illitization of detrital

I-S in the lake and soil, and dissolution of other clays (kaolinite and chlorite) in the hypersaline lake.

Interlayer K, octahedral Fe and octahedral + interlayer Mg increase with intensity of illitization

(increase range 0.32�0.63, 0.68�1.67, 0.18�0.24 per O10(OH)2, respectively), first in the

increasingly saline lake, and latterly as a result of wetting and drying in a gley soil. In the soil

environment, reduction of Fe(III) to Fe(II) resulted in increased layer charge but, as by this stage very

few smectite interlayers remained, this did not result in an equivalent increase in illite. Laser particle-

size analysis, supported by SEM observation, shows the existence of a bimodal distribution of clay

particle size (maxima at 0.2 and 1.5�1.8 mm) in which the finer fraction increases largely in thepedogenically affected samples, probably due to particle break-up caused by seasonal wetting and

drying. This ‘dual action’ illitization, first in a hypersaline lake and latterly through wetting and

drying, may be responsible for both the intensity of illitization and exceptionally high (for the Solent

Group) Fe content of the authigenic illite. The chemical characteristics of the illitic I-S and the illite

end-member correspond to glauconite. Hence, this is an example of onshore, non-pelletal glauconite

formation.

KEYWORDS: hypersaline lake, Isle of Wight, glauconite, low-temperature illitization, wetting and drying.

The Solent Group (Isle of Wight, Fig. 1) is a

succession of brackish and freshwater clays, sands

and thin freshwater limestones of Late Eocene to

Early Oligocene age, with a maximum thickness of

200 m, preserved only in the central part of the

Hampshire Basin, in southern Hampshire and the

Isle of Wight. The Solent Group represents the

youngest Palaeogene strata present onshore in the

UK. An extensive review of previous work on the

Tertiary deposits of the Isle of Wight has been

published by Daley (1999) and detailed accounts of

the sections exposed at Whitecliff Bay are given in

Gale et al. (2006).* E-mail: [email protected]: 10.1180/claymin.2010.045.1.35

ClayMinerals, (2010) 45, 35–49

# 2010 The Mineralogical Society

The Headon Hill Formation (the oldest part of

the Solent Group) is conspicuously cyclic; each

cycle includes: (a) laminated, shelly silts and silty

clays deposited in quasi-marine, brackish and

fluviatile environments, overlain by (b) units

including numerous palaeosols developed in fresh-

water silts and silty clays (Daley, 1973, 1999;

Huggett et al., 2001; Gale et al., 2006). The interval

described here is the youngest of these cycles. Most

of the Headon Hill Formation palaeosols are gleys

that have latterly undergone seasonal wetting and

drying. The interval described in this study is the

marl at the top of the Seagrove Member of the

Headon Hill. It differs from the older palaeosols in

that it consists of lacustrine marl (Daley, 1973,

1999), deposited close to the shoreline (Armenteros

et al., 1997) and affected by pedogenesis only in

the upper part. The lower part, up to 40 cm, has a

faint lamination, indicating sub-aqueous deposition.

At 40�50 cm, calcite pseudomorphs after gypsum

are present. Gypsum would have formed during

periods when evaporation exceeded precipitation

(White, 1997, p. 29). From 50�100 cm, slick-

ensiding that increases in intensity towards the top

is present. Slickensiding is an indicator of

pedogenic modification caused by movement of

peds (a block or crumb of soil) during wetting and

drying (Fitzpatrick, 1980, p. 180). Hence wetting

and drying increases in intensity through this

section. The marl ends with an organic-rich layer

a few cm thick that may indicate plant colonization

of the sediment, though no in situ roots were

observed. The colour of the marl changes from

green to grey at 110 cm. Calcite nodules are present

throughout the clay marl.

Authigenic clay minerals formed at or close to the

Earth’s surface are likely to be sensitive indicators

of the geochemical environment of deposition and

authigenesis. The spatial and temporal distribution of

pre-burial authigenic clay minerals in continental

settings is an important record of palaeoenviron-

mental and palaeoclimatic conditions (e.g. Singer,

1984; Singer & Stoffers, 1980; Gale et al., 2006).

The mechanism inferred for smectite illitization in

palaeosols (Robinson & Wright, 1987; Huggett et

al., 2001) is one of seasonal wetting and drying.

This has been proposed based on experimental work

(e.g. Mamy & Gaultier, 1975; Eberl et al., 1986;

Sucha & Siranova, 1991) where smectite subjected

to wetting and drying in the presence of K, NH4 or

Cs has been shown to undergo partial layer collapse.

Abundant slikensides in the Solent Group palaeosols

indicate that wetting and drying has occurred, while

red-green mottling indicates partial oxidation or

reduction which may be a consequence of wetting

and drying cycles (Huggett et al., 2001), reflecting

wetter winters and drier summers (Gale et al., 2006).

In a previous study (Huggett & Cuadros, 2005)

of authigenic illite-rich illite-smectite (I-S) from

pedogenically modified sediments from the Headon

Hill Formation we demonstrated that the driving

mechanism for what appears to be irreversible

Fe(III) reduction is wetting (reducing) and drying

(oxidizing) cycles in gley soil. However, the

FIG. 1. Location map of the Isle of Wight and the sampled section at Whitecliff Bay (WB). Only the late Tertiary

outcrop is shown.

36 J. M. Huggett and J. Cuadros

samples from this study were from several sections

and we were not able to detect clearly any

intermediate stages in the illitization process. Our

study also included one sample (named ‘44 m’ in

the previous study) significantly more Fe-rich and

more illitic than any other. Dating of this clay

(Huggett et al., 2001) indicates that it formed at

around the time of deposition (~34 Ma).

Consequently, for the detailed study of the

illitization process and the cause of the large Fe

enrichment, we selected a 1.1 m thick interval of

marl at the boundary of the Seagrove Member and

the overlying Bembridge Limestone Formation (this

is also the Eocene-Oligocene boundary) that

includes this highly illitic sample. The chemical

characterization of the illitic end-member in the

sequence shows it to be glauconite.

MATER IALS AND METHODS

Samples

Eleven samples of unweathered material were

taken at 10 cm intervals over a 1.1 m section of

marl at the very top of the Headon Hill Formation

between a limestone with green-grey clay laminae

and the Bembridge Limestone Formation (Fig. 2).

The lowest sample is 20 cm (0 cm is an arbitrary

datum), the highest 120 cm. A sample analysed

previously from a height of 97 cm above the base

of the marl (sample 44 m of Huggett et al., 2001,

and Huggett & Cuadros, 2005) proved to have the

most intense illitization and highest Fe content of

any of the Solent Group samples analysed at that

time; because of this, this sample was also included

in the present study, making the total number of

samples 12.

XRD and sample treatment

The untreated, whole-rock samples were analysed

by means of X-ray powder diffraction (XRD) to

determine their mineral composition. They were

ground with acetone in a rod mill, dried and gently

ground in an agate mortar before being side-packed

into sample holders and scanned at a rate of 1 s per

0.02º step width, using 0.3 mm slits from 5 to

65º2y. For further analysis, calcite was removed

with 30% acetic acid and the insoluble residue

washed three times with distilled water. The <2 mm

FIG. 2. Lithological log of the sampled section with sample positions (distance in cm to an arbitrary datum) and

their ‘illite’ content shown graphically.

Glauconite from lacustrine/palaeosol sediments 37

fraction was separated using centrifugation. The

sodium dithionite method (Smith, 1994) was used

to remove possible Fe or Al (oxyhydr)oxide.

Oriented mounts for XRD were prepared by

allowing a few drops of the clay slurry to dry on

a glass slide. Samples were analysed in the range

2�50º2y in a Philips PW1710 diffractometer at

45 kV and 40 mA using a Cu anode, a graphite

secondary monochromator, fixed slit and a 0.1 mm

receiving slit. The samples were scanned at

0.015º2y step size and 8 s/step. The analyses were

carried out in an air-dry state (20�25ºC, 50�60%relative humidity) and after overnight glycolation at

60ºC in a glycol atmosphere.

XRD modelling

The traces of the glycolated, oriented mounts

were modelled with NEWMOD (Reynolds &

Reynolds, 1996; Moore & Reynolds, 1997), a

programme that allows calculation of 00l profiles

for end-members and mixed-layer phyllosilicates

with different interlayer complexes. The NEWMOD

calculations included I-S of different compositions

and illite. The variables used to obtain the best

match with the experimental patterns were % illite

layers in the I-S phases, layer ordering (R) in the

I-S stacking sequence, Fe and K abundance, and

size of the coherent scattering domain. The

orientation of the particles was set at s* = 30º by

best match with the experimental patterns, where

s* is the standard deviation from a 0º angle (layers

perfectly parallel to the substrate) in a Gaussian

distribution.

Inductively coupled plasma atomic emission

spectrometry (ICP-AES)

Major and minor elements were analysed by ICP-

AES, after first removing Fe and Al oxides by the

sodium dithionite method. Powdered samples were

fused with LiBO2, then dissolved in dilute HNO3.

The concentrations of selected major and trace

elements were determined using a Varian Vista Pro

spectrometer with a CCD detector. Analytical errors

vary between 0.4 and 5% of measured values.

Fe(II) analysis

A split of the calcite- and oxide-free <2 mmfraction was also analysed for structural Fe(II). For

this, samples were digested in HF/H2SO4 and the

resulting solution titrated against potassium

permanganate.

Scanning electron microscopy (SEM) and

energy dispersive X-ray analysis (EDX)

Fresh, gold-coated, fracture surfaces of each rock

sample were examined in a Philips Field Emission

LV30 Scanning Electron Microscope (SEM) to

observe any changes in particle morphology with

illitization. For two samples (20 cm and 97 cm)

quantitative EDX analyses were collected from the

carbon-coated <2 mm fraction mounted on carbon

stickers on a glass slide, using a JEOL 5900LV

equipped with an Oxford Instruments INCA EDX

detector. Chemical analyses were calibrated using

mineral standards. The EDX analyses were

performed because these two samples showed Na

contamination, possibly from NaCl. Due to a lack

of samples for further ICP-AES analyses, EDX

analyses were obtained, which confirmed low-Na

contents, and these values were used in the

subsequent calculations.

Structural formulae

Bulk and EDX chemical analyses were trans-

formed into structural formulae on the O10(OH)2basis (per half formula unit: p.h.f.u.). Some Al was

assigned to the tetrahedral sheet so Si + Al = 4.

This provides accurate results given that quartz

contamination in the <2 mm fraction is always

minor. The remaining Al, Fe, Mn, Ti and Mg were

assigned to the octahedral sheet, up to a maximum

of two atoms. If this resulted in more than two

atoms, the necessary Mg was assigned to the

interlayer, with K, Na and Ca.

Laser particle size analysis (LPSA)

Particle size analysis was carried out on the

insoluble residue after calcite removal and prior to

<2 mm fraction separation, on all samples except

20 cm and 97 cm. Samples were dispersed in

deionized water and further dispersed in an ultrasound

bath for 1 min. They were analysed using a Malvern

Masters izer 2000 version, in the range

0.02�2000 mm. Relative proportions of particle-size

ranges were quantified by measuring the areas under

the size distribution maxima. The areas were

measured by curve-fitting procedures. As the experi-

mental distribution peaks have irregular shapes,


several peaks were used to fit each experimental peak

to obtain a good coverage of the area.

Infrared analysis

All samples except 97 cm were analysed using a

Perkin Elmer Spectrum One FTIR spectrometer in

transmission mode, in the range 4000�250 cm�1,

with a resolution of 16 cm�1, and accumulation of

eight scans per sample. The samples were prepared

as KBr pellets with 1�3 mg clay/200 mg KBr.

RESULTS

XRD qualitative analysis of powder mounts

The whole-rock mineralogy is predominantly

calcite, plus clays and minor quartz. Goethite was

only detected in the illite-rich samples from 70 to

110 cm inclusive, and potassium feldspar is present

in samples 20�60 cm inclusive.

XRD analysis of oriented mounts

The experimental and simulated patterns of the

glycolated oriented mounts (<2 mm fraction, after

calcite removal) are shown in Fig. 3. The calculated

background intensity in the low-angle region is

greater than the experimental one due to the

dependence of the effective size of the coherent

scattering domain on the diffracting angle (Plancon,

2002), a feature that NEWMOD does not

incorporate. We found that the sensitivity of the

calculations to Fe content in the I-S phases

decreased with illite proportion, and thus we do

not attach great significance to the calculated Fe

contents of the smectite-rich I-S phases. The only

clays present are illite and I-S, and traces of

kaolinite in sample 110 cm. Goethite is frequently

present (peak at 21º2y), whereas calcite (peak at

29.5º2y) and quartz (sharp peaks at 21 and 26.7º2y)appear in sample 97 cm. We modelled the patterns

using a limited number of components (Table 1)

TABLE 1. Parameters in the best fits of the NEWMOD calculations of oriented, glycolated mounts.

Sample Phase %I Fe N Nave Wt.% Tot %I

120 cm I-S 95 0.25 1-14 7 92 91I-S 40 0.7 1-6 4 8

110 cm I-S 95 0.4 1-14 7 94 92I-S 40 0.7 1-6 4 6

100 cm Illite 100 0.8 1-15 8 100 10097 cm I-S 70 0.8 1-7 4 12 96

Illite 100 0.8 1-5 2 15Illite 100 0.8 1-18 15 73

90 cm I-S 95 0.7 1-14 7 100 9580 cm I-S 95 0.7 1-14 7 100 9570 cm I-S 93 0.7 1-14 7 100 9360 cm I-S 90 0.6 1-14 7 80 80

I-S 40 0.7 1-6 4 2050 cm I-S 87 0.25 1-14 7 50 64

I-S 40 0.7 1-6 4 5040 cm I-S 85 0.25 1-14 7 70 72

I-S 40 0.7 1-6 4 3030 cm I-S 85 0.25 1-14 7 70 72

I-S 40 0.7 1-6 4 3020 cm I-S 85 0.25 1-14 7 85 78

I-S 40 0.7 1-6 4 15

%I, Tot %I = percent illite layers in the phase and the sample, respectivelyFe = in the corresponding phase per O10(OH)2N, Nave = range of number of layers and average number of layers in the coherent scattering domainOther parameters: R0, K in illite = 0.8 per O10(OH)2, illite d value = 9.98 A, smectite d-value = 16.9 A


FIG. 3. Experimental and calculated XRD patterns of the oriented, glycolated clay mounts.


but we believe that the samples consist of a mixture

of numerous I-S phases, within the range of

smectite and illite compositions given in Table 1.

Our model is not intended as a completely accurate

description of the complex samples but it is a good

approximation to their clay mineral composition. As

demonstrated previously for I-S from the Solent

Group (Huggett & Cuadros, 2005), R0 ordering

predominates in the entire range of I-S composition,

which points to a low-temperature illitization

process, as opposed to burial diagenesis illitization

that commonly results in R>1 for illite >~60%. The

modelled smectite-rich component has 40% illite

layers, while the illite-rich component has

85�100% illite layers. Where the lacustrine marl

has been pedogenically modified the smectite-rich

I-S is much reduced in abundance or absent, and

the illite-rich component is, on average, both more

abundant and more illitic (Table 1). The features of

the XRD traces of the air-dry mounts (not shown)

are perfectly compatible with the results obtained

from the glycolated mounts.

All samples with goethite have >90% illite.

Quartz is present in all bulk samples (powder

XRD) but the ratio of illite:quartz increases with %

illite in the clay fraction by more than an order of

magnitude from ~ 2 to 14. This implies that the

proportion of quartz is smallest in the most illitized

samples.

Chemical analyses

The chemical analyses confirm that the clay is

Fe-rich, and that K is the principal interlayer cation

(Table 2). The K content correlates well with the

illite content determined from XRD (Fig. 4). There

is a good negative correlation between K and Si

(Fig. 5a), indicating that illitization produces an

increase of the tetrahedral charge (from 0.24 to 0.58

p.h.f.u.). There is also a good negative correlation

between K and octahedral Al (Fig. 5b), mirrored by

a positive correlation between K and total (ferric +

ferrous) Fe (Fig. 5c). Thus, illitization was accom-

panied by Fe-for-Al substitution in the octahedral

sheet. There is a broad increase in total Mg

(octahedral + interlayer) with increasing K content

(Fig. 5d). Layer charge and K are also well

correlated (Fig. 5e). There is therefore more K,

Fe(II) and total Fe in the pedogenically modified

samples than in the lacustrine samples. It was noted

that the only two samples that were not green in the

bulk sample (110 and 120 cm) became green after

removal of Fe and Al oxides from the <2 mmfraction. From this we infer that colour in these

sediments is not related to clay composition. The

octahedral occupancy is 1.96�2.00 ions and R3+ is

1.65�1.74. The calculated charge of the interlayer

cations is 0.59�0.89 p.h.f.u. and is balanced by thelayer charge.

SEM and LPSA analysis

Assessment of the lateral size of individual

particles in SEM images suggests that the most

illite-rich samples (70�120 cm) are dominated by

particles ~0.1�0.2 mm (Fig. 6a). The more smec-

tite-rich samples appear to be dominated by two

particle sizes; ~0.1�0.2 mm and ~1�2 mm(Fig. 6b). Given that this SEM assessment is a

visual estimate of the clay particle dimensions, it is

remarkable that the LPSA data (Fig. 7) are quite

consistent with the SEM observations. The LPSA

particle-size distributions are bimodal, with maxima

at ~0.2 and 1.5�1.8 mm (Fig 7). Both maxima have

shapes approaching that of log-normal distributions,

much broader for the larger particle-size maximum.

IR analysis

The IR spectra are typical of dioctahedral Fe-rich

I-S. They show no evidence of interlayer NH4+ (no

band at 1450 cm�1). The OH-stretching region does

not provide usable information due to overlap with

a very prominent OH band from hydration water.

FIG. 4. Potassium vs. illite content in the clay samples.

The correlation corresponds to a 95% confidence

interval.


The OH-bending region displays features that

confirm the chemical results. The samples have

bands at the following approximate frequencies:

918 cm�1 (Al2OH), 875 cm�1 (AlFeOH) and

817 cm�1 (Fe2OH; Fig. 8). Also, a weak doublet

appears in some samples at ~795 and 775 cm�1,

corresponding to quartz. These band assignments

are fairly well established (Russell & Fraser, 1994).

The intensity of the Al2OH band in samples

20�60 cm is greater than that of Fe2OH.

However, for samples 70�120 cm (97 cm not

analysed) the Fe2OH band is more intense than

Al2OH. The AlFeOH band is always intense. These

results are in agreement with the chemical results,

as there is a sharp Fe increase and Al decrease in

samples 70�120 cm (Table 2). Figure 8 shows an

example of each sample group. The OH-bending

bands are wide, a typical characteristic of illitic clay

(Russell & Fraser, 1994).

D I SCUSS ION

The illitization

Huggett & Cuadros (2005) found that, for samples

from the Solent Group as a whole, all of which have

medium to large amounts of Fe, illite was formed in

soils subjected to seasonal wetting and drying that

caused Fe reduction during the wet periods, a

process that has been described in modern soils

(Siyuan & Stucki, 1994; Stucki, 1997). As complete

Fe re-oxidation did not occur during the dry seasons,

there was a net process of progressive reduction.

This conclusion was supported by analysis of Fe(II),

TABLE 2. Chemical composition of the <2 mm fraction of the studied samples and structural formulae calculated

on the basis of O10(OH)2.

Sample 120 110 100 97 90 80 70 60 50 40 30 20

% illite by XRD 91 92 100 96 95 95 93 80 64 72 72 78SiO2 45.10 43.45 43.00 44.70 43.75 44.77 44.44 45.42 44.90 45.31 46.04 45.96Al2O3 8.55 8.97 8.30 9.60 8.32 8.50 9.73 11.80 12.70 12.70 13.20 14.95MgO 2.07 1.93 1.75 2.11 1.78 1.73 1.67 1.61 1.58 1.62 1.51 1.75Fe2O3 24.12 23.67 22.33 22.81 25.28 23.05 22.24 15.20 10.97 10.63 10.36 9.82FeO 1.33 2.97 2.40 2.99 2.72 2.58 1.17 1.02 1.03 1.53 1.26 1.29MnO 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01TiO2 0.39 0.19 0.17 0.10 0.18 0.18 0.18 0.27 0.32 0.34 0.36 0.22CaO 0.12 0.49 0.39 0.19 0.34 0.56 0.64 0.91 1.23 1.17 1.31 1.17Na2O 0.18 0.18 0.15 0.15 0.18 0.28 0.18 0.19 0.23 0.18 0.22 0.25K2O 5.38 5.82 5.93 6.39 5.74 5.82 5.26 4.34 3.43 3.65 3.11 4.00

Sum 87.27 87.69 84.43 89.05 88.30 87.47 85.51 80.77 76.41 77.14 77.37 79.41

Structural formulaeSi 3.52 3.42 3.50 3.45 3.43 3.51 3.51 3.66 3.74 3.75 3.76 3.68Al tet. 0.48 0.58 0.50 0.55 0.57 0.49 0.49 0.34 0.26 0.25 0.24 0.32Al oct. 0.30 0.26 0.30 0.33 0.20 0.30 0.42 0.78 0.99 0.98 1.04 1.09Mg oct. 0.17 0.13 0.16 0.15 0.11 0.16 0.17 0.19 0.20 0.20 0.18 0.21Fe(III) 1.42 1.41 1.37 1.33 1.49 1.36 1.32 0.92 0.69 0.66 0.64 0.59Fe(II) 0.09 0.20 0.16 0.19 0.18 0.17 0.08 0.07 0.07 0.11 0.09 0.09Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ti 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.01Mg intl 0.07 0.10 0.05 0.10 0.09 0.04 0.03 0.00 0.00 0.00 0.00 0.00Ca 0.01 0.04 0.03 0.02 0.03 0.05 0.05 0.08 0.11 0.10 0.11 0.10Na 0.03 0.03 0.02 0.02 0.03 0.04 0.03 0.03 0.04 0.03 0.04 0.04K 0.54 0.59 0.62 0.63 0.58 0.58 0.53 0.45 0.36 0.39 0.32 0.41Sum oct. 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.98 1.96 1.97 1.96 1.99Total Fe 1.50 1.60 1.53 1.52 1.67 1.53 1.40 0.99 0.76 0.77 0.72 0.68Oct charge 0.24 0.32 0.31 0.33 0.28 0.32 0.24 0.29 0.36 0.37 0.35 0.32Intl charge 0.72 0.89 0.81 0.88 0.85 0.81 0.72 0.63 0.62 0.62 0.59 0.65


which increased with illite content. The chemical

data also confirmed that there is no evidence of the

usual process of Al-for-Si substitution in the

tetrahedral sheet, or Mg-for-(Al+Fe) substitution in

the octahedral sheet in the sample suite then

analysed, both of which processes could have

produced the layer charge increase necessary for

illitization. Modelling of oriented mount XRD

patterns by Huggett & Cuadros (2005) produced

several I-S phases of varying illite content. Their

results required the most illitic I-S phase in each

sample to have a smaller Fe content than the less

illitic components. The exception to this result was

the sample from the Seagrove Member (greatest Fe

content among samples from Huggett & Cuadros,

2005), where Fe is high and the same for all I-S

phases, for which result there was no explanation.

The present data allow a more complete view of the

processes producing illitization in the Solent Group,

explain the ‘anomalous’ sample of Seagrove

Member marl from the previous studies, its high

degree of illitization, the large Fe content of the

illitic clay and the absence of detrital kaolinite and

chlorite.

FIG. 5. Chemical correlations in the studied clays, indicating chemical modification accompanying illitization.

The only weak correlation is that of Mg vs. K (d).


The samples in this study are different from those

of Huggett & Cuadros (2005) in that illitization is

correlated with tetrahedral Al-for-Si substitution and

octahedral Fe-for-Al substitution (Fig. 5a�c). Thus,illitization in these samples is driven or influenced

by a different (or additional) process. There is

evidence of two types of depositional environment in

the sampled section. The lower part of the section,

below 50 cm, is lacustrine sediment (Daley, 1973,

1999), with no evidence of pedogenic modification.

Illitization of smectite in this interval therefore

occurred in a subaqueous hypersaline environment,

without any subsequent modification by pedogenic

processes. The presence of calcite pseudomorphs

after gypsum at 40�50 cm indicates increased

salinity relative to the lacustrine environment.

Illitization of smectite has previously been reported

from hypersaline lacustrine environments with alka-

line potassic pore fluid (Singer & Stoffers, 1980;

Deconinck et al., 1988). Potassium enrichment is

reported to have occurred through evaporation and

marine ingression in Berriasian lacustrine sediments

(Deconinck et al., 1988). Both these processes may

apply to the Seagrove Member lakes, which would

have lain close to the coastline (Armenteros et al.,

1997). Illitization in saline waters is accompanied by

Al-for-Si substitution and chemical changes in the

octahedral sheet, (Mg-for-(Al+Fe) substitution and

transformation from dioctahedral to trioctahedral

clays) driven by water chemistry (Singer &

Stoffers, 1980; Hover & Ashley, 2003).

The second type of environment in this study

corresponds to the section above 50 cm, where

slickensiding indicates increasing intensity of

pedogenesis, culminating in a thin layer with

organic matter in the top (sample 120 cm). It can

be inferred that the hypersaline lake dried up and

became a soil environment. Such an environment

would favour illitization driven by Fe reduction, as

in the samples of Huggett & Cuadros (2005).

Figure 9 shows that, in the present study, Fe(II)

remains approximately constant in the range

60�90% illite (0.3�0.55 K p.h.f.u.), which

corresponds to samples 20�80 cm, and then

increases sharply for samples 90�120 cm, where

there is a small increase of illite (90�100%) or K(0.55�0.63 atoms p.h.f.u.). The samples with the

greater Fe(II) content correspond to those with most

intense signs of pedogenesis processes. The two

environments (hypersaline lake and soil) produced

different processes influencing illitization, which

are described below.

All samples in this study experienced the hyper-

saline environment producing illitization, which was

more intense, generally, from the bottom to the top

of the lithological section. For this reason, all

samples show a correlation of Si, Al and Fe vs. K

or illite (Fig. 5). There is also a slight increase in

Mg content with increased K content, consistent

with hypersaline clay authigenesis (Fig. 5d). Such

illitization produced large illite contents in the upper

samples. After this process, the upper samples

(60�120 cm) also underwent pedogenesis and

concomitant wetting and drying processes, which

caused Fe reduction (Fig. 9). This process caused

further illitization, although of little extent because

the illite % was already large (~90%). Figure 5

FIG. 6. Fracture surface SEM images of the marl.

(a) Sample 100 cm, with 100% illite and grain size

predominantly ~0.1�0.2 mm. (b) Sample 40 cm, one ofthe more smectite-rich samples with 71.5% illite

measured by XRD, and grain size dominated by two

particle sizes; ~1�2 mm and ~0.1�0.2 mm. The

appearance of this sample is strikingly different to

that of shown in (a).


shows that data points are divided in two groups;

corresponding to those that experienced only

hypersaline clay authigenesis of lower intensity (K

< 0.5 atoms p.h.f.u.) and those that experienced

FIG. 7. Plots of the particle-size distribution measured by laser granulometry. The change of the area proportions

of the two peaks in the bimodal distribution correlates with the occurrence of pedogenic activity at the upper part

of the sequence.


more intense hypersaline clay authigenesis and

wetting and drying (K > 0.5 atoms p.h.f.u.). The

latter data points have a slightly increased dispersion

in Fig. 5a, b and e. This is probably due to the

overprinting of illitization driven by Fe reduction

over hypersaline clay authigenesis. In Fe-reduction

illitization there is no Al-for-Si substitution and the

illitization process is a very heterogeneous one

occurring in the solid state (Huggett & Cuadros,

2005), which slightly blurs the trends produced by

hypersaline clay authigenesis. In summary, the ‘dual

action’ illitization, first in a hypersaline lake and

latterly through wetting and drying, may be

responsible for the intensity of illitization and

exceptionally high (for the Solent Group) Fe

content of the authigenic illite. Consistent with

this, dating of the illite from the sample at 97 cm

indicated that all the illite was formed at around the

time of deposition (Huggett et al., 2001).

Clays of the mica family formed at low

temperature are characterized by large Fe contents

(e.g. Odin, 1988; Huggett, et al., 2001, and

references therein; Huggett, 2005). Most low-

temperature Fe-rich clay is marine but there are a

few accounts of neoformed ferric illite in lacustrine

or lagoonal environments (Singer & Stoffers, 1980;

Deconinck et al., 1988; Hay et al., 1991) and

palaeosols (Robinson & Wright, 1987; Ingles &

Ramos-Guerrero, 1995). However it is not apparent

from the published literature whether hypersaline

lakes or soils provide a more favourable environ-

ment for Fe fixation. The distribution of data points

in two clusters correlating with the hypersaline lake

and the palaeosol in Fig. 5 suggests that a second

phase of Fe and K increase took place in the soil

environment, at the top of the section. This is

consistent with our observation that the data point

with an intermediate Fe-content value (0.99 atoms

p.h.f.u.) was the first that was affected by

pedogenesis, and that the low intensity of slick-

ensiding indicates that pedogenesis for this sample

was less intense than in the overlying Fe-rich clay.

If the Fe-enrichment process happened entirely

within the lacustrine environment, with salinity

increasing with time, a continuous trend in the data

points might be expected, rather than two data

clusters on a trend (Fig. 5c). Thus, Fe enrichment

was probably produced in the lake and soil

environments and it is not possible to assess their

relative contribution to this process.

Further evidence that two processes operated in

the transformation of these samples is provided by

the particle-size data. The very wide maximum at

1.5�1.8 mm suggests a detrital component, with a

longer and more complex history, whereas the

narrow maximum at ~0.2 mm suggests a specific

event (Fig. 7). Up to 60 cm the I-S particle size

distribution is overwhelmingly dominated by the

maximum at 1.5�1.8 mm. Above 60 cm the

maximum at 0.2 mm becomes more prominent.

This sample marks the transition to samples

overprinted with soil processes. Precisely, wetting

and drying causes particle rupture and generates

smaller crystals. Besson et al. (1983) found that Cs-

FIG. 8. Part of the IR spectra of two selected samples,

showing the OH-bending bands corresponding to

AlOHAl (918 cm�1), AlOHFe (875 cm�1) and

FeOHFe (817 cm�1). There is a change in the relativeintensities of the AlOHAl and FeOHFe bands that

match their different Al and Fe contents.

FIG. 9. Illite and K content vs. Fe(II) content. Fe(II)

increases in the samples with the greatest illite content,

corresponding to those which suffered pedogenesis.

The implication is that the final stage of illitization in

these samples was caused by Fe reduction.


and K-smectite affected by wetting and drying

cycles had particles more than one order of

magnitude smaller than before the treatment.

Huggett & Cuadros (2005) also found evidence of

reduced lateral dimensions (a and b crystallographic

dimensions) of I-S crystals from palaeosols in the

Solent Group. Thus, seasonal wetting and drying in

a soil environment can explain the different

particle-size distribution below and above 60 cm.

Particle size provides further insight by comparison

with illite % (Fig. 10). A plot of the area under the

0.2 mm maximum (finer fraction) vs. illite % shows

a shallow slope in the range 64�100 % illite and

then a sharp increase with no illitization.

Tentatively, the plot can be interpreted as the

result of two processes occurring in the two

environments experienced by the samples. In the

hypersaline lake, neoformation of illite or illite-rich

I-S may have generated the finer fraction and

caused a slight increase of this fraction towards the

top, as the salinity of the waters increased. Then, in

the soil environment, particle break-up caused the

large increase of the fine fraction. Alternatively, the

dual particle-size distribution below 60 cm may be

that of the pre-illitized clay.

The absence of K feldspar above 60 cm suggests

that it has been dissolved in the pedogenic

environment and has provided K for the final

illitization that occurred in this environment

(resulting in 90�100% illite clay). Dissolution of

K feldspar in the soil is very likely because, in the

wet season, porewater would have been of small

ionic content (relative to the hypersaline lake). The

proportions of this mineral (<5 wt.% by XRD semi-

quantitative analysis) present in the lower part of

the section may be indicative of what was formerly

present above 60 cm.

The Seagrove Member marl has the only clay

assemblage in the formation that comprises only

illite and I-S. Throughout the rest of the Headon

Hill Formation, minor amounts of kaolinite and

chlorite occur in the clay assemblage. The absence

of these minerals from the Seagrove Member marl

may be a consequence of dissolution of these

detrital clays. Kaolinite may have dissolved in the

hypersaline lake because it is destabilized by high-

pH and ion concentration. Chlorite may have also

been dissolved in the lake, although chemical

conditions in hypersaline waters are probably not

far from the chlorite stability field (Garrels &

Christ, 1965; their Fig. 10.8). Chamley (1989)

points out that chlorite is usually absent or scarce

in evaporative environments not subjected to burial

diagenesis, from which it may be inferred that they

are outside the stability field of chlorite.

Defining the clay

Previously, Fe-rich 10 A clay from the brackish

and freshwater sediments of the Headon Hill

Formation have been described as Fe-rich illite

(Huggett et al., 2001). As a result of this study of

Seagrove Member marl, we have been able to

identify an interval with illite as the only clay

(~100 cm). This is conveniently also the interval

with least quartz contamination of the clay, and

therefore the best suited to correctly identifying

precisely the Fe-rich clay on the basis of chemistry

and charge distribution. Glauconite is defined by

Bailey (1980) as an Fe-rich dioctahedral clay with

tetrahedral Al usually >0.2 atoms per O10(OH)2,

octahedral R3+ >1.2 atoms, with Fe3+>>Al +

Mg>>Fe2+, and d060 >1.510 A. The structural

formulae in this work and previous measurement

of d060 (Huggett & Cuadros, 2005) indicate that all

these conditions are met by the Seagrove Member

10 A clay. The clay also conforms to the layer

charge and octahedral charge requirements for

glauconite. The 10 A clay in the Seagrove

Member marl has an interlayer charge of

0.80�0.89 and an octahedral charge of

0.30�0.33 p.h.f.u. (assessments from values in

Table 2). These values are within the chemical

FIG. 10. Relative proportion of the area under the peak

corresponding to the finer fraction (from laser

granulometry) vs. the illite percent.


and charge fields of glauconite as plotted by

Weiszburg et al. (2004; their fig. 9). For charge

0.8�0.85 p.h.f.u., our samples are glauconite

according to the definitions of both the AIPEA

(Bailey, 1980, 1986) and IMA (Rieder et al., 1998);

for charge >0.85 our samples are glauconite

according to the AIPEA and ‘unclassified’

according to the IMA. In situ pelletal authigenic

glauconite has previously been reported from very

shallow marine (Huggett & Gale, 1997) and

lagoonal environments (El Albani et al., 2005),

but we are not aware of any reports of glauconite,

pelletal or non-pelletal from a lacustrine setting.

The illite in the Seagrove Member is therefore a

rare occurrence of non-marine glauconite.

CONCLUS IONS

The lacustrine marl has a clay assemblage with a

range of I-S compositions that, for the sake of

modelling, has been simplified to one to three I-S

phases of increasing illite (more specifically,

glauconite) composition. Where the lacustrine

marl has been pedogenically modified the smec-

tite-rich mixed-layer clay is much reduced in

abundance or absent, and the illite-rich component

is both more abundant and more illitic. In one

sample (100 cm) the clay is pure glauconite. These

assemblages are a consequence of illitization of

detrital illite-smectite and dissolution of other clays

(kaolinite and chlorite) in the hypersaline waters.

Throughout the marl, the following processes

occurred in one or two of the geological

environments (hypersaline lake and soil) experi-

enced by the clays: increased layer charge through

Al substitution for Si in the tetrahedral sheet (lake),

slight Mg-for-Al substitution in the octahedral sheet

(lake), octahedral Fe-for-Al substitution (lake and

soil), K fixation in interlayer sites (lake and soil),

and reduction of Fe(III) to Fe(II) (soil). This ‘dual

mechanism’ has resulted in the most ‘illitic’, Fe-

rich neoformed clay in the Headon Hill Formation.

Following several nomenclature recommendations

(Bailey, 1980, 1986; Rieder et al., 1998) the Fe-rich

neoformed clay is glauconite. This is therefore a rare

occurrence of non-marine glauconite.

ACKNOWLEDGMENTS

Andy Gale from the University of Portsmouth is

thanked for assistance with sampling and David Wray

from the University of Greenwich is thanked for

carrying out the LPSA. Jean Francois Deconinck and

an anonymous reviewer are thanked for reviewing the

manuscript and their comments.

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