the nature and origin of authigenic smectites in some recent marine ...

Clay Minerals (1983) 18, 239-252
THE NATURE AND ORIGIN OF AUTHIGENIC
SMECTITES IN SOME RECENT MARINE
SEDIMENTS
f. G. COLE AND H. F. SHAW
Department of Geology, Imperial College, Prince Consort Road, London SW7, UK
(Received 21 February 1983; revised 12 April 1983)
A B S T R A C T: Three principal modes of formation are apparent for authigenic smectites in
Recent marine sediments: alteration of volcanic rocks and glass, low-temperature combination
of biogenic silica and Fe oxyhydroxides, and direct precipitation from hydrothermat fluids. The
latter two mechanisms are discussed with reference to new evidence from studies of sediments
from the Bauer Deep of the equatorial eastern Pacific and the Atlantis II Deep in the Red Sea. In
the Atlantis II Deep sediments, three sub-environments of smectite formation from hydro-
thermal fluids are recognized. In two of them nontronites are formed, whilst in the third an
Fe-poor smectite, intermediate in composition between beidellite and montmorillonite, occurs.
In general, clay mineral assemblages in Recent marine sediments are largely detrital in
origin and reflect the nature of soil-clay minerals formed by weathering on the adjacent
continents. The distributions of the main clay mineral groups in the oceans are
well-documented (Biscaye, 1965; Griffin et al., 1968; Rateev et aL, 1969; Windom, 1976).
Smectites, which are the predominant clay minerals in large areas of the Pacific, Indian
and South Atlantic Oceans (Fig. 1), present an important exception to the pattern of
detrital supply. Although a significant proportion of the smectites are detrital, the majority
form authigenically in the ocean sediments where a variety of formation mechanisms are
evident.
This paper presents a summary of the nature and origin of authigenic smectites in
Recent marine sediments and singles out two particular environments of authigenic
smectite formation which have been the subject of detailed analytical study (Cole, 1982)
and which represent two dominant sources of authigenic smectite in Recent marine
sediments.
NATURE OF AUTHIGENIC SMECTITES IN RECENT MARINE
SEDIMENTS
Nontronite, montmorillonite and saponite, in that order of apparent abundance, have been
reported as authigenic phases in Recent marine sediments. Examples of some authigenic
smectites from Recent marine sediments which have been reported in the literature are
given in Table 1.
One striking feature of authigenic smectites in marine sediments is that their layer
charges are generally higher than those of detrital smectites formed by continental
(~) 1983 The Mineralogical Society

240 T. G. Cole and H. F. Shaw
c..t\~,~,

Smectites in Recent sediments
TABLE l, Nature of smectites in modern marine sediments.
Clay mineral Location Interlayer cations Reference
Nontronite Bauer Deep C ao.oTNa o. 16Ko . 37 Dymond &
Eklund (1978)
Cao.23Nao.v2Ko.22 McMurtry & Yeh
(1981)
Cao.loNao.4oKo.lz Cole (1982)
Galapagos C a o. uNao.47Ko.41 Corliss et al. (1978)
C ao.ozNao.39Ko.42 Corliss et al. (1978)
Fe-rich montmorillonite
Saponite-Nontronite
Saponite
Red Sea Cao.ogNavooKo.o7 Bischoff (1972)
C ao.o3Nao.8oK o. 14 Bischoff (1972)
C ao.o9Na o. 94K0 . 32 Bischoff ( 1972)
241
C a o. loNav loKo . 18 Bischoff (1972)
Gulf&Aden Not known Cann et al. (1977)
Mid-Atlantic
(FAMOUS)
Not known Rona et al. (1976)
NE Pacific Cao.o2Nao.36Ko.14 Aoki et al. (1974)
Cao.o6Nao.38Koq2 Aoki et al. (1974)
Cao.osNao.a6Ko.14 Aoki et al. 0974)
Caoq4Nao.42Ko.12 Aoki et al. (1974)
SE Pacific Cao.oTNao.86Koq 1 Aoki et al. (1979)
Pacific (DOMES) Cao.28Nao.azKo.44 Hein et al. (1979)
St. Pauls Rocks Cao.osNao.21Ko.4z Melson & Thompson
(Atlantic)
(1973)
St. Pauls Rocks
(Atlantic)
Cao.t0Nao.29Ko.42 Melson & Thompson
(1973)
Peru Trench Cao.34Nao.6oKo,24 Scheidegger & Stakes
(1977)
weathering. In some cases the reported layer charges exceed the defined limit for smectites
(1.2 equivalents of charge per 020(OH)4 formula unit).
These apparently excessive layer charges could simply arise because no account was
taken of the presence of amorphous iron and other oxides and fine-grained mineral
impurities in the analysis sample when the structural formula was calculated. Also,
authigenic smectites in Recent marine sediments, particularly the nontronites, often occur
in environments containing higher than average concentrations of trace elements for
pelagic clays. The trace elements may become incorporated in the smectite structure
during formation but they are not always accounted for in calculations of layer charge.
ORIGIN OF AUTHIGENIC SMECTITES IN RECENT MARINE
SEDIMENTS
The origin of authigenic smectites in Recent marine sediments may be summarized in
terms of three separate mechanisms.
Alteration of volcanic rock fragments and glass
Significant proportions of basalts recovered during the DSDP and IPOD drilling
programmes have been found to be altered to various clay minerals. Iron-bearing saponites
are the most widespread, occurring as veins in the groundmass, infilling vesicles, and

242 T. G. Cole and H. F. Shaw
replacing phenocrysts and glass of the altered basalts (Banks, 1972; Melson &
Thompson, 1973; Scheidegger & Stakes, 1977; Vallier & Kidd, 1977; Seyfried et'al.,
1978; Kurnosov et al., 1982). The saponites form by the reaction of basalts with juvenile
water or seawater under reducing alkaline conditions at temperatures ranging from 400~
to normal ocean-bottom temperatures (Kurnosov et al., 1982).
Kurnosov et al. (1982) also concluded that under oxidizing conditions at relatively low
temperatures (

o
o
eo
Smeetites in Recent sediments 243
- -_ 9 ee
O.
t~
; o7oo %
110 ~ 100 ~ 90 ~ 80 ~
FIG. 2. Location of the Bauer Deep, southeast Pacific.
..
2.., 9 -
;;...
I',~:, 2o:
!l .....
P IL? , "
i . . . . . 9
3. ' 9
2::'.- .'
',.' 9 ,
i:"... *o~
,..... . . .
The Bauer Deep sediments represent an off-ocean ridge metalliferous deposit having a
trace element content similar to the metalliferous sediments of the adjacent East Pacific
Rise when the latter are expressed on a carbonate-free basis. The Bauer Deep nontronite is
therefore an example of an authigenic smectite associated with higher than average pelagic
clay concentrations of trace elements.
Scanning electron microscopy (SEM) of sediment specimens from the Bauer Deep
shows that the siliceous microfossils are well-preserved in the surface horizons of the
sediment (Fig. 3a). However, the microfossils undergo corrosion during burial, with
infilling and replacement of the skeletal structures by nontronite (Fig 3b). The microfossils
appear to provide a substrate for the authigenic growth of nontronite (Fig. 3c), suggesting
1.5:" "

244 T. G. Cole and H. F. Shaw
FIG 3. Scanning electron micrographs of sediments from the Bauer Deep. (a) Well-preserved
siliceous microfossils in surface sediments (x 1200); (b) Corrosion and nontronite infiIling of
siliceous microfossils in buried sediments (x 1200); (c) Nontronite infilling of siliceous
microfossils (x 3200).
that the silica source for the clay mineral is biogenic in origin. Qualitative elemental
analysis of the infilling clay material indicated that it was composed predominantly of Si
and Fe.
The iron necessary for the formation of the nontronite originates from the East Pacific
Rise as a hydrothermal precipitate. This is transported, along with other trace elements, as
X-ray amorphous Fe- (and Mn-)oxyhydroxide flocs which are ponded in the Bauer Deep
(Dymond & Veeh, 1975; Lonsdale, 1976; Heath & Dymond, 1977; 1981; Edmond et al.,
1982; Cole, 1982). The Fe-oxyhydroxide probably becomes associated with the biogenic
opal in the Bauer Deep sediments by coordination of Fe(III) to silanol groups (SiO-) at
the surfaces of the dissolving microfauna to form an Fe(III)-Si complex which is the
precursor of the nontronite. Harder (1976; 1978) has synthesized Fe-rich layer-silicates,

Smeetites in Recent sediments 245
including nontronite, at low temperatures (

246 T. G. Cole and H. F. Shaw
35 ~ 36 ~ 37 ~ 38 ~ 319~ 40 ~
I I I I I
Oceanographer Deep
Kebrit Deep
f
.Gypsum Deep
=ma Deep
Nereus Deep
41 ~ 42 o
I I
Thetis Deep ~ o //~Atlantis II Deep
Wando Basin ~ ,Chain Deeps
Valdivia Dee
P~
--ll~e/ )~Djidda
I i- ee_____~Albatross Deep
DeepJ e ~ a
Discovery j Deep
Erba Deep I |
Port Sudan ~////)
Port Sudan DeeD
Suakin Dee
9 Deeps with brine & metalliferous sediment.
9 Deeps with metalliferous sediment only.
| Deeps with brine only.
FIG. 4. Location of the Atlantis II Deep, Red Sea.
work by Bischoff (1972) showed that the nontronites from these sediments had a typical
composition of
9 . 3+ OH nH O
Cao.09Nal.ooKo.07(Fe~+aFe~+3Mno.18Zno.41Cuo llMgo.41) (S16.74Alo.48Feo.80)O20( )4. 2 ~
From the previous work of Bischoff (1972) the nontronites appeared to be poorly
crystalline. However, recent XRD analyses carried out by Cole (1982) indicate that the
nontronites are 'well crystallized' (v/p ratio 0.80-0.85).
Scanning electron microscope examination of the nontronites showed that they had a
flaky, often 'honeycombed', texture (Fig. 5a) typical of smectites (Bohor & Hughes,
1971). However, the nontronite in the sulphide/silicate/amorphous facies may also occur
with a fibrous texture (Fig. 5b). This is a relatively unusual morphology for smectites
(Kurnosov et al., 1982).
E
--26"
29 ~
28 ~
27 ~
--25 ~
--24 ~
22 ~
21 ~
20 ~
19 ~
118 ~
I17 ~
16 ~
15 ~

Smectites in Recent sediments 247
FIG. 5. Scanning electron micrographs of sediments from the Atlantis II Deep. (a) Flaky texture
of nontronite in sulphide/silicate/amorphous facies ( 570); (b) Fibrous texture of nontronite in
sulphide/silicate/amorphous facies (x2400); (c) Flaky/fibrous texture of nontronite in
silicate/carbonate/oxide facies (x4000); (d) Flaky texture of montmorillonite-beidellite in
sulphate/sulphide/silicate/oxide facies (x 1330).
The formation of the nontronite in the sulphide/silicate/amorphous facies is thought to
occur by sorption of silica supplied by the incoming hydrothermal brines onto
Fe-oxyhydroxide which settled from the upper brine pool layers; an Fe(III)-Si complex
analogous to the precursor of the Bauer Deep nontronite results. This mechanism is
supported by oxygen isotope data which indicate that the nontronite has a formation
temperature in the range 90-140~ (Cole, 1982). This rather wide temperature range may
be accounted for in terms of precise sample location in the facies but, overall, it is
intermediate between the temperature of the incoming brine, which may be as high as
250~ (Shanks & Bischoff, 1977), and the temperature of the deep brine pool which is
about 60~ (Hartmann, 1980). The temperature range reflects mixing of isotopically light
SIO2, which equilibrated at high temperatures with sub-surface brines, and isotopically

248 T. G. Cole and H. F. Shaw
heavier Fe-oxyhydroxide, which equilibrated at lower temperatures in the upper brine pool
layers.
This nontronite is the dominant constituent of the sulphide/silicate/amorphous facies
which is forming at the present day over most of the Atlantis II Deep in the anoxic layers
of the brine pool, and represents the thickest and most continuous facies in the Deep
overall (Backer & Richter, 1973; Hackett & Bischoff, 1973). Consequently, the nontronite
in this facies is quantitatively one of the most important phases in the metalliferous
sediments as a whole.
The second environment of smectite formation in the Atlantis II Deep is also
characterized by nontronite formation but in association with poorly crystallized
Fe(III)-oxide and manganosiderite; this association was defined as the silicate/carbonate/
oxide facies by Cole (1982). X-ray diffraction analysis of bulk samples and clay fractions
of the sediments from this facies showed the nontronite to have similar characteristics to
the nontronite from the anoxic facies described above. The nontronite was also well
crystallized with a v/p ratio of 0.85-0.90.
The Fe-rich nature of the nontronite was confirmed by chemical analysis during SEM
examination of the sediments. The SEM analysis also showed that the nontronite again
exhibited both flaky and fibrous morphologies similar to the nontronites from the anoxic
facies (Fig. 5c).
The occurrence of a nontronite in oxic sediments having this mineral association has
been reported from the Atlantis II Deep (Goulart, 1976) but it has not previously been
distinguished from the nontronite of the anoxic sulphide/silicate/amorphous facies.
The origin of the nontronite in this facies is probably similar to that of the nontronite
from the anoxic material but, owing to its occurrence in oxic sediments, its formation
mechanism may differ in detail. For example, the nontronite in the oxic sediments may
incorporate a higher Fe: Si ratio than the nontronite occurring in the anoxic sediments.
This would reflect its association with sediments which characterize the lower-temperature
oxic end of the mineral fractionation sequence of the geothermal system, at the periphery
of the Atlantis II Deep. This hypothesis is again supported by oxygen isotope data which
indicate a formation temperature for the nontronite of the silicate/carbonate/oxide facies
of ~ 80 ~ C (Cole, 1982). This temperature is below the formation temperature range of the
nontronite in the anoxic sediments but it is sufficiently high for a similar formation
mechanism to be applicable.
The third environment of smectite formation is characterized by an Fe-poor smectite.
Detailed X-ray diffraction analyses, including the application of the Greene-Kelly test
(MacEwan & Wilson, 1980), indicated that this smectite had a composition intermediate
between that of montmorillonite and beidellite. The montmorillonite-beidellite showed
unusual behaviour on heating. Complete collapse did not occur at 400~ and heating to
>500~ was necessary for this. The Fe-poor nature of the smectite was also confirmed
from chemical analysis of the clay during scanning electron microscopy. Fe-poor varieties
of smectite in the Atlantis II Deep have not previously been reported.
This variety of smectite is well crystallized with a v/p ratio of 0.85-0.90. However,
unlike the authigenic nontronites from the Atlantis II Deep described above, the
montmorillonite-beidellite only shows a flaky morphology with no evidence of fibrous
texture (Fig. 5d). The montmorillonite-beidellite occurs associated with anhydrite and
sulphides and localized occurrences of hematite; this was defined as the sulphate/
sulphide/silicate/oxide facies by Cole (1982). This facies represented the lowermost 2 m
section underlying 70 cm of sulphide/silicate/amorphous facies (uppermost unit) in a

Smectites in Recent sediments 249
single 2.7 m gravity core. The detailed mineralogy of this facies reflects a rather complex
depositional history where the minerals did not all form contemporaneously. However, the
mineralogical data indicate that formation of this facies was associated with active brine
discharge and that the core was probably recovered at the site of an extinct vent. This is
supported by oxygen isotope data for the smectite of this facies which indicate a formation
temperature in the range 160-200~ (Cole, 1982). This is approaching the estimated
temperature of the subsurface brine just prior to discharge (Shanks & Bischoff, 1977).
The mineralogical and isotopic evidence available suggest that this variety of smectite
formed by direct precipitation of Si and A1, both supplied from a discharging hydrothermal
brine, at the site of a once-active vent. The very high formation temperature range of the
montmorillonite-beidellite would appear to exclude any significant involvement of, or
exchange with, other possible sources of oxygen besides that associated with the
brine-derived Si and A1, such as dissolved 02 or H20-oxygen derived from elsewhere in the
brine pool--unless they became incorporated into the montmorillonite-beidellite in the
vicinity of the brine discharge. Therefore the formation-temperature range of this smectite
probably reflects the temperature range in which dissolved SiO2 in the incoming brine
becomes supersaturated and fixes its isotopic composition by precipitation from solution.
Incorporation of the SiO 2 into the montmorillonite-beidellite in the vicinity of the brine
discharge vent would then enable this clay mineral to retain the high-temperature imprint
of the SiO 2.
The montmorillonite-beidellite represents a quantitatively important phase in the
sulphate/sulphide/silicate/oxide facies, but owing to its unique association with active
brine discharge it is probable that neither this smectite species nor the facies itself are of
quantitative importance in the Atlantis II Deep as a whole; the anoxic nontronite and the
sulphide/silicate/amorphous facies are, respectively, the more commonly occurring
smectite variety and silicate-containing facies.
CONCLUSIONS
From the above discussion it is evident that a variety of di- and tri-octahedral smectites
can form authigenically. Three principal mechanisms account for their formation:
alteration of volcanic rock fragments and glasses, low-temperature combination of
Fe-oxyhydroxides and biogenic silica, and direct precipitation from hydrothermal fluids.
The nontronites are the most widespread of the authigenic smectites. They form by all
three mechanisms outlined above over a wide range of temperatures and physico-chemical
conditions. In particular, isotopic, chemical, and mineralogical data have shown that in the
Bauer Deep of the southeast Pacific, well-crystallized nontronites are generated at normal
ocean-bottom water temperatures of 2-3~ under generally oxic conditions by the
combination of Fe-oxyhydroxides and biogenic silica. In the Atlantis II Deep of the Red
Sea, formation of well-crystallized nontronite also occurs but by direct precipitation from
hydrothermal fluids (brines) over a wide range of temperatures, and in both oxic and
anoxic environments. In the oxic environment the formation temperature is 80~ while in
the anoxic environment formation temperatures in the range 90-140~ have been
measured. Although not a subject of analytical attention in the present work, nontronite
formation by low-temperature (

250 T. G. Cole and H. F. Shaw
The necessity of an Fe-oxyhydroxide precursor for nontronite formation is obvious
with regard to the mechanism involving low-temperature combination of Fe-oxyhydroxide
and biogenic silica, but it is apparent that a similar precursor is also required in the
mechanism involving precipitation of nontronite from hydrothermal fluids, in particular the
hydrothermal brines of the Atlantis II Deep. Indirect support for this requirement in the
case of nontronite precipitation from the Atlantis II Deep brines is apparent from the
exceptional occurrence of the Fe-poor montmorillonite-beidellite in the Atlantis II Deep
associated with active brine discharge. In the vicinity of the hydrothermal vents, the
conditions would be too anoxic to host an Fe-oxyhydroxide precursor for possible
nontronite formation and, as observed, under these circumstances precipitation of an
Fe-poor smectite occurs. Conversely, formation of the Fe-poor smectite in the Atlantis II
Deep does not appear to compete with the nontronite formation as it is not found
intermixed with the nontronites. This is probably due to the predominance of the
nontronite formation mechanism and because the ALsupply of the discharging brines is
only limited. The extent of Fe-poor smectite formation is therefore restricted to localized
occurrences only, associated with active brine discharge, and, except at brine discharge
sites, nontronite precipitation involving an Fe-oxyhydroxide precursor appears to be the
dominant mode of smectite formation in the Atlantis II Deep.
Clearly, where nontronite formation in the Recent marine environment does involve an
Fe-oxyhydroxide precursor, this sets an important constraint on the range of redox
conditions under which the formation reaction can take place, at least in its early stages.
Later stages appear to be less constrained by redox conditions, although Harder (1976;
1978) concluded from laboratory synthesis experiments that nontronite precipitation is
achieved at surface temperatures by nucleation under oxidizing conditions followed by
ageing under reducing conditions. The dominant smectite formation environment in the
Atlantis II Deep, characterized by the anoxic nontronite, is compatible in terms of redox
conditions both with the overall conditions and the trend of redox conditions proposed by
Harder (I 976, 1978). Possibly, in the Bauer Deep and other similar generally oxic regions
where nontronite formation by combination of Fe-oxyhydroxide and opal occurs, anoxic
microenvironments may be generated within the interiors of the siliceous structures which
could promote their infilling and replacement by the nontronite.
Although nontronites are formed during the alteration of basalts, the Fe-bearing
saponites are the smectites that characterize the reactions of basalts with juvenile water or
seawater. According to Kurnosov et al. (1982) saponites have a very wide range of
formation temperatures from 400~ down to ocean-bottom temperatures but appear to
form only under reducing alkaline conditons.
Additional, more detailed investigation of the mineralogy and the environments of
formation of authigenic smectites is required to define more closely the physico-chemical
conditions that control their generation. In particular, stable-isotope geothermometry
should be more widely used on carefully separated and concentrated smectite samples to
provide temperatures of formation.
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RESUME: II existe trois modes de formation de smectites authig~nes dans les s6diments
matins recents: I'alt~ration de roches et verres volcaniques, la combinaison fi basse temperature
de silice biog~ne et d'oxhydrate de fer, la precipitation directe au sein de fluides hydrothermaux.
Les deux derniers m6canismes sont discut~s en faisant appel ~i des resultats nouveaux dus fi
I'~tude de sMiments de la Depression de Bauer du Pacifique Equatorial Oriental et de la
Depression Atlantis II de la Mer Rouge. Les s6diments de la D~pression Altantis II permettent
de mettre en 6vidence 3 sous groupes de formation de smectite fi partir de fluides hydrothermaux.
Dans 2 de ceux-ci on forme de la nontronite, tandis que dans la 3e appara~t une smectite pauvre
en fer de composition interm6diaire entre une beidellite ct une montmorillonite.
K U R Z R E F E R A T : Drei haupts~ichliche Bildungsprozesse for authigeoe Smectite in rezenten
marinen Sedimenten sind: Verfinderung vulkanischer Gesteine und Glfiser, Reaktion von
biogener Kiesels~iure mit Fe oxyhydroxiden bei niedrigen Tempcraturen und direkte Ausf'Mlung
aus hydrothermalen L6sungen. Anhand neuerer Ergebnisse von Untersuchungen des Bauer-
Tief (6stlicher ~iquatorialer Pazifik) und des Atlantis ll-Tief (Rotes Meet) werden die letztcren
zwei Mechanismen diskutiert. In den Sedimenten des Atlantis ll-Tief sind drei unterschiedliche
Bildungsmilieus ffir Smectitc aus hydrothermalen L6sungen zu finden: In zwei davon bilden sich
Nontronite, w~hrend im dritten ein eisenarmer Smectit gebildet wird. der in der Zusammenset-
zung zwischen Beidellit und Montmorillonit steht.
R ESUMEN: Existen tres principales modos de formacion de esmectitas autigenicas en
sedimentos marinos recientes: alteracion de rocas volcfinicas y vidrios, combinaci6n a baja
temperatura de silice bi6gene y oxihidr6xidos de hierro, y precipitaci6n directa a partir de flfiidos
hidrotermales. Se discuten los dos flltimos mecanismos teniendo en cuenta las recientes
evidencias obtenidas en el estudio de los sedimentos de la depresion Bauer en el Pacifico cste
ccuatorial y de la depresi6n Altantis II en el mar Rojo. En los sedimentos de la depresion
Atlantis II aparecen ires subentornos de formaci6n de esmectitas a partir de flflidos hidro
termales. En dos de ellos se forman nontronitas, mientras qucen el tercero aparece una esmectita
pobre en hierro, de composici6n intermedia entre beidellita y montmorillonita.